diff --git a/app/controllers/tournaments_controller.rb b/app/controllers/tournaments_controller.rb
index d11667d..9fa7f18 100644
--- a/app/controllers/tournaments_controller.rb
+++ b/app/controllers/tournaments_controller.rb
@@ -5,6 +5,7 @@ class TournamentsController < ApplicationController
   before_action :authenticate_user!, only: %i[create update destroy]
   before_action -> { require_owner! @tournament.owner }, only: %i[update destroy]
   before_action :validate_create_params, only: %i[create]
+  before_action :validate_update_params, only: %i[update]
   rescue_from ActiveRecord::RecordNotFound, with: :render_not_found_error
 
   # GET /tournaments
@@ -63,10 +64,20 @@ class TournamentsController < ApplicationController
 
   # PATCH/PUT /tournaments/1
   def update
-    if @tournament.update(tournament_params)
-      render json: @tournament
-    else
-      render json: @tournament.errors, status: :unprocessable_entity
+    Tournament.transaction do
+      if only_playoff_teams_amount_changed
+        @tournament.instant_finalists_amount, @tournament.intermediate_round_participants_amount =
+          TournamentService.calculate_default_amount_of_teams_advancing(
+            params['playoff_teams_amount'].to_i,
+            @tournament.stages.find_by(level: -1).groups.size
+          )
+      end
+      if @tournament.update(tournament_params)
+        render json: @tournament
+      else
+        render json: @tournament.errors, status: :unprocessable_entity
+        raise ActiveRecord::Rollback
+      end
     end
   end
 
@@ -113,4 +124,26 @@ class TournamentsController < ApplicationController
 
     render json: { error: 'Invalid teams array' }, status: :unprocessable_entity
   end
+
+  def only_playoff_teams_amount_changed
+    params['playoff_teams_amount'] &&
+      params['instant_finalists_amount'].nil? &&
+      params['intermediate_round_participants_amount'].nil?
+  end
+
+  def validate_update_params
+    return if only_playoff_teams_amount_changed
+
+    playoff_teams_amount = params['playoff_teams_amount'].to_i || @tournament.playoff_teams_amount
+    instant_finalists_amount = params['instant_finalists_amount'].to_i || @tournament.instant_finalists_amount
+    intermediate_round_participants_amount = params['intermediate_round_participants_amount'].to_i ||
+                                             @tournament.intermediate_round_participants_amount
+
+    return if instant_finalists_amount + (intermediate_round_participants_amount / 2) ==
+              playoff_teams_amount
+
+    render json: {
+      error: 'playoff_teams_amount, instant_finalists_amount and intermediate_round_participants_amount don\'t match'
+    }, status: :unprocessable_entity
+  end
 end
diff --git a/app/interactors/add_group_stage_to_tournament.rb b/app/interactors/add_group_stage_to_tournament.rb
index d1d28e2..23e81de 100644
--- a/app/interactors/add_group_stage_to_tournament.rb
+++ b/app/interactors/add_group_stage_to_tournament.rb
@@ -10,6 +10,9 @@ class AddGroupStageToTournament
     begin
       group_stage = GroupStageService.generate_group_stage(groups)
       tournament.stages = [group_stage]
+      tournament.instant_finalists_amount, tournament.intermediate_round_participants_amount =
+        TournamentService.calculate_default_amount_of_teams_advancing(tournament.playoff_teams_amount,
+                                                                      group_stage.groups.size)
       context.object_to_save = tournament
     rescue StandardError
       context.fail!
diff --git a/app/models/stage.rb b/app/models/stage.rb
index 9246352..1f36b21 100644
--- a/app/models/stage.rb
+++ b/app/models/stage.rb
@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class Stage < ApplicationRecord
+  enum state: %i[playoff_stage in_progress finished]
+
   belongs_to :tournament
   has_many :matches, dependent: :destroy
   has_many :groups, dependent: :destroy
diff --git a/app/models/tournament.rb b/app/models/tournament.rb
index 3813f08..ec71609 100644
--- a/app/models/tournament.rb
+++ b/app/models/tournament.rb
@@ -10,6 +10,8 @@ class Tournament < ApplicationRecord
   validates :name, presence: true
   validates :code, presence: true, uniqueness: true
 
+  validate :playoff_teams_amount_is_positive_power_of_two
+
   alias_attribute :owner, :user
 
   after_initialize :generate_code
@@ -24,4 +26,11 @@ class Tournament < ApplicationRecord
       break if errors['code'].blank?
     end
   end
+
+  def playoff_teams_amount_is_positive_power_of_two
+    return if (Utils.po2?(playoff_teams_amount) && playoff_teams_amount.positive?) || playoff_teams_amount.zero?
+
+    errors.add(:playoff_teams_amount,
+               'playoff_teams_amount needs to be a positive power of two')
+  end
 end
diff --git a/app/serializers/stage_serializer.rb b/app/serializers/stage_serializer.rb
index dc76a83..56cb6d0 100644
--- a/app/serializers/stage_serializer.rb
+++ b/app/serializers/stage_serializer.rb
@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 class StageSerializer < ApplicationSerializer
-  attributes :level
+  attributes :level, :state
 
   has_many :matches
   has_many :groups
diff --git a/app/serializers/tournament_serializer.rb b/app/serializers/tournament_serializer.rb
index 9805797..25582e1 100644
--- a/app/serializers/tournament_serializer.rb
+++ b/app/serializers/tournament_serializer.rb
@@ -1,7 +1,8 @@
 # frozen_string_literal: true
 
 class TournamentSerializer < SimpleTournamentSerializer
-  attributes :description
+  attributes :description, :playoff_teams_amount,
+             :instant_finalists_amount, :intermediate_round_participants_amount
   has_many :stages
   has_many :teams
 
diff --git a/app/services/group_stage_service.rb b/app/services/group_stage_service.rb
index 592f6ad..504ff24 100644
--- a/app/services/group_stage_service.rb
+++ b/app/services/group_stage_service.rb
@@ -9,7 +9,7 @@ class GroupStageService
       raise 'Groups need to be equal size' unless (groups.flatten.length.to_f / groups.length.to_f % 1).zero?
 
       groups = groups.map(&method(:get_group_object_from))
-      Stage.new level: -1, groups: groups
+      Stage.new level: -1, groups: groups, state: :in_progress
     end
 
     def get_group_object_from(team_array)
diff --git a/app/services/tournament_service.rb b/app/services/tournament_service.rb
new file mode 100644
index 0000000..b01ef02
--- /dev/null
+++ b/app/services/tournament_service.rb
@@ -0,0 +1,14 @@
+# frozen_string_literal: true
+
+class TournamentService
+  class << self
+    def calculate_default_amount_of_teams_advancing(playoff_teams_amount, amount_of_groups)
+      # the amount of whole places that advance in a group (e. g. all 1rst places of every group instantly go through)
+      instant_finalists_amount = (playoff_teams_amount.floor / amount_of_groups.floor) * amount_of_groups.floor
+      # the amount of teams that still need to play an intermediate round before advancing to playoffs
+      intermediate_round_participants_amount = (playoff_teams_amount - instant_finalists_amount) * 2
+
+      [instant_finalists_amount, intermediate_round_participants_amount]
+    end
+  end
+end
diff --git a/db/migrate/0000_create_schema.rb b/db/migrate/0000_create_schema.rb
index f1d4a3f..61bba1f 100644
--- a/db/migrate/0000_create_schema.rb
+++ b/db/migrate/0000_create_schema.rb
@@ -51,7 +51,9 @@ class CreateSchema < ActiveRecord::Migration[5.2]
       t.string :code, null: false, index: { unique: true }
       t.string :description
       t.boolean :public, default: true
-      t.integer :playoff_teams_amount
+      t.integer :playoff_teams_amount, default: 0
+      t.integer :instant_finalists_amount, default: 0
+      t.integer :intermediate_round_participants_amount, default: 0
 
       # relation to owner
       t.belongs_to :user, index: true, null: false, foreign_key: { on_delete: :cascade }
@@ -61,6 +63,7 @@ class CreateSchema < ActiveRecord::Migration[5.2]
 
     create_table :stages do |t|
       t.integer :level
+      t.integer :state, default: 0
 
       t.belongs_to :tournament, index: true, foreign_key: { on_delete: :cascade }, null: false
 
diff --git a/spec/controllers/tournaments_controller_spec.rb b/spec/controllers/tournaments_controller_spec.rb
index c3313bd..06e05dc 100644
--- a/spec/controllers/tournaments_controller_spec.rb
+++ b/spec/controllers/tournaments_controller_spec.rb
@@ -194,6 +194,42 @@ RSpec.describe TournamentsController, type: :controller do
             expect(@group_stage_tournament.playoff_teams_amount)
               .to eq(create_group_tournament_data[:playoff_teams_amount])
           end
+
+          context 'playoff_teams_amount unacceptable' do
+            shared_examples_for 'wrong playoff_teams_amount' do
+              it 'fails' do
+                expect(response).to have_http_status(:unprocessable_entity)
+              end
+              it 'returns the correct error message' do
+                expect(deserialize_response(response)[:playoff_teams_amount].first)
+                  .to eq('playoff_teams_amount needs to be a positive power of two')
+              end
+            end
+
+            context 'is not a power of two' do
+              before do
+                post :create, params: create_group_tournament_data.merge(playoff_teams_amount: 18)
+              end
+
+              it_should_behave_like 'wrong playoff_teams_amount'
+            end
+
+            context 'isn\'t positive' do
+              before do
+                post :create, params: create_group_tournament_data.merge(playoff_teams_amount: -16)
+              end
+
+              it_should_behave_like 'wrong playoff_teams_amount'
+            end
+
+            context 'isn\'t positive nor a power of two' do
+              before do
+                post :create, params: create_group_tournament_data.merge(playoff_teams_amount: -42)
+              end
+
+              it_should_behave_like 'wrong playoff_teams_amount'
+            end
+          end
         end
 
         it 'renders a JSON response with the new tournament' do
@@ -282,6 +318,100 @@ RSpec.describe TournamentsController, type: :controller do
           expect(response).to have_http_status(:ok)
           expect(response.content_type).to eq('application/json')
         end
+
+        context 'any variable relevant for group stage to playoff transition changed' do
+          before(:each) do
+            @filled_tournament = create(:group_stage_tournament)
+            apply_authentication_headers_for @filled_tournament.owner
+          end
+
+          it 'fails when only instant_finalists_amount is changed' do
+            put :update, params: { id: @filled_tournament.to_param }.merge(instant_finalists_amount: 29)
+            expect(response).to have_http_status(:unprocessable_entity)
+          end
+
+          it 'fails when only intermediate_round_participants_amount is changed' do
+            put :update, params: { id: @filled_tournament.to_param }.merge(intermediate_round_participants_amount: 29)
+            expect(response).to have_http_status(:unprocessable_entity)
+          end
+
+          it 'fails when parameters don\'t match' do
+            put :update, params: { id: @filled_tournament.to_param }.merge(intermediate_round_participants_amount: 29,
+                                                                           instant_finalists_amount: 32)
+            expect(response).to have_http_status(:unprocessable_entity)
+          end
+
+          it 'succeeds when all three are changed correctly' do
+            put :update, params: { id: @filled_tournament.to_param }.merge(intermediate_round_participants_amount: 2,
+                                                                           instant_finalists_amount: 1,
+                                                                           playoff_teams_amount: 2)
+          end
+
+          context 'only playoff_teams_amount is changed reasonably but update fails' do
+            before do
+              allow_any_instance_of(Tournament)
+                .to receive(:update)
+                .and_return(false)
+            end
+
+            it 'returns unprocessable entity' do
+              put :update, params: { id: @filled_tournament.to_param }.merge(playoff_teams_amount: 8)
+              expect(response).to have_http_status(:unprocessable_entity)
+            end
+
+            it 'doesn\'t change playoff_teams_amount' do
+              expect do
+                put :update, params: { id: @filled_tournament.to_param }.merge(playoff_teams_amount: 8)
+                @filled_tournament.reload
+              end
+                .to_not(change { @filled_tournament.playoff_teams_amount })
+            end
+
+            it 'doesn\'t change instant_finalists_amount' do
+              expect do
+                put :update, params: { id: @filled_tournament.to_param }.merge(playoff_teams_amount: 8)
+                @filled_tournament.reload
+              end
+                .to_not(change { @filled_tournament.instant_finalists_amount })
+            end
+
+            it 'doesn\'t change intermediate_round_participants_amount' do
+              expect do
+                put :update, params: { id: @filled_tournament.to_param }.merge(playoff_teams_amount: 8)
+                @filled_tournament.reload
+              end
+                .to_not(change { @filled_tournament.intermediate_round_participants_amount })
+            end
+          end
+
+          context 'only playoff_teams_amount is changed to something reasonable' do
+            before do
+              put :update, params: { id: @filled_tournament.to_param }.merge(playoff_teams_amount: 8)
+              @filled_tournament.reload
+            end
+
+            it 'succeeds' do
+              expect(response).to have_http_status(:ok)
+            end
+
+            it 'changes playoff_teams_amount' do
+              expect(@filled_tournament.playoff_teams_amount).to eq(8)
+            end
+
+            it 'adapts instant_finalists_amount' do
+              expect(@filled_tournament.instant_finalists_amount).to eq(8)
+            end
+
+            it 'adapts intermediate_round_participants_amount' do
+              expect(@filled_tournament.intermediate_round_participants_amount).to eq(0)
+            end
+          end
+
+          it 'fails when playoff_teams_amount is higher than the amount of teams participating' do
+            put :update, params: { id: @filled_tournament.to_param }.merge(playoff_teams_amount: 783)
+            expect(response).to have_http_status(:unprocessable_entity)
+          end
+        end
       end
 
       context 'as another user' do
diff --git a/spec/factories/stages.rb b/spec/factories/stages.rb
index c2d1e2b..8027cd7 100644
--- a/spec/factories/stages.rb
+++ b/spec/factories/stages.rb
@@ -5,6 +5,7 @@ FactoryBot.define do
     tournament
     factory :group_stage do
       level { -1 }
+      state { :in_progress }
       transient do
         group_count { 4 }
         match_factory { :group_match }
diff --git a/spec/factories/tournaments.rb b/spec/factories/tournaments.rb
index b4a522a..9a8a7d3 100644
--- a/spec/factories/tournaments.rb
+++ b/spec/factories/tournaments.rb
@@ -10,6 +10,11 @@ FactoryBot.define do
     end
     after(:create) do |tournament, evaluator|
       tournament.teams = create_list(:team, evaluator.teams_count, tournament: tournament)
+      tournament.playoff_teams_amount = (tournament.teams.size / 2)
+      tournament.instant_finalists_amount = (tournament.playoff_teams_amount / 2)
+      tournament.intermediate_round_participants_amount = ((tournament.playoff_teams_amount -
+                                                            tournament.instant_finalists_amount) * 2)
+      tournament.save!
     end
 
     factory :stage_tournament do
diff --git a/spec/interactors/add_group_stage_to_tournament_interactor_spec.rb b/spec/interactors/add_group_stage_to_tournament_interactor_spec.rb
index 6e009df..1b4f90b 100644
--- a/spec/interactors/add_group_stage_to_tournament_interactor_spec.rb
+++ b/spec/interactors/add_group_stage_to_tournament_interactor_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe AddGroupStageToTournament do
+RSpec.describe AddGroupStageToTournament, type: :interactor do
   let(:empty_tournament_context) do
     AddGroupStageToTournament.call(tournament: @empty_tournament, groups: @groups)
   end
@@ -14,6 +14,7 @@ RSpec.describe AddGroupStageToTournament do
     @group_stage_tournament = create(:group_stage_tournament, stage_count: 0, group_count: 0)
     @group_stage = create(:group_stage)
     @groups = Hash[1 => create_list(:team, 4), 2 => create_list(:team, 4)].values
+    @tournament_service_defaults = [78_345, 2_387]
   end
 
   context 'GroupStageService mocked' do
@@ -24,13 +25,28 @@ RSpec.describe AddGroupStageToTournament do
     end
 
     context 'empty tournament' do
+      before do
+        allow(class_double('TournamentService').as_stubbed_const(transfer_nested_constants: true))
+          .to receive(:calculate_default_amount_of_teams_advancing)
+          .with(@empty_tournament.playoff_teams_amount, @group_stage.groups.size)
+          .and_return(@tournament_service_defaults)
+      end
+
       it 'succeeds' do
         expect(empty_tournament_context).to be_a_success
       end
 
       it 'adds group stage to the tournament' do
-        test = empty_tournament_context.tournament.stages.first
-        expect(test).to eq(@group_stage)
+        expect(empty_tournament_context.tournament.stages.first).to eq(@group_stage)
+      end
+
+      it 'sets default for instant_finalists_amount' do
+        expect(empty_tournament_context.tournament.instant_finalists_amount).to eq(@tournament_service_defaults.first)
+      end
+
+      it 'sets default for intermediate_round_participants_amount' do
+        expect(empty_tournament_context.tournament.intermediate_round_participants_amount)
+          .to eq(@tournament_service_defaults.second)
       end
     end
   end
diff --git a/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb b/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb
index ca60cf9..0ee2cbc 100644
--- a/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb
+++ b/spec/interactors/add_playoffs_to_tournament_interactor_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe AddPlayoffsToTournament do
+RSpec.describe AddPlayoffsToTournament, type: :interactor do
   let(:group_stage_tournament_context) do
     AddPlayoffsToTournament.call(tournament: @group_stage_tournament)
   end
diff --git a/spec/interactors/populate_match_below_interactor_spec.rb b/spec/interactors/populate_match_below_interactor_spec.rb
index 8b751e0..6f87038 100644
--- a/spec/interactors/populate_match_below_interactor_spec.rb
+++ b/spec/interactors/populate_match_below_interactor_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe PopulateMatchBelow do
+RSpec.describe PopulateMatchBelow, type: :interactor do
   before do
     @match = create(:match)
     @objects_to_save = [create(:match), create_list(:match_score, 2)]
diff --git a/spec/interactors/save_application_record_object_to_database_interactor_spec.rb b/spec/interactors/save_application_record_object_to_database_interactor_spec.rb
index ce2edb3..46f6d72 100644
--- a/spec/interactors/save_application_record_object_to_database_interactor_spec.rb
+++ b/spec/interactors/save_application_record_object_to_database_interactor_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe SaveApplicationRecordObject do
+RSpec.describe SaveApplicationRecordObject, type: :interactor do
   before do
     @tournament = create(:tournament)
   end
diff --git a/spec/interactors/update_groups_group_scores_interactor_spec.rb b/spec/interactors/update_groups_group_scores_interactor_spec.rb
index 83d1068..6db8ba9 100644
--- a/spec/interactors/update_groups_group_scores_interactor_spec.rb
+++ b/spec/interactors/update_groups_group_scores_interactor_spec.rb
@@ -1,6 +1,6 @@
 # frozen_string_literal: true
 
-RSpec.describe UpdateGroupsGroupScores do
+RSpec.describe UpdateGroupsGroupScores, type: :interactor do
   before do
     @group = create(:group)
     @group_scores = create_list(:group_score, 2)
diff --git a/spec/services/group_stage_service_spec.rb b/spec/services/group_stage_service_spec.rb
index c7a850e..2a41658 100644
--- a/spec/services/group_stage_service_spec.rb
+++ b/spec/services/group_stage_service_spec.rb
@@ -1,31 +1,35 @@
 # frozen_string_literal: true
 
-RSpec.describe GroupStageService do
+RSpec.describe GroupStageService, focus: true do
   before do
     @teams1 = create_list(:team, 4)
     @teams2 = create_list(:team, 4)
     @prepared_groups = Hash[1 => @teams1, 2 => @teams2].values
   end
-  describe '#generate_group_stage method' do
+  describe '#generate_group_stage' do
+    let(:prepared_groups_groupstage) do
+      GroupStageService.generate_group_stage(@prepared_groups)
+    end
+
     it 'returns a stage object' do
-      group_stage = GroupStageService.generate_group_stage(@prepared_groups)
-      expect(group_stage).to be_a(Stage)
+      expect(prepared_groups_groupstage).to be_a(Stage)
+    end
+
+    it 'assigns the correct state' do
+      expect(prepared_groups_groupstage.state).to eq('in_progress')
     end
 
     it 'returns a stage object with level -1' do
-      group_stage_level = GroupStageService.generate_group_stage(@prepared_groups).level
-      expect(group_stage_level).to be(-1)
+      expect(prepared_groups_groupstage.level).to be(-1)
     end
 
     it 'adds the provided groups to the stage' do
-      group_stage_teams = GroupStageService.generate_group_stage(@prepared_groups).teams
-      expect(group_stage_teams).to match_array(@prepared_groups.flatten)
+      expect(prepared_groups_groupstage.teams).to match_array(@prepared_groups.flatten)
     end
 
     it 'adds GroupScore objects for every team present in the group' do
-      group_stage = GroupStageService.generate_group_stage(@prepared_groups)
-      teams_in_group_scores = group_stage.groups.map { |g| g.group_scores.map(&:team) }.flatten
-      expect(teams_in_group_scores).to match_array(@prepared_groups.flatten)
+      expect(prepared_groups_groupstage.groups.map { |g| g.group_scores.map(&:team) }.flatten)
+        .to match_array(@prepared_groups.flatten)
     end
 
     it 'raises exception when given different sizes of groups' do
diff --git a/spec/services/tournament_service_spec.rb b/spec/services/tournament_service_spec.rb
new file mode 100644
index 0000000..65abd35
--- /dev/null
+++ b/spec/services/tournament_service_spec.rb
@@ -0,0 +1,18 @@
+# frozen_string_literal: true
+
+RSpec.describe TournamentService do
+  describe '#calculate_default_amount_of_teams_advancing' do
+    before do
+      @instant_finalists_amount, @intermediate_round_participants_amount =
+        TournamentService.calculate_default_amount_of_teams_advancing(32, 5)
+    end
+
+    it 'accurately calculates @instant_finalists_amount' do
+      expect(@instant_finalists_amount).to eq(30)
+    end
+
+    it 'accurately calculates @intermediate_round_participants_amount' do
+      expect(@intermediate_round_participants_amount).to eq(4)
+    end
+  end
+end