diff --git a/app/models/tournament.rb b/app/models/tournament.rb
index 3813f08..002436d 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_isnt_positive_power_of_two,
+               'playoff_teams_amount needs to be a positive power of two')
+  end
 end
diff --git a/db/migrate/0000_create_schema.rb b/db/migrate/0000_create_schema.rb
index 14ffb52..390edfa 100644
--- a/db/migrate/0000_create_schema.rb
+++ b/db/migrate/0000_create_schema.rb
@@ -51,7 +51,7 @@ 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
 
diff --git a/spec/controllers/tournaments_controller_spec.rb b/spec/controllers/tournaments_controller_spec.rb
index c3313bd..506581e 100644
--- a/spec/controllers/tournaments_controller_spec.rb
+++ b/spec/controllers/tournaments_controller_spec.rb
@@ -194,6 +194,29 @@ 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
+            it 'is not a power of two' do
+              post :create, params: create_group_tournament_data.merge(playoff_teams_amount: 18)
+              expect(response).to have_http_status(:unprocessable_entity)
+              expect(deserialize_response(response).values.first.first)
+                .to eq('playoff_teams_amount needs to be a positive power of two')
+            end
+
+            it 'isn\'t positive' do
+              post :create, params: create_group_tournament_data.merge(playoff_teams_amount: -16)
+              expect(response).to have_http_status(:unprocessable_entity)
+              expect(deserialize_response(response).values.first.first)
+                .to eq('playoff_teams_amount needs to be a positive power of two')
+            end
+
+            it 'isn\'t positive nor a power of two' do
+              post :create, params: create_group_tournament_data.merge(playoff_teams_amount: -42)
+              expect(response).to have_http_status(:unprocessable_entity)
+              expect(deserialize_response(response).values.first.first)
+                .to eq('playoff_teams_amount needs to be a positive power of two')
+            end
+          end
         end
 
         it 'renders a JSON response with the new tournament' do
@@ -234,10 +257,17 @@ RSpec.describe TournamentsController, type: :controller do
       end
 
       context 'with invalid parameters' do
-        it 'renders an unprocessable entity response' do
-          put :create, params: { teams: [1, 2, 3] }
-          expect(response).to have_http_status(:unprocessable_entity)
+        shared_examples_for 'renders unprocessable entity response' do
+          it 'renders an unprocessable entity response' do
+            expect(response).to have_http_status(:unprocessable_entity)
+          end
         end
+
+        before do
+          put :create, params: context
+        end
+
+        context ''
       end
 
       context 'with empty team objects' do