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