An American football field is about 300ft×160ft, which translates to 91.44m×48.8m = 4462.272m^2
An association football field is about 105m×68m = 7140m^2.
An NHL hockey rink is about 200ft×85ft, which translates to 60.96m×25.5m = 1554.48m^2
12×7140/4462.272 = 19.2
12×1554.48/4462.272 = 4.18...