There are 231 cubic inches per 1 u.s. gallon.
Your tank: 36" x 18" x 25" = 16,200 cubic inches
16,200 divided by 231 = 70. 2
You have a 70 gallon tank.
Edit: I'm thinking you measured the height from the bottom of the plastic trim to the top. In which case the actually height of the tank is 24". In this case the math comes out to approximately 67 gallons. The traditional 36" x 18" x 24" tank is classified as a standard 65 gallon tank.