How do you work out sprocket sizes for bmx?

Answer 1

Divide your rear sprocket into your front sprocket. Like 25x9. 25 divided by 9 = 2.77. so 2.77 to 1 is the drive ratio. If you go with a smaller sprocket like a 24 in the front but still a 9 in the back, you are lowering the gear ratio to 2.66 to 1. So your bike will be slower but easier to pedal.

If you go with a 25 in the front and a 10 in the back, you end up with a 2.5 to 1 ratio.

Don't confuse gear ratio with gear inches, that's a calculation including the wheel and sometimes the circumference of the tire.