To calculate how fast the earth spins, we need to know a few details. To figure out the speed of the earth at the equator, we need to know the circumference of the earth at the equator. The circumference of the earth at the equator is 25, 040 miles (40,070 KM). The earth makes a full revolution once a day-about every 24 hours (an exact day is 23 hours 56 minutes 04. 09053 seconds). Once we know the circumference and time required for one revolution, we can simply divide 25040/24. The answer to how fast the earth spins is at the equator is 1,038 miles per hour (1674.66km/hr).