I found this:
Determine the number of sit-ups you can perform in 1 minute. Follow the pace of your normal workout.
Calculate the number of calories you burn per minute performing sit-ups. As a baseline, a 100-pound individual burns 6 calories per minute doing sit-ups. For every 10 pounds more you weigh, add 1 calorie per minute. For example, a 110-pound individual burns 7 calories per minute, a 120-pound individual burns 8 calories per minute, and so on.
Divide the number of calories you burn per minute of sit-ups by the number of sit-ups you can perform in 1 minute. This is the number of calories you burn per sit-up. For example, here is the calculation for a 120-pound individual: 8 calories per minute / 30 sit-ups per minute = 0.27 calories per sit-up.