Sit-ups alone are not an effective way to burn any amount of fat, unless they are combined with a diet and exercise program that burns more calories than are taken in each day. AZ Central explains that sit-ups are useful for building and toning muscle but not for losing fat.
Aerobic exercises such as running, swimming or riding a bike burn more calories and fat than anaerobic exercises like sit-ups. However, anaerobic exercises are also useful since they build muscle, which raises the body's base metabolic rate. The most effective weight-loss programs combine aerobic and anaerobic exercises, and include in a low-calorie diet.