A sit up does burn calories but maybe not as many as you might think. One sit up burns on average 2 calories. The calories burned are based on the health and fitness of the person.
When an overweight individual with little muscle tone in the abdominal region does a sit up they may burn more calories than someone who is fit and healthy with a six-pack. The reason being is that it requires more energy from the overweight individual to burn calories than the person who is fit and healthy. There is more of struggle for a person to do a sit up that is overweight which as result burns more calories.
One sit up engages a variety of muscles, including the stomach, back and even arms and legs. A sit up generally does not require the heart rate to increase, so it is not considered to be cardiovascular fitness, but can be implemented along with a good cardio program in order to lose weight.
Once the heart rate is up from cardiovascular exercise, a person can burn more calories when doing sit ups than if the heart rate is at rest when the sit up occurs. Sit ups can burn calories if done in combination with cardio exercise and strength training and should be done in addition to other forms of exercise.