I wrote a Hypercard program for Macs on this subject about 10 years ago.

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Losing weight is all mathematics.
Calculate how many calories your body uses up in 24 hours.
Calculate how many calories you eat.
If you eat more calories than you burn up, you'll gain weight.
If you want to lose weight, eat less calories or exercise more to burn more calories.

ie, there's about 3500 calories in one pound of fat. (one pound = 9.74351079 times 10 to the power of 15 calories)
You probably burn up about 100 calories per hour just sitting around doing nothing.
So if you want to maintain your current weight, eat exactly 2400 calories per day and sit around doing nothing.
If you eat only 1400 calories per day, it'll take you 3.5 days to lose 1 pound.
If you burn an extra 1400 calories per day, you'll lose the same amount.