Quick Answer — What Does Biking to Work Actually Add Up To?
Calories burned cycling follow the standard MET (metabolic equivalent) formula: calories per hour = MET value × body weight in kg. A moderate pace (12-14 mph) has a MET value of about 8, so a 170 lb (77 kg) rider burns roughly 616 calories per hour at that pace — about 47 calories per mile at 13 mph.
On the emissions side, EPA's gasoline combustion factor is 8,887 grams of CO2 per gallon burned. Divide that by your car's MPG to get grams per mile avoided by biking instead.
Quick reference: A 5-mile one-way commute (10 miles round trip), ridden 5 days a week at a moderate pace by a 170 lb rider whose car gets 25 mpg, burns about 123,400 calories per year (roughly 35 lb of fat-equivalent energy, though real weight change depends on total diet, not just this) and avoids about 0.92 metric tons of CO2 per year — similar in scale to about a fifth of a typical passenger vehicle's total annual emissions.