Quick answer: 30 minutes at 100 W burns about 237 kcal for a 160 lb (72.6 kg) rider by the ACSM leg cycling equation, or about 215 kcal by the work and efficiency method. At 150 W the two methods nearly meet: about 318 and 323 kcal.
Two methods, one workout
ACSM leg cycling equation. The American College of Sports Medicine estimates oxygen use on a cycle ergometer as VO2 = 1.8 x work rate / body mass + 7, with work rate in kg-m per minute (watts x 6.12) and the 7 covering rest (3.5) plus the cost of turning unloaded pedals (3.5). The ACSM table gives its range as 300 to 1,200 kg-m/min, which is 50 to 200 W. Oxygen is converted at 4.9 kcal per litre.
Work and efficiency. Watts times seconds is the mechanical work you did. Divide by 4,184 to turn joules into kcal, then by gross efficiency, the share of food energy that reaches the pedals. Research summarised in Sports Medicine Open (2019) puts gross efficiency mostly around 18 to 20% at 150 W; this page uses 20%.
Example worked by hand
100 W is 612 kg-m per minute. For a 72.6 kg rider: 1.8 x 612 / 72.6 + 7 = 22.2 ml of oxygen per kg per minute. That is 1.61 litres a minute, or 7.9 kcal a minute at 4.9 kcal per litre: about 237 kcal in 30 minutes. The work route: 100 W x 1,800 s = 180 kJ; 180,000 / 4,184 / 0.20 = 215 kcal.
Why they disagree
The ACSM equation includes body mass, so a heavier rider at the same watts gets a higher number (a 200 lb rider at 100 W for 30 minutes comes out at about 255 kcal). The work method ignores body mass because the bike measures only what reached the flywheel. At low power the ACSM fixed terms dominate, so at 50 W the gap is wide (156 against 108 kcal for the 160 lb rider); at 200 W the work method is higher (430 against 399).
Before you rely on it
- Gym bike watt readings depend on calibration; some read high, some low.
- If your bike shows only a level or resistance number, not watts, this page cannot convert it, because levels differ by machine.
- All figures are estimates of energy use, not a plan for losing weight.
Riding outside? Use the cycling calorie calculator. Compare a session on the rowing calorie calculator. Training for a triathlon? Add the bike leg to the triathlon calculator.
Sources: ACSM's Guidelines for Exercise Testing and Prescription, Table 7.3 metabolic calculations (as reproduced by Texas Tech University); ACSM's Guidelines for Exercise Testing and Prescription (Wolters Kluwer); Matomaki, Linnamo and Kyrolainen (2019), cycling mechanical efficiency, Sports Med Open. Rules checked . Found a problem? Tell us.
Questions
How many calories does 30 minutes on a stationary bike burn?
At 100 W, about 237 kcal for a 160 lb rider by the ACSM equation and about 215 kcal by the work method. At 150 W, about 318 to 323 kcal.
Are exercise bike calorie displays accurate?
They are estimates. Many use a fixed body weight or their own efficiency figure, and the watt reading itself depends on calibration. Compare sessions on the same bike rather than across machines.
How do I convert watts to calories on a bike?
Multiply watts by seconds for joules, divide by 4,184 for kcal of work, then divide by gross efficiency (about 0.20). One hour at 100 W is about 430 kcal.
Does my weight matter on a stationary bike?
In the ACSM equation it does, because oxygen cost is scaled per kg. In the work method it does not, since the bike measures only the power you produce.