Every other calorie calculator asks you to describe your ride with an adjective. Was it moderate? Vigorous? You pick a word, and the word decides the answer. There is a better way, and if your bike shows watts you already have everything it needs.
A power meter measures the work you did. Watts are joules per second, so a ride is nothing more than watts multiplied by seconds. Ride at 180 watts for 90 minutes and you produced 180 × 5,400 = 972,000 joules, or 972 kilojoules. There is no estimating in that sentence. It is the same arithmetic that bills your electricity.
The shortcut worth remembering: your average watts × 3.6 is your kilojoules per hour. Hold 200 watts for an hour and you have done 720 kJ. Hold 120 watts for an hour and you have done 432 kJ.
Kilojoules are not calories, though. They are the work that reached the pedals, and your body is not a perfect machine — most of what you burn leaves as heat. The fraction that becomes forward motion is called gross efficiency, and in trained cyclists it has been measured at around 20%, with the usual range quoted as 20% to 25%.
| Step | Arithmetic | Result |
|---|---|---|
| Mechanical work | 180 W × 5,400 s ÷ 1,000 | 972 kJ |
| Energy your body spent, at 22% efficiency | 972 ÷ 0.22 | 4,418 kJ |
| Turned into calories | 4,418 ÷ 4.184 | 1,056 kcal |
Now look at what happens across the whole efficiency range, because this is the useful part.
| Gross efficiency | kJ → kcal multiplier |
|---|---|
| 20% | × 1.195 |
| 22% | × 1.086 |
| 23.9% | × 1.000 |
| 25% | × 0.956 |
At 23.9% efficiency the multiplier is exactly 1.000. The two conversions — dividing by your efficiency and dividing by the 4.184 kJ in a calorie — very nearly cancel each other out. That is why cyclists say 1 kJ is about 1 kcal, and why it is close enough to be useful. It is not a rule of thumb someone made up. It is what the arithmetic does.
Which is why the calculator at the top of this page scores that 90-minute ride at 972 kcal while the worked table above, using 22%, came to 1,056. Both are right. The gap between them is not an error, it is the honest width of the answer, and it is the band we print under every result. No calculator that gives you one confident number has that width. It just hides it.
Use your average power, not your normalized power. Normalized power is deliberately weighted towards the hard bits, so it is always higher than the average, and it would overstate the work you actually did. For energy, the plain average is the right number.
One rider, 78 kg, 90 minutes. Here is what the ride cost, measured — and here is what a MET calculator says it cost, depending on which word the rider picks. The gap is the whole problem.
Between 929 and 1,162 kcal across the 20-25% efficiency range. One number, one narrow band, no adjectives.
The same 90 minutes comes out anywhere from 835 to 1,351 kcal. That is a 516-calorie spread — 62% of the smallest answer — and it is decided entirely by which of three words the rider chooses.
Worse: in the published MET tables, the 8.8 and the 11.0 rows are both labelled "vigorous effort". There is no wording that tells a rider which one they were in. And they are 270 calories apart.
The measured answer, 972 kcal, sits in the middle of that spread. Which is exactly the point: a MET estimate is not wrong on average. It is wrong on your ride, in a direction you cannot know, by an amount you cannot check.
This is the question most people arrive with, so here is the honest version of the answer — and the reason indoor riders are in a better position than they think.
Almost every stationary bike shows a watts number. Gym bikes, spin bikes, smart trainers, the cheap magnetic ones: they all display power, because resistance is what they control. That means the indoor rider, who assumes they have the worst data, actually has the best. You do not need to guess your intensity. Read the number off the screen.
A worked example. 75 kg rider, 45 minutes, display showing 110 watts on average.
Between 284 and 355 kcal across the efficiency range. Subtract the roughly 59 kcal you would have burned anyway over those 45 minutes and the session added about 238 kcal.
Now the same session through the MET route. At 110 watts the rider falls into the published band for 101-160 watts, which carries a MET of 8.8. For a 75 kg rider over 45 minutes, that comes out at 520 kcal — 75% more than the watts say.
Nothing went wrong. The band is 101 to 160 watts wide, so a rider at the bottom of it and a rider at the top of it get the same figure, even though one is doing 58% more work. At 110 watts you are near the bottom, and the band’s single value flatters you.
One caveat about the console. Bikes without a strain gauge do not measure your power, they infer it from flywheel speed and the resistance setting. That is an estimate, not a measurement, and it is not calibrated to you. If the console’s calorie figure is far from what this page gives you, the disagreement is probably in the watts, not in the arithmetic.
The most searched question in this whole area is whether an indoor bike will take weight off. Here is what the arithmetic says, and then the four reasons the scale will not agree with it.
Start with the honest version of the calorie figure. The number at the top of this page is gross: it includes the energy you would have spent existing for those hours anyway, which runs at roughly one calorie per kilo per hour. For a 75 kg rider that is about 79 kcal an hour, sitting still. For weight-loss arithmetic you want what the ride added, so we subtract it.
Then the constant. A kilo of body weight is usually treated as about 7,700 kcal — the same figure as the familiar 3,500 calories per pound. Divide your weekly net calories by 7,700 and you have a rate.
| Outdoors | On a stationary bike | |
|---|---|---|
| The ride | 90 min at 180 W, 78 kg | 45 min at 110 W, 75 kg |
| Gross | 972 kcal | 297 kcal |
| Minus resting | −123 kcal | −59 kcal |
| Net per ride | 849 kcal | 238 kcal |
| Rides per week | 3 | 5 |
| Per week | 2,547 kcal | 1,190 kcal |
| Over four weeks | 10,188 kcal | 4,760 kcal |
| Ceiling | about 1.3 kg | about 0.6 kg |
This is arithmetic about energy, and nothing more. It is not a diet, it is not a target, and it is not advice about what to eat. If you are planning a serious change in how you eat, that is a conversation with a professional, not with a calculator.
What the numbers do say is worth hearing: five 45-minute stationary sessions a week, kept up for a month, is about six-tenths of a kilo of arithmetic. Not nothing, and not the transformation the internet promises. The riders who get somewhere are the ones who make it a routine they repeat, which is a training problem before it is a calorie problem.
We can do better than argue from first principles, because we can check. For every ride in our data with a real power meter, we know exactly how much work it took. We also know how far it went and how long it lasted, which is all a MET calculator gets. So we can run both and compare.
We ran the comparison the way that is most generous to the MET formula. We fed it a measured average speed rather than an adjective, and we used the highest published estimate of how much energy a rider actually spends per unit of work. Even then, the formula overestimates on seven rides out of ten, and misses by more than 20% on six.
The number that matters is not the average error, though. An average error is a bias, and a bias can be corrected with a constant. What we found is spread: the middle half of rides lands anywhere between 2.5% under and 40% over. The formula does not know which half of that band your ride is in. Your power meter does.
Worth saying plainly, because a number you trust too much is worse than no number.
It does not measure your metabolism. Gross efficiency varies between riders and within the same rider on different days. The 20-25% range on your result is the honest width of that, and nobody can narrow it for you without a lab.
It does not know if your power meter is right. A meter reading 3% high gives you calories 3% high. On a stationary bike without a strain gauge, the error can be far larger than that, because the watts are inferred rather than measured.
It does not predict weight loss. It converts energy into an arithmetic ceiling. Appetite, sleep, how much you move on the days you do not ride, and what you actually eat all sit between that ceiling and the scale.
It does not use normalized power, and neither should you here. NP is weighted towards the hard efforts and is always above the average, so it would inflate the work figure.
It is not a nutrition plan. It says nothing about what you should eat, before or after the ride, and it should not be read as though it did.
The MET routes are estimates by design. They come from published tables that describe an average person doing an average version of an activity. You are not that person, and neither is the ride you just did.
The published work behind the numbers above, named so you can check it.
It depends almost entirely on the watts, not on you. An hour at 120 watts is 432 kJ, which is roughly 432 calories. An hour at 200 watts is 720 kJ, so roughly 720 calories. The shortcut is average watts × 3.6 for kilojoules per hour, and kilojoules are close enough to calories to use directly. If you have no power figure, an hour of steady riding for most people lands somewhere between 400 and 700 calories, and that width is exactly why we would rather you used watts.
Not exactly, but close enough to be useful, and for an honest reason. A calorie is 4.184 kilojoules, so the kilojoules you produced should be divided by 4.184 — but first they have to be divided by your efficiency, because only a fifth to a quarter of what you burn reaches the pedals. Those two divisions nearly cancel. At 23.9% gross efficiency they cancel exactly. If you are at the efficient end of the range, at 25%, the shortcut runs about 5% high. If you are at 20%, the true figure is about 20% above what the shortcut gives you. Either way it is close, which is more than any effort adjective can promise.
On the watts route, barely. Producing 200 watts costs about the same whether you weigh 60 kg or 90 kg, because the work is the work. On the speed and effort routes it matters a lot, because a heavier rider moving at the same speed is doing more work against gravity and rolling resistance. That is why this page asks for your weight on two routes and treats it as optional on the third — and it is a good illustration of how different the two methods really are.
Take five 45-minute sessions a week at a genuine 110 watts. That is about 238 calories a session once you subtract what you would have burned anyway, so 1,190 a week, so about 4,760 over a month. At roughly 7,700 calories per kilo, that is about six-tenths of a kilo of arithmetic in four weeks. Real results are usually smaller, because appetite rises and you move less on the days in between. It is a real effect, and it is slower than the internet suggests.
At lower intensities a larger share of the energy comes from fat, which is true and is often where the claim stops. But weight change follows total energy across the week, not which fuel a given hour used. The reason Zone 2 earns its place is different and better: it is the intensity you can repeat four or five times a week without needing to recover from it, so it produces more total energy than a couple of sessions hard enough to wreck the rest of your week.
Because it is answering a slightly different question with a slightly different constant. Devices differ in the efficiency they assume, in whether they add heart-rate-derived estimates when the power drops out, and in whether they report gross calories or only what the exercise added. If two numbers are within about 15% of each other, both are defensible. If they are 50% apart, one of them is estimating from something other than watts.
That is a nutrition question rather than a calculator question, and this page will not answer it. What the calculator can tell you is that the headline figure is gross: it includes roughly one calorie per kilo per hour that you would have spent anyway. If you are using the number to plan anything, use the net figure in the weight-loss block, not the big one at the top.
Longer, almost always, because energy is power multiplied by time and time is the term you can extend the furthest. Doubling your power for twenty minutes is very hard and adds a few hundred kilojoules. Adding an hour at your normal pace is easy by comparison and adds more. This is also why the riders who lose weight on a bike tend to be the ones who ride often rather than the ones who ride hard.
