Cycling Power to Weight Calculator

How this is calculated

WATTS PER KILO IS POWER DIVIDED BY MASS, and the only interesting question is WHICH mass. Quoting your figure against your body weight alone is the convention for comparing riders, and it is the wrong number for predicting a climb - gravity lifts the bike too. Both are shown here, and the total is the one that decides how fast you go up. ⚠️ W/kg MATTERS ON CLIMBS; RAW WATTS MATTER ON THE FLAT. This is the single most useful thing to understand about the number. On a steep climb almost all your power goes into lifting mass, so the ratio decides the outcome. On the flat, air resistance dominates and it scales with frontal area rather than with weight - which is why a large, heavy rider with a big engine will ride away from a light climber on a flat road and lose to them twenty minutes into a mountain. A rider is not simply 'stronger' or 'weaker' than another; it depends entirely on the terrain. THE CLIMBING RATE IS DERIVED, NOT LOOKED UP. Lifting mass at a steady rate takes power equal to mass times gravity times vertical speed, so vertical speed equals power over mass over gravity. Converting to metres per hour gives VAM = W/kg x 3600 ÷ 9.81, which is W/kg x 367. That figure is pure physics and assumes every watt becomes height. ⚠️ SO THE IDEAL FIGURE IS ALWAYS OPTIMISTIC. Drivetrain friction, rolling resistance and air resistance all take a share even uphill, and the steeper the climb the smaller the air-resistance share becomes. A loss of around 10-15% is a reasonable working figure on a genuinely steep climb and more on a shallow one, where you are moving faster and pushing more air. It is an input here because it depends on the gradient, the surface and the bike. THE WEIGHT QUESTION HAS AN ACTUAL ANSWER. Rather than arguing about whether a lighter bike is worth it, the tool reports the seconds a kilogram saves on the climb you enter. On a 500 metre climb it is a matter of seconds - which is either meaningful or trivial depending on whether you are racing, and is a far better basis for the decision than marketing copy. ⚠️ POWER FIGURES NEED A DURATION TO MEAN ANYTHING. Twenty minutes of 250 W and an hour of 250 W are different athletes. Use a number you can genuinely hold for the length of the climb you are calculating - a five-minute peak used against a forty-minute climb will produce a time you cannot ride. MEASURED POWER VARIES BETWEEN METERS. Different power meters and trainers disagree, sometimes by several percent, and comparing your figure against someone else's is comparing two devices as much as two riders. Your own numbers over time on the same meter are the reliable comparison.

Common questions

Should I use my weight or my weight plus the bike?

Both, for different purposes, which is why the tool shows both. Quoting watts per kilo against body weight alone is the convention when riders compare themselves, and it is fine for that. But it is the wrong number for predicting a climb, because gravity lifts the bike, the bottles, the tools and the food as well as you. On the defaults that is the difference between 3.57 and 3.21 W/kg - about ten percent, and ten percent is a lot of climbing time. Use the total whenever you want a time rather than a talking point.

Why does W/kg matter on climbs but not on the flat?

Because different forces dominate. On a steep climb almost all your power goes into lifting mass against gravity, so power divided by mass decides the outcome almost entirely. On the flat, gravity is doing nothing and air resistance takes nearly everything - and air resistance scales with frontal area and speed, not with weight. That is why a big powerful rider will ride away from a light climber on a flat road and lose to them twenty minutes up a mountain, with neither of them having changed. Neither rider is simply stronger; the terrain decides which number counts.

Where does the rate of climb come from?

Physics rather than a lookup table. Lifting mass at a steady rate requires power equal to mass times gravitational acceleration times vertical speed, so vertical speed is power divided by mass divided by 9.81. Converting metres per second into metres per hour multiplies by 3600, which leaves the tidy result that vertical metres per hour is W/kg multiplied by about 367. You can check the tool against that in your head. What the physics does not include is friction and air resistance, which is why the ideal figure is always optimistic and a loss percentage is applied on top.

What loss percentage should I use?

Around 10 to 15 percent on a genuinely steep climb, and more on a shallow one. The reason is that the losses are not fixed: drivetrain friction and rolling resistance are roughly constant, but air resistance depends on how fast you are moving, and on a shallow gradient you are moving considerably faster for the same power. So a long drag at four percent loses far more to the air than a wall at twelve percent does. It is an input rather than a constant because it depends on the gradient, the surface, the bike and how you are sitting on it.

Is a lighter bike actually worth it?

The tool answers that in seconds rather than opinions - it reports what one kilogram off the total saves on the climb you entered. The honest summary is that on a typical climb it is a handful of seconds, which is either meaningful or trivial depending entirely on whether you are racing. Two things worth weighing against it: a kilogram off the rider costs nothing and does the same job, and money spent on tyres or position often buys more speed on the flat than the same money spent on frame weight buys uphill. But at least the trade is now a number rather than a feeling.

Does the power figure need a duration?

Absolutely, and using one without a duration is the most common way to get an unrideable answer. Twenty minutes at 250 W and an hour at 250 W are different athletes, and a five-minute peak power used against a forty-minute climb will produce a predicted time nobody could ride. Use a figure you can genuinely hold for the length of the climb in question - if the climb takes forty minutes, use something close to your hour power rather than anything shorter. The arithmetic cannot tell that you have given it a number you cannot sustain.

Why does my figure not match my friend's?

Partly fitness and partly hardware. Different power meters and smart trainers disagree with each other, sometimes by several percent, so comparing your number against someone else's compares two devices as well as two riders. Weighing yourself at different times of day adds more noise than people expect. The comparison that actually means something is your own figures over time on the same meter, weighed the same way - that removes the device from the equation and leaves the training, which is the thing you were trying to measure.

Is testing my power safe?

A maximal or threshold test is a hard effort, and this tool cannot see you. If you have a heart or lung condition, high blood pressure, are returning from illness or injury, are pregnant, or have not trained in a while, get the go-ahead from a clinician before testing rather than after. Stop for chest pain, dizziness or unusual breathlessness. Indoors on a trainer is a safer place to go deep than a public road, where the same effort competes with traffic and cornering for your attention.

Take it further with AI

Copy this into ChatGPT or Claude with your own numbers filled in. It hands over the figures this calculator worked out, so the answer is built on real arithmetic instead of a guess.

I used the Cycling Power to Weight Calculator at https://www.bfcbrilliance.com/tools/cycling-power-to-weight-calculator.

What I entered:
- Power you can hold (W): ___
- Your weight (kg): ___
- Bike and everything on it (kg): ___
- Losses to friction and air (%): ___
- Vertical metres of the climb (m): ___

What it calculated:
- Watts per kilo - your body weight: ___
- Watts per kilo - you and the bike: ___
- Rate of climb, pure physics: ___
- Rate of climb after losses: ___
- Time up that climb: ___

Use those figures as given — they are already worked out, so please don't recalculate or estimate your own. Help me turn them into a plan: what to buy or do, in what order, roughly what it should cost, and the mistakes people most often make with this job.

Keep this general and do not give medical advice — where it matters, tell me what to raise with a clinician.

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