Instrument 03
Watts per Kg Calculator: Cycling Power to Weight and Your FTP
Relative power is the number that makes two riders of different sizes comparable on a hill.
A watts per kg calculator does one division, and the division is the easy part. What the resulting power to weight figure means depends entirely on how long the effort lasted, which is why this one asks for a duration as well and reads the answer against the ceiling of professional cycling for that length of effort.
Watts per kg calculator: compute relative power
Relative power
0 W/kg
Ceiling of published professional performance at this duration:0
Reading a cycling power to weight figure honestly
The bands below are reference ranges. They are not certificates. They describe roughly where trained populations sit for a twenty-minute effort, which is the most commonly quoted test duration of the several in use. Move that duration and every row moves with it, upward for shorter efforts and downward for longer ones.
| Group | W/kg |
|---|---|
| Untrained | below 2.5 |
| Recreational | 2.5 – 3.5 |
| Trained club rider | 3.5 – 4.3 |
| Strong amateur racer | 4.3 – 5.0 |
| Elite amateur | 5.0 – 5.7 |
| Professional | 5.7 – 6.4 |
| Above the published record | beyond 6.4 |
Where an FTP figure fits on the scale
FTP is meant to approximate the power sustainable for about an hour, so it belongs on the hour row, not the twenty-minute one. Testing protocols that take ninety-five percent of a twenty-minute effort produce a number close to it, but the two are estimates of different things and drift apart under load.
A rider with an FTP of 290 watts at 72 kilograms sits at just over four watts per kg. That is a competent racing amateur. The same 290 watts at 58 kilograms is five watts per kg and a different rider entirely on any climb steeper than a few percent, despite the identical wattage.
Why the ceiling matters more than the band
Reference bands describe populations. The ceiling describes physics and the historical record together, and it is the more useful line when a specific result is in question. A figure below it is merely impressive; a figure above it says the inputs or the recording deserve a second look before anything else.
Used this way a watts per kg calculator is a sanity check rather than a score. The number it returns is only as good as the power figure fed into it, and a meter reading drifting by five percent moves the power to weight result by the same amount without anything on the page looking wrong.
That is also why a watts per kg calculator is the wrong tool for comparing two riders on different courses. It compares efforts instead of people, and an FTP taken in different conditions a month apart is really two separate measurements of two different days, rather than one clean before-and-after comparison.
Body weight is the half of the division riders treat as fixed, and it is not. It swings by a kilogram or more across a day on hydration alone, which moves a seventy-kilogram rider's relative power by about one and a half percent before any training effect exists at all. Weigh at the same hour, in the same state.
The same caution applies to the power half. A twenty-minute test paced by feel tends to start too hard and fade, and the resulting average understates what a properly paced effort would have held. Two tests are worth more than one, and a figure that falls between them is closer to the truth than either alone.
Watts per kg questions
What does the watts per kg calculator actually divide?
Power in watts by rider body weight in kilograms, without the bike. Equipment mass matters for climbing speed and belongs in a full physical model, but the convention for comparing athletes counts the body alone. Keep to that convention or the figure you produce will not be comparable with anyone else’s at all.
Should I use FTP or the power from one specific effort?
Either, as long as you attach the matching duration. FTP is conventionally an hour-equivalent figure, so read it against the hour row. A twenty-minute test produces a higher number that is not directly comparable with it. The same cycling wattage means very different things at five minutes and at sixty, so the duration has to travel with the figure.
Why does the plausible ceiling fall as the effort gets longer?
Because the energy systems supplying the work change. Short efforts draw heavily on stored phosphates and glycolysis; long ones depend on sustained aerobic supply. The result is a curve, not a line, and any honest power to weight comparison has to fix the duration first, before it attempts to compare anything at all.
Is a high watts per kg figure always an advantage?
Only where gravity dominates. On a steep climb relative power decides almost everything. On flat roads absolute watts matter far more, because air resistance scales with frontal area, not with mass, which is the reason heavy riders tend to win sprints and light riders tend to win in the mountains.
To estimate power where no meter existed, use the plausibility check. For climbing rate alone, the VAM calculator avoids the aerodynamic guess entirely. On how calculated figures diverge from meters, see estimated against measured power.