Wondering why hills feel so much harder than your watts suggest? Your power to weight ratio cycling number explains it: divide your sustainable watts by your body mass in kilograms, and you get your climbing currency.
This guide is part of our wider series on training with power.
| Item | Value |
|---|---|
| Formula | FTP divided by body weight in kilograms |
| Example rider Pat | 345 W / 81.4 kg = 4.2 W/kg |
| Example rider Andy | 320 W / 69.5 kg = 4.6 W/kg |
| Zwift race category A | Above 4.0 W/kg |
| Zwift race category B | 3.2 to 3.9 W/kg |
| Zwift race category C | 2.5 to 3.1 W/kg |
Figures come from Stages Cycling and TrainerRoad. Category cut-offs and guidance can change, so check the sources before you rely on them.
What watts per kilogram actually measures
Power to weight ratio is expressed as watts of cycling power produced per kilogram of body weight, abbreviated as W/kg. It is a simple division, and it turns a raw power number into something you can compare between riders of different sizes.
Most commonly, the watts in W/kg refer to your sustainable power, or FTP. TrainerRoad puts the calculation plainly: divide your FTP by your weight in kilograms. You can use the same method for other durations, by dividing your best power over a chosen time by your body weight.
The idea is easiest to see with two riders. Pat produces 345 W at 81.4 kg, which is 4.2 W/kg. Andy produces 320 W at 69.5 kg, which is 4.6 W/kg.
Pat is putting out more power at threshold. Andy is putting out more power relative to his mass, so on a long, steep climb he would generally have the advantage, other factors being comparable.
Why climbers care so much
Gravity is the reason. When the road tilts upward, you are lifting your own mass as well as pushing through the air, and every extra kilogram is more to lift. TrainerRoad notes that larger riders tend to be more powerful than smaller cyclists in absolute terms, but lighter riders require less power to overcome inertia and propel themselves forward, especially uphill.
That is why the same engine feels very different on different roads. Here is how the terrain tends to decide which number matters:
- Long, steep climbs: W/kg is the most useful predictor of who goes up fastest.
- Flat or rolling roads: aerodynamics and raw watts matter more.
- Mixed courses: both matter, and the balance depends on how much of the route points upward.
The TrainerRoad article is blunt about this. It says that on flat or rolling terrain, aerodynamics and raw power play a much bigger role than watts per kilogram. It adds that this is why larger and more powerful riders tend to win flat and windy races, while smaller climbers shelter in the pack.
The same source also points out that only on long and steep climbs does power-to-weight become the most important variable. Terrain like that is relatively uncommon in amateur cycling races. If your local rides are mostly flat, your absolute watts and your position on the bike may deserve more attention than your ratio.
Reading your number: Zwift race categories as a yardstick
Stages Cycling describes an older Zwift categorisation system, in which indoor race categories were not set by race results the way they are in real-world racing, and Zwift may now use a different system. They are based on historical W/kg performances over a short threshold-style effort.
The categories it lists are shown below.
| Category | W/kg range |
|---|---|
| A | Above 4.0 |
| B | 3.2 to 3.9 |
| C | 2.5 to 3.1 |
| D | Below 2.4 |
These ranges are quoted from Stages Cycling; the figures can change, so check the source before you rely on them.
Treat these as a rough yardstick rather than a verdict. They come from one platform, as described in a single Stages Cycling article, and the platform may have changed its scheme since. Compare your own number against the ranges, then check the current rules wherever you plan to race.
There is a useful lesson in how Zwift handles misplaced riders, too. Stages notes that if someone self-selects a lower category and then posts an extraordinary performance, such as finishing a 40 minute “C” race with an average power of 4.0 W/kg, they can be disqualified from the results and reminded to upgrade. In other words, the ratio is trusted as a fair measure of who belongs with whom.
Looking beyond a single FTP number
One ratio does not describe a rider. Stages suggests charting your mean maximal power in W/kg, which can help you determine your strengths and weaknesses.
Its example shows a rider whose best 5s average was 15.61 W/kg, whose best 1m average was 6.56 W/kg, and who managed 4.6 W/kg for 5m and 4.0 W/kg for threshold power. The same rider looks like a sprinter, a climber or an endurance rider depending on which duration you read.
For climbing, the longer durations tell you the most. A short hill is judged by your short-effort W/kg, and one that takes much longer is judged closer to your threshold figure. Tracking several durations is more honest than quoting one headline number.
If you use a stress model to plan your weeks, the same logic applies: the numbers are only as good as the FTP behind them. Our guide to training stress score explains how that figure feeds into load calculations.
How to improve your power to weight ratio
TrainerRoad says there are two ways to improve your power-to-weight ratio: by increasing power, and by decreasing weight. It also says that almost every athlete can improve the ratio with structured training and a sensible diet.
The power side is the safer and more reliable place to start. As you get fitter, your sustainable power increases, and your ratio improves with it. Steady work just below threshold is a common way to build that base, and our piece on sweet spot training covers one popular approach.
The weight side needs more care. A higher ratio can reflect more lean muscle mass and a stronger aerobic engine, according to TrainerRoad, but cutting weight too hard can cost you the very power you are trying to divide. If you want to change your body composition, talk to a qualified health professional rather than chasing a number.
- Measure your current FTP and your body weight in kilograms.
- Divide one by the other and write the result down.
- Repeat the calculation for the durations that match your local climbs.
- Retest every so often, and compare like with like, using the same method each time.
Remember that your ratio is also a snapshot. It moves when either side of the fraction moves, so a drop in W/kg is not automatically a drop in fitness. It may simply be a heavier week.
Common mistakes when comparing W/kg
- Ignoring terrain. A high ratio does not guarantee a win on a flat, windy course.
- Mixing durations. A short-effort ratio and a threshold ratio are different numbers and should not be compared directly.
- Trusting a stale FTP. An out-of-date FTP estimate makes the ratio less representative of your current fitness.
- Treating category tables as fixed. They belong to a platform and can be revised.
Used with those limits in mind, W/kg is a handy shorthand. It tells you how well your engine suits the climbs you ride, and it gives you a clear target when you plan your training.
Frequently asked questions
How do I calculate my power to weight ratio?
Show answer
Divide your FTP by your body weight in kilograms. For example, 320 W at 69.5 kg gives 4.6 W/kg, as in the Stages Cycling example.
Does W/kg matter on flat roads?
Show answer
Much less. On flat or rolling roads, drag and absolute watts decide more than your ratio does, which is why heavier, powerful riders often shine there.
What W/kg puts me in Zwift category A?
Show answer
Stages Cycling lists category A as above 4.0 W/kg. Check Zwift’s own rules, because the cut-offs may have changed since that article.
Should I lose weight to climb faster?
Show answer
Improving the ratio can come from raising power or lowering weight, but power is the safer lever. If you are thinking about weight change, speak to a qualified health professional first.
