Watts or weight for W/kg: which lever actually moves, and what each one costs

The arithmetic, and the exponent it gets wrong

Watts per kilogram divides power by mass raised to the power of one. Cycling does not work that way. Swain put the mass exponent for climbing at 0.79 and for air resistance near one third [Swain 1994], so the ratio overstates what a kilogram buys you uphill and misdescribes the flat almost entirely.

The shortcut is just the ratio's own slope. A 75-kilogram rider at 250 watts sits at 3.33 W/kg; take a kilogram off at unchanged power and the figure moves to 3.38, the same move 3.3 extra watts would make. The slope steepens with where you already are, so a rider at 4.5 W/kg gets 4.5 watts from each kilogram — and that is the rider with the least mass spare to give.

Now the exponent. Swain modelled the two forces that dominate a cyclist's energy cost and found that neither scales with mass the way W/kg assumes [Swain 1994]. Going uphill, the mass exponent is 0.79 rather than 1.0, because the bicycle is a fixed load that makes up a proportionally larger share of a small rider's total mass. Against air resistance on the flat, frontal drag scales at roughly mass to the one-third power, so a large rider carries only slightly more drag in absolute terms and much less of it per kilogram. Swain's conclusion was that small riders hold a real advantage climbing and a real disadvantage in flat time trials, and that field data supported both.

Field data in cyclists sharpened the flat half of that. Nevill and colleagues pooled three time-trial studies, 79 riders in total, and found the scaling that best predicted cycling speed was VO2max divided by mass to the power of -0.32 — almost exactly the exponent Swain derived for sub-maximal cycling [Nevill et al. 2005]. Dividing by mass to the power of one, which is what W/kg does, is not a mild simplification on flat terrain. It is the wrong exponent by a factor of three, in the direction that flatters light riders in the events where they are slower.

The denominator does not move on its own

The one-kilogram-for-3.3-watts trade holds only if the watts survive whatever removed the kilogram. In a randomised comparison of two rates of weight loss in elite athletes, lean body mass rose 2.1% in one group and did not move in the other, for the same total mass lost [Garthe et al. 2011].

Garthe and colleagues randomised 24 elite athletes to lose body weight at two different rates while all of them did four resistance sessions a week [Garthe et al. 2011]. Both groups lost about 5.5% of body weight. The group that took longer over it cut energy intake by 19% and increased lean body mass by 2.1%; the faster group cut intake by 30% and finished with lean mass unchanged. Same mass off the scale, different composition underneath it. Two limits before carrying that into cycling: these were strength- and power-sport athletes, and the outcomes were a one-repetition maximum, a 40-metre sprint and a countermovement jump, not sustained power on a bike. Read it as evidence that the two halves of the ratio are coupled, not as a cycling result.

The health side is less ambiguous than the performance side. The International Olympic Committee's 2023 consensus on Relative Energy Deficiency in Sport treats sustained low energy availability as a cause of harm to bone, endocrine and immune function, with performance decrement on the same list [Mountjoy et al. 2023]. The performance half of that is genuinely unsettled in men: a scoping review found six studies covering 103 male athletes, and performance outcomes were positively, negatively or not at all affected depending on which study you read [Hänisch et al. 2025]. That heterogeneity is a reason for caution rather than a licence. The outcomes nobody disputes are the ones that are not about watts.

So we will be direct about what this page is not. AdaptCycling does not set weight targets, does not publish a goal weight, and does not put a number on how fast mass should come off. Body composition is a conversation between a rider and a qualified professional who can see their whole history, not something a training app should prescribe from a form field. What a training app can do honestly is work on the other half of the ratio.

The numerator has the better evidence, and it can add mass

The randomised trials that improved cycling performance moved watts, not mass. In one of them the intervention that worked was heavy leg strength training — the lever most likely to add a kilogram while making a rider measurably faster over 40 minutes [Vikmoen et al. 2016].

The clearest counterexample to 'lighter is always better' in the cycling literature is a strength study. Vikmoen and colleagues added heavy leg work to 11 weeks of endurance training in well-trained female cyclists, 19 in all, and only the group that lifted improved 40-minute mean power, cycling economy and the fraction of VO2max they could sustain [Vikmoen et al. 2016]. Whatever that work did to those riders' mass, it did not stop them producing more power for 40 minutes.

Watts also transfer everywhere. Twenty watts helps on a climb, in a flat time trial, in a breakaway and on the last lap of a criterium. A kilogram helps where gravity is the dominant cost and does close to nothing where air resistance is — which, on Swain's exponents, is most of the riding most amateurs do [Swain 1994]. The two levers are not two routes to the same place. One is general and one is terrain-specific.

The measurement problem the wider guide to raising FTP opens on applies here too. The 20-minute test varies about 2.9% between attempts in trained cyclists [Borszcz et al. 2020], so on a 250-watt rider anything under roughly 20 watts sits inside one re-test's noise. W/kg inherits that noise and adds its own: body mass swings a kilogram or more across a day with hydration and glycogen, so a ratio that moved 0.05 between two mornings has told you nothing about either half of itself.

What to measure instead of the ratio

Decide which kind of effort you actually care about before optimising a ratio built for one of them. W/kg describes sustained climbing. Absolute watts describe flat time trialling, criterium racing and every sprint. Most riders need both numbers and track only one.

A 20-minute climb at 8% is close to the pure case W/kg was built for: gravity dominates, and the rider producing more watts per kilogram arrives first. A flat 40-kilometre time trial is close to the opposite case, where the field data put the useful exponent near 0.32 [Nevill et al. 2005] and the heavier rider at the same absolute power wins. Rolling terrain, criteriums and group rides sit between the two, usually much nearer the flat case than riders assume.

That is why a single ratio makes a poor diagnostic. A power profile that places your watts per kilogram against a population at 5 seconds, 1 minute, 5 minutes and 20 minutes splits the question into four, and the answer is usually that one coordinate is dragging down the result you actually want. A rider whose 20-minute figure is respectable and whose 1-minute figure sits twenty percentile points lower does not have a weight problem. They have a duration they have never trained.

Practically: track the numerator on its own, because it is the half you can attribute to training. Weigh under the same conditions or do not weigh at all, since inconsistent measurement manufactures trends that are not there. And give any change 8 to 12 weeks before judging it, for the same reason a threshold gain needs that long — anything shorter is mostly the measurement.

Quick answers

Sources cited in this guide

1. 01
   Swain 1994. The influence of body mass in endurance bicycling. Medicine & Science in Sports & Exercise.
2. 02
   Nevill et al. 2005. Scaling maximal oxygen uptake to predict cycling time-trial performance in the field: a non-linear approach. European Journal of Applied Physiology.
3. 03
   Garthe et al. 2011. Effect of two different weight-loss rates on body composition and strength and power-related performance in elite athletes. International Journal of Sport Nutrition and Exercise Metabolism.
4. 04
   Mountjoy et al. 2023. 2023 International Olympic Committee's (IOC) consensus statement on Relative Energy Deficiency in Sport (REDs). British Journal of Sports Medicine.
5. 05
   Hänisch et al. 2025. Effects of low energy availability on performance in male athletes: A scoping review. Journal of Science and Medicine in Sport.
6. 06
   Vikmoen et al. 2016. Strength training improves cycling performance, fractional utilization of VO2max and cycling economy in female cyclists. Scandinavian Journal of Medicine & Science in Sports.
7. 07
   Borszcz et al. 2020. Reliability of the Functional Threshold Power in Competitive Cyclists. International Journal of Sports Medicine.

More inside How to increase your FTP

Other articles in this series

1. 01

   How long does it take to raise your FTP by 20 watts?

   Twenty watts is near the smallest gain one 20-minute re-test can prove. What the trials observed, and why 20 W differs at 200 W and at 350 W.

   →