Two riders can post the exact same 4.0 watts per kilogram on a lab test. They can still finish minutes apart on race day. A 2025 study found the “right” way to weigh power against body weight isn’t fixed. It swings from an exponent of 0.35 on flat roads to 0.89 on steep climbs. That’s nowhere near the flat 1.0 that plain W/kg assumes.
The Number Every Cyclist Chases, and Why It Falls Apart
Watts per kilogram (W/kg) is your FTP, the highest power you can hold for about an hour, divided by your body weight in kilograms. Ride a 20-minute max effort, average 280 watts, weigh 70kg, and your FTP lands near 266 watts. That’s 3.8 W/kg.
Coaches, apps, and Strava segments all treat this one number as gospel. Heavier riders get told to lose weight. Lighter riders get told they’re “built for climbing.” The number gets used the same way for a flat crit and a mountain stage.
Here’s the problem. W/kg divides power by mass raised to exactly 1.0. Physicists call that the mass exponent. It’s the value body weight gets raised to before it’s divided into your watts. Plain W/kg picks 1.0 and never changes it, no matter the terrain.
What 2025 Research Found About Watts Per Kilogram
Researchers Horvath and Andersson built five rider profiles from data on 144 World Tour and Pro Continental cyclists. They tested those profiles on real and hypothetical time-trial courses. Then they solved for the mass exponent that best predicted actual finishing times on each one.
The results moved a lot.
On flat ground with no wind, the model fit was 0.38, not 1.0. Translation: on a flat course, body weight barely matters. Raw wattage does almost all the work.
Even two “similar” Grand Tour time trials didn’t match each other. The 2024 Giro d’Italia stage 7 came in at 0.49. The 2024 Tour de France stage 21 came in at 0.61. Same category of race. Different course shape. Different answer.
The Physics: Why Drag and Gravity Don’t Play by the Same Rules
Here’s the metaphor that makes this click. Riding into a headwind is like pushing a shopping cart through water. A bigger cart displaces more water, but not in direct proportion to its size. Climbing a hill is different. It’s like carrying that same cart straight up a ladder. Every extra kilogram costs you one extra kilogram of lifting. No discount.
Aerodynamic drag scales with body mass to roughly the one-third power (mass^0.33). Aerobic capacity, your engine size, scales closer to mass^0.67. On flat ground, a bigger rider’s engine grows faster than their drag penalty does. Raw power wins.
Gravity doesn’t negotiate. It acts on mass at exactly mass^1.0. That’s why the exponent climbs toward 1.0 as the road tilts up. It’s also why steep climbs are the one place plain W/kg gets close to right.
Close, but not exact. Even on a 7-degree climb, the optimal exponent was 0.89. Still short of the full 1.0 that W/kg bakes in.
Flat and Rolling Courses: Raw Watts Beat Raw W/kg
If your goal race is flat or rolling, chasing a higher W/kg number by cutting weight is often the wrong project. The 2025 data puts the optimal exponent for flat, no-wind conditions at 0.38. Headwind conditions push it even lower, to 0.37.
A rider who drops 4kg but loses 15 watts in the process usually gets slower on that kind of course.
This lines up with older research. A 2006 study by Nevill and colleagues modeled flat time trials with a mass exponent around 0.34 to 0.48. That model explained up to 96.3% of the variance in finishing times. Flat racing rewards engine size, not scale weight.
Weight matters less than the scale suggests.
The Gradient Tipping Point: 4.5% for Amateurs, 7.5% for Pros
So when does weight actually start to dominate? Engineering estimates put the crossover, the gradient where power-to-weight overtakes raw aerodynamics, at roughly 4 to 4.5% for amateur riders. For elite and pro riders, it’s closer to 7.5 to 8%.
That gap exists because faster riders push through more air at any given gradient. A pro climbing at 25 km/h fights more drag than a recreational rider climbing the same hill at 14 km/h. The pro needs a steeper road before weight takes over as the deciding factor.
A 2008 field study tested this on a real 5.3km, 5.4%-gradient hill climb. It found an even steeper relationship in practice, with a mass exponent near 1.24 once rolling resistance and combined rider-and-bike mass were counted. Real roads are messier than lab models. They tend to punish extra weight even more once you’re actually climbing.
Same 4.0 W/kg, Two Very Different Races
Take two fictional riders, Elena and Dario. Elena is a 58kg climber producing 232 watts at threshold. Dario is a 78kg rouleur producing 312 watts. Both test out at exactly 4.0 W/kg.
On a sustained 7% mountain-top finish, where the optimal exponent runs close to 0.89, Elena and Dario are genuinely close to even. That’s the one case where W/kg tells something close to the truth.
Drop them onto a flat 40km time trial instead, and the picture flips.
On the flat time trial, Dario’s bigger raw-watt engine gives him close to a 20% edge. That’s an edge the shared “4.0 W/kg” number hides completely. By the steep climb, the gap has nearly closed. At the exact exponent plain W/kg assumes, they’re dead even. That’s exactly why the flat FTP test made them look identical in the first place.
Same number. Completely different race.
A course-blind plan built around “raise my W/kg” would have Dario chasing weight loss that barely helps on his actual target course. The smarter move, backed by the exponent research, is simple: keep building raw watts, and leave his weight alone.
So, Train for Watts or Weight? A Course-Specific Answer
The honest answer: it depends on the road, not a single number from a 20-minute test. If your A-race is flat or rolling, chase FTP and raw watts. Weight loss buys you almost nothing there, and it can cost you power you actually need.
If your A-race is a sustained climb, weight starts to pay off. Real money, since the exponent there runs closer to 0.9. It’s still not the full kilogram-for-kilogram trade that plain W/kg implies. But it’s close enough to matter.
One number can’t describe every course. That’s also why platforms that track your fitness, fatigue, and form scores as separate signals, instead of one flat training-load figure, give you a clearer picture of what’s actually happening.
How AthleteOS Reads Your Actual Race Course
Most tools report one flat W/kg number pulled from your last FTP test and stop there. AthleteOS instead pulls your target race’s actual elevation and gradient profile. It weighs your watts and weight goals against that specific course, not a generic number.
A rider training for a flat century gets pushed toward raw watts. A rider building toward a mountainous gran fondo gets a plan that treats weight as a real lever, not an assumption. It’s the same course-aware thinking behind how AthleteOS tracks your aerobic engine over time, through your drift ratio across long rides, rather than a single snapshot number.
If you want training matched to your goal race’s actual demand curve instead of a generic percentage of FTP, sign up for AthleteOS and connect your race course.
The scale doesn’t know what road you’re racing. Your training plan should.