Why W/kg Is Lying to You
The metric every cyclist obsesses over — and what it's missing
You've been dropped on the flat by a rider who has worse W/kg than you. You know it because you've both compared numbers. You're pushing more watts per kilogram of body weight — and they're still riding away from you.
This isn't a mystery. And it isn't unfair. It's physics. W/kg is telling you an incomplete story, and most cyclists don't know what the missing chapters are.
Let's fix that.
Where W/kg Comes From — And What It's Actually For
Watts per kilogram became the dominant cycling metric for one specific reason: it predicts climbing performance extremely well.
When you're riding uphill, the dominant resistance force is gravity acting on your mass. The heavier you are, the harder you have to work to lift yourself up the same gradient. So if two riders both produce 300W on a 10% gradient, the lighter rider climbs faster because they're lifting less weight.
In that context, W/kg is the right number to look at. It's not lying to you on a climb. It's giving you exactly the information you need.
The problem starts when you take a climbing metric and try to apply it everywhere else.
W/kg was designed for climbers. Using it to evaluate performance on flat roads is like using a compass to measure temperature — it's not that the tool is wrong, it's that you're asking it the wrong question.
What Happens When the Road Flattens Out
On flat roads, the dominant resistance force changes. Gravity is no longer your main enemy — aerodynamic drag is.
Air resistance increases with the square of your speed. At 40 km/h, you're fighting roughly four times more drag than at 20 km/h. At racing speeds, somewhere between 70 and 90% of your total resistance is aerodynamic. Gravity — and therefore your body mass — becomes a relatively minor factor.
This is where W/kg starts to mislead you.
Consider two riders. Rider A weighs 60 kg and produces 4.0 W/kg — 240W. Rider B weighs 80 kg and also produces 4.0 W/kg — 320W. On a steep climb, they'll perform similarly. On a flat road at racing speed, Rider B is going to ride away from Rider A. Not because they're fitter in any meaningful sense — but because they're pushing 80W more into the pedals, and on the flat, absolute watts matter far more than watts per kilogram.
This is the scenario that plays out every weekend on group rides, every race on rolling terrain, and every Zwift race on a flat route. The leaderboard sorted by W/kg tells you one thing. The finishing order tells you another.
The Aerodynamic Variable Nobody Talks About
Mass is only part of what W/kg is missing. The other missing variable is aerodynamic drag — specifically, your CdA.
CdA (the product of drag coefficient and frontal area) captures how aerodynamically efficient you are. A rider in a tucked TT position on a low-stack road bike has a very different CdA from a rider sitting upright on a sportive bike — even if they're exactly the same height, weight, and power output.
Two riders with identical W/kg and identical body mass can produce dramatically different speeds on flat roads based purely on their position and equipment. The more aerodynamic rider is faster — even if their power numbers are identical.
W/kg is blind to this entirely. It doesn't know what position you're in. It doesn't know whether you're on a TT bike or a mountain bike. It doesn't know whether you're tucked or upright. It only knows how many watts you're producing per kilogram of body mass.
On a climb, that's fine. On anything else, it's missing the variable that might matter most.
The Allometric Problem — Why Mass Doesn't Scale Linearly
There's a deeper mathematical issue with W/kg that's worth understanding if you want to really see why the metric breaks down on flat and rolling terrain.
When researchers study the relationship between power output and speed on flat roads across different body sizes, they find that mass doesn't affect performance in a simple 1:1 ratio. The relationship is allometric — meaning the effect of mass on flat-road performance scales closer to mass to the power of 0.67, not mass to the power of 1.0 as W/kg implies.
What this means in practice: the penalty for extra body mass on flat terrain is significantly lower than W/kg suggests. A heavier rider doesn't lose as much flat-road speed per extra kilogram as the W/kg metric implies, because they're also typically producing more absolute power and their additional mass contributes relatively little to aerodynamic drag.
British Cycling used allometric scaling in talent identification testing for exactly this reason — because it gives a more accurate picture of performance potential across different body types than W/kg alone.
The best predictor of flat-road cycling performance combines power, mass, and aerodynamic drag. W/kg only uses two of those three — and uses one of them incorrectly.
So What Should You Use?
The answer isn't to throw out W/kg. For climbing performance, it remains the right metric. If you're preparing for a mountain sportive or targeting KOMs on long climbs, W/kg is exactly what you should be monitoring.
But for a more complete picture of your performance — one that tells you how you'll perform on flat roads, in bunch rides, in mixed terrain racing, and against riders of different body types — you need something that accounts for all three variables: power, mass, and aerodynamics.
This is the thinking behind the PSTS score (Performance Speed Translation Score) built into the CycleCoach MAP & Power Profile Calculator. PSTS takes your power output, your body mass, and your aerodynamic profile and combines them into a single performance index — one that predicts flat and rolling performance far more accurately than W/kg alone.
It answers the question W/kg can't: not just how many watts per kilogram you produce, but how fast those watts will actually make you go.
The calculator actually produces two PSTS scores, and both matter — because performance is context-dependent.
Your MAP-based PSTS reflects your ceiling performance: the speed and power you can express in a short, intense effort. Think of a hard surge in the final kilometres, a late-race attack, or the kind of explosive effort that opens a gap. If you want to know whether you can go with a breakaway when it goes, MAP PSTS is the number to look at.
Your FTP-based PSTS reflects your sustained performance: whether you can hold race pace for an extended period, survive in a long breakaway, or maintain speed across a flat stage without fading. If the question is whether you can stay with a group at 45 km/h for 30 minutes, FTP PSTS answers it.
Two riders with identical W/kg can have very different MAP and FTP PSTS scores — and that gap tells you something important about your rider type and where your training investment should go.
A rider whose MAP PSTS is significantly higher than their FTP PSTS is a punchy, anaerobic rider — capable of explosive efforts but potentially vulnerable over sustained flat race pace. They'll make the break but may struggle to stay away. Training priority: aerobic durability and threshold work.
A rider whose FTP PSTS is close to or exceeds their MAP PSTS is a diesel engine — highly efficient at sustained pace but with a lower ceiling for short explosive efforts. They'll stay in a breakaway all day but might get shelled when the accelerations come. Training priority: short power and neuromuscular work.
Reference ranges for both scores run from recreational through masters-competitive to World Tour level, so you can see where you sit — and which score to target first.
What This Means for Your Training
Understanding the limits of W/kg has practical implications for how you train and race.
If you're a lighter rider who has been chasing W/kg gains and wondering why you still get dropped on flat group rides — the answer may not be your fitness at all. It may be your position, your equipment, or simply the physics of flat-road cycling working against you. Improving your aerodynamics could give you more flat-road speed than another training block would.
If you're a heavier rider who has been demoralised by seeing lower W/kg numbers than lighter training partners — your actual flat-road and rolling performance may be considerably better than W/kg suggests. You might be the right rider for a different kind of event than you think.
And if you're training for mixed-terrain events — road races, gran fondos, gravel — using W/kg as your sole performance benchmark is giving you an incomplete picture of how your fitness will translate to race day.
The most effective training targets the right variables for the right terrain. W/kg matters enormously on climbs. On flat and rolling roads, absolute power and aerodynamic efficiency matter more.
The Bottom Line
W/kg is not a bad metric. It's an incomplete one — and the gap between what it measures and what you actually want to know grows wider the flatter the road gets.
A complete performance picture combines your power output, your body mass (scaled correctly, not linearly), and your aerodynamic profile. When you have all three, you can start to understand not just how fit you are, but how fast you'll actually ride — and what to change to go faster.
Want to see your PSTS score alongside your MAP-based training zones, VO2max estimate, and power profile?
The CycleCoach MAP & Power Profile Calculator calculates all of it — including your PSTS score and how it compares to reference ranges from recreational through to World Tour level. Enter your MAP test result and key power data to get your full performance picture.