FTP vs W/kg: which cycling metric actually matters?

Cycling culture has largely settled on watts per kilogram as the number that counts. That is correct on a climb and quietly misleading everywhere else — and the riders most often misled by it are the bigger ones, who conclude they are weaker than they are.

The two numbers answer different questions

FTP is the power you can hold for a long, steady effort, measured in watts. W/kg is that same number divided by your body weight. Because one is derived from the other, riders tend to treat them as two views of the same fitness. They are not. They answer separate questions:

• FTP answers: how much work can I do against the road? That governs how fast you move on flat ground and how much you can contribute on the front of a group.
• W/kg answers: how much work can I do relative to what I have to lift? That governs how fast you go uphill and lets riders of different sizes compare engines fairly.

Both are real. Which one is relevant depends entirely on what is slowing you down at that moment — and that is a question about physics, not about fitness.

Why gravity and air resistance behave differently

On a sustained climb, most of your power goes into lifting the whole system — you and the bike — against gravity. That demand scales strongly with total mass, so for a given gradient, climbing speed tracks power divided by mass. Add a kilogram and you need proportionally more watts to hold the same speed. That is the whole case for W/kg, and on a long climb it is a strong one.

On flatter roads the picture changes, because the dominant resistance is no longer gravity but air. Aerodynamic drag depends on how much of you the wind has to get around — your effective drag area — and that does not rise in proportion to body mass. A rider who is 25 kg heavier is not pushing 25 kg more air.

The practical consequence: two riders at the same W/kg are much more closely matched on a steep climb, while the heavier rider has substantially more absolute power available on flatter terrain — and only a modest drag penalty to spend it against. That is the mechanism behind a pattern most club riders already recognize — the rider who gets dropped on every climb is the one driving the pace on the flat run home. Neither observation says anything about who trains harder.

Body size is not the main lever here, though. Riding position, bike setup, clothing, and shoulder width all affect drag area more than mass does, which is why a well-positioned smaller rider routinely beats a larger one on flat roads. And on rolling terrain neither effect dominates cleanly, which is why mixed-terrain racing rewards riders who are strong on both measures.

Same W/kg, very different riders

The clearest way to see the gap is to hold power-to-weight constant and vary the rider:

All three are identical on a power-to-weight chart. On a flat 40 km time trial they are not remotely equivalent, and the 100 W between the first and last rider is the reason. Run your own numbers through the W/kg calculator, then read what counts as a good FTP for the watts side of the same picture.

Which number to track for your riding

A practical default: train to FTP, judge yourself on whichever number matches your terrain. FTP is what sets your training zones and what actually responds to a training block, so it is the one to watch week to week. Beyond that:

• Hilly sportives and climbing goals — W/kg is the number that predicts your day. Absolute watts matter mainly for getting to the bottom of the climb with something left.
• Flat time trials and triathlon — absolute FTP, and your position on the bike. Power-to-weight is close to irrelevant here, and time spent improving aerodynamics usually beats time spent losing weight.
• Flat and rolling group rides — absolute watts keep you in the group; W/kg decides whether the short rises hurt.
• Mixed-terrain road racing — both, honestly. Selections usually happen on the climbs, but you need the watts to still be there when they do.
• General fitness — pick one and stay consistent. If your weight is stable, the two move together and tracking both adds nothing.

Whichever you follow, make sure the FTP underneath it is current — a stale number puts every zone in the wrong place. The cycling power zone calculator turns it into watt ranges once you have it.

Where raw watts genuinely win

Worth stating plainly, because the W/kg framing has become default enough that heavier riders routinely undersell themselves. Absolute power is the number that matters when you are:

• riding into a headwind or driving a paceline;
• time trialling on flat or rolling courses;
• closing a gap or bridging on flat roads;
• sprinting, where the relevant figure is peak watts, not threshold;
• holding a fast group on the flat, where the pace is set by the watts the strongest riders can produce.

A rider with a 340 W FTP has a genuinely strong engine. Reading that as mediocre because the power-to-weight chart says otherwise is a measurement error, not a fitness one.

How SmarterTraining thinks about this

Training prescription runs on absolute power, not on the ratio. Your intervals are set in watts relative to your FTP, so that is the number SmarterTraining builds sessions around and updates as your fitness changes. Power-to-weight is a lens for interpreting where you stand, not an input to what you should ride on a given Tuesday.

Takeaway

Takeaway: W/kg wins when you are lifting your own mass, FTP wins when you are pushing through air, and the reason is physics rather than preference. Train to FTP because that is what sets your zones and moves with training. Judge yourself against the number that matches the roads you actually ride.