Power-to-weight ratio: what is W/kg, and why does it matter?

If you’ve ever read any books about professional cycling, you’ll probably have read about power-to-weight ratio: riders at the peak of their abilities killing themselves on a benchmark climb to ascertain the magic figure. And if you’ve spent any time sweating indoors with any of the current crop of training apps you may have come across it there too. So what’s it all about? And as a non-professional rider, how important is it? Read on to find out and you can watch us demonstrate what it means in the real world in our video below too.

What is power-to-weight ratio?

It’s not a clever name or anything: it’s literally just the ratio of your power to your bodyweight. It’s normally expressed in watts per kilogram (W/kg). At this point someone will normally point out in a nasal voice that the kilogram is a unit of mass, not of weight, but unless you’re doing some of your training on Earth and some on the moon, that’s not likely to be an issue, since gravity is a constant, near as dammit. Those people need to take a long, hard look at themselves. But I digress.

Your weight is, well, your weight. And the figure you use for your power output in watts is generally your functional threshold power, of FTP. That’s the power you can nominally put out for a whole hour. Most of the indoor training apps currently available will have some kind of FTP test for you to complete so you can assign yourself a number; it’s generally not much fun finding out, but it’s not complicated.

How do I measure it?

Well, you’ll need a pair of bathroom scales, but you likely already have them. And then you’ll need a bike thing that measures power. That might be a power meter on your bike or it might be an indoor trainer that can give you a repeatable number. Then you just get the one number and divide it by the other one. Easy!

What’s a good power-to-weight ratio?

Well, higher is better. I’m a pretty big rider at 92kg, and my FTP is around 315W, giving me a power-to-weight ratio of about 3.4W/kg. That’s fairly average. I’m a good enough rider to make it off the bottom rung of the racing ladder into the third cats, but I’m not exactly pulling up any trees there. It’s accepted Tour de France lore that to be in with a shout of winning the race you need to have a power-to-weight ratio approaching 6.5W/kg. So a pro rider who weighs 65kg would need to be capable of putting out over 420W for an hour. Yikes!

Does it matter?

Obviously none of us are going to win a Grand Tour, but that doesn’t mean that your power-to-weight ratio is a meaningless metric. In short: yes, it matters. It matters most when you’re going uphill, and the steeper a climb, the more it’s about your power-to-weight, which is why it’s so important in a Grand Tour: that’s where the race is usually decided. And it matters most there because you’re moving slowly.

If you’re riding a bike then you’re working all the time against certain forces that will slow you down. There’s mechanical friction: the rolling resistance of the tyres on the road, the bearings in the hubs and pedals, friction losses between cleat and pedal, that sort of thing. Then there’s wind resistance, as you push the air out of the way as you move through it. And if you’re riding uphill, you’re also working against gravity.

There’s a clever tool here where you can input numbers for all the variables* and get an idea of how many watts would be required to ride at a given speed. By working up a couple of scenarios you can see how these different forces come into play.

First, let’s assume I’m riding along on the flat, on a still day, at a reasonable speed, say 35km/h. For a 92kg rider on the hoods, the calculation suggests that about 278W of input is required to keep going at that speed. We’re assuming you’re on your own here, as drafting another rider makes an enormous difference; we won’t go into that here!

Of that 278W, by far the biggest component is air resistance at 225W – four fifths of the total. That’s because cyclists aren’t particularly aerodynamic, and air resistance increases in a cubic relationship with velocity: if you go twice as fast, there’s eight times as much air resistance. That’s why time triallists spend so much time obsessing over their bikes and riding position. It really matters to be aero when you’re going quick.

Secondly, let’s pretend I’m grinding up a 12% hill at 8km/h. That’s also going to require me to put out 278W of power, but a lot has happened to the mix. Air resistance is almost negligible at less than 3W, but the work I’m doing against gravity to climb the hill is 260W, 93% of the total. When I’m on a steep hill, moving slowly, it’s basically all about me working against gravity. On a steep climb, it’s pretty close to being a direct relationship between the power from my legs and my speed up the climb.

It’s interesting to see how weight affects the speed of riders of different weights on the flat, and on climbs.

Take, for example, Liam and I. Liam’s a svelte 63kg and I’m almost half as much again. His FTP is lower than mine at 298W, but because he’s a lot lighter his power-to-weight is much higher at over 4.7W/kg, and he generally smokes me on the hills.

On the flat, all other things being equal, that 298W would mean that Liam was rolling along at around 36.5km/h. For my 315W I’d be able to manage nearly 37km/h, because on the flat it’s more to do with your absolute power than power-to weight. So even though my W/kg number is lower, I could theoretically go a bit faster. If I managed to gap him, I might be able to pull away.

However, on our 12% climb I’m dead in the water: my 315W buys me about 9km/h, and Liam’s up at nearly 12km/h, over 30% faster than me. The climbs are where it matters, and the steeper they are, the more it’s about power-to-weight.

What can I do to get better?

If you want to go faster, then there are two things you can do. You can put out more power, or you can reduce the amount of mass that’s working against gravity, either by losing weight or spunking a huge wad of cash on a really, really light bike. It’s worth pointing out here that for mortals like us, your power-to-weight using your whole system weight – you and the bike – is sometimes more meaningful, because you can realistically save weight on your bike too. For professional riders, everyone’s on a UCI-limit 6.8kg bike (or near enough), so it’s not really a variable. If you’re a hillclimber, and you can shed a couple of kilos by sawing bits off and drilling holes in things**, that can make a significant difference to your time up a climb.

For an average rider, though, most of the easy gains are with you, not the bike. You can make an effort to lose weight, and you can embark on a training programme to increase the power you can put out. In reality those two things often go hand in hand: as you spend more time training, your weight often decreases. When I was originally training to race my weight dropped by around 10kg, and my FTP increased from about 280W to over 300W. Those kind of gains just aren’t available by swapping your wheels for a set of nicer ones. Not everyone has 10kg to shed, but there’s usually something that could go…

It’s not all about power-to-weight though

A final thought: your numbers can take you so far, and in some disciplines – time trialling, individual track pursuits – they’re very important, but a lot of the time it’s one factor in among many others. Pacing – knowing how hard you can push for how long – is vital in a range of situations.

Take a sportive; if you’re fit enough to get round in three hours but you go off much too hard from the start and blow up spectacularly halfway, then it might take you four, or you might end up being sick in a hedge and never make it to the finish, not that I’d know what that was like.

If you’re in a crit race then it doesn’t matter if your power-to-weight numbers are the highest of the whole field if you’re in 20th place coming out of the final bend for the sprint: you’re not going to cross the line first. Racecraft and tactics are a huge part of doing well in that discipline. Then there’s nutrition, and your technique, and conditioning, and peaking at the right time... The list goes on. Power-to-weight is a useful thing to know, but it’s not the be-all and end-all of riding your bike.

*The figures I used were:

Frontal area A: 0.4 and coefficient of drag Cd: 1 (from https://www.cyclingpowerlab.com/CyclingAerodynamics.aspx)

Coefficient of rolling resistance Crr 0.005 and air density Rho 1.22601 (both default figures)

**This is not recommended