Power to weight ratio: watts per kilogram explained and how to boost yours

Unless you only ever ride on pancake-flat surfaces, improving your power-to-weight ratio is a must

(Image credit: Russ Ellis)

'Power-to-weight ratio' is an oft quoted phrase in cycling - especially by cyclists who find themselves struggling when the terrain goes up. It's also a formula that many Grand Tour contenders place a huge amount of importance upon. Andrew Hamilton explains just why power-to-weight is important and how you can improve yours.

One of my friends is an avid petrol-head who builds and races cars for a living. He often jokes: “Money can’t buy you happiness, but it can buy you more horsepower — and that’s the same kind of thing.” As cyclists, we can’t buy more power, of course — we have to train our muscles and bodies to produce it, and that’s not easy. While improving your aerobic fitness can increase the amount of power your muscles can produce, human physiology means there’s a limit to the gains that can be achieved this way.

Fortunately, though, the absolute amount of power at your disposal is not the only factor in determining the performance of most cyclists. The amount of mass you have to move around — i.e. your bodyweight — is vitally important too. This is because accelerating mass or moving mass uphill against the force of gravity requires power. It follows therefore that if you have less mass to lug around, you need less power to move it.

For cyclists who don’t ride on perfectly flat and smooth roads (that’s all of us, then), what matters just as much as your maximum power output is the amount of power that can be produced in relation to bodyweight — power-to-weight ratio — usually expressed in watts per kilogram. To work out your power-to-weight ratio figure, simply divide your maximum power output (in watts) by your body mass in kilograms (kg). For example, an 80kg rider with a maximum sustainable power output of 280 watts has a power-to-weight ratio of 3.5 watts per kilo (commonly abbreviated as 3.5W/kg or 3.5W.kg-1).

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Power-to-weight ratio matters because it’s a great predictor of performance. Take two cyclists: Cyclist A can sustain a maximum power output of 250W while Cyclist B can only manage 225W. On a perfectly flat, smooth indoor track (where gravity is not an issue) we can confidently predict that A will be faster than B. On an undulating road, however, power-to-weight begins to matter more. If both cyclists weigh 80kg, A will still be faster. But if A weighs 80kg and B weighs 68kg, cyclist A’s power-to-weight ratio is 3.13W/kg, while B’s is 3.31W/kg. On a flat road, there might not be much in it, but head into the hills and it is cyclist B who will be pulling away.

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Table: Power-to-weight ratio/watts per kilogram for a range of rider weights and power outputs


Understanding power-to-weight ratio/watts per kilogram

Since power-to-weight ratio is determined by the simple formula power (watts) ÷ mass (kg), hopefully even the most non-mathematical readers can appreciate that there are three ways to increase your power-to-weight ratio:

  • Increase your power output while keeping your weight constant
  • Keep your power output constant while decreasing your weight
  • Increase your power output while also decreasing your weight.

It also follows that if your power output increases but your weight increases too, your power-to-weight ratio might not improve at all. The same is true of cyclists who lose weight but suffer a drop-off in maximum power — something we’ll return to. Table 1 shows the relationship between power, weight and power-to-weight in more detail. Looking at Table 1, notice how power-to-weight ratios rise as power output rises and bodyweight falls — i.e. higher and further to the right in this table. Notice too how any given power-to-weight ratio (we’ve highlighted 3W/kg) can be achieved at much lower absolute power outputs when the rider’s mass is low. For example, a 50kg rider churning out just 150 watts has the same power-to-weight ratio as a 90kg rider churning out 270.

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Now suppose this 90kg rider wants an improved power-to-weight ratio. If he or she sheds 10kg (down to 80kg), power-to-weight ratio jumps from 3.0 to 3.4W/kg — that’s a bigger improvement than staying at the same weight and working on aerobic fitness to increase power output to 300W. This underlines why shedding excess body mass (fat) is so effective at boosting performance — even if your aerobic fitness remains the same.

Power to weight

Photo: Russ Ellis

How good is a good power-to-weight ratio?

What constitutes a ‘good’ power-to-weight ratio? Well, this depends on the time period and the level at which you’re riding. Dr Andrew Coggan, an internationally acclaimed exercise physiologist, has compiled some typical power-to-weight ratios, which are shown in the Table below.

Table: Typical power-to-weight ratios for different level cyclists

Rider type5 mins20 mins1 hour

It’s not surprising to observe that the pros have superior power-to-weight ratios regardless of time period. What’s more intriguing is that compared to amateur and recreational riders, the typical one-hour power-to-weight ratio of a pro rider is only fractionally lower than the 20-minute figure. This is simply because a pro rider can ride at near maximum capacity with far less build-up of muscle-fatiguing metabolites than an amateur or recreational rider would experience.

Spiral bridge: a rare and dramatic structure. Photo: Andy Jones

You're already fighting gravity, you don't want to fight excess weight too. Photo: Andy Jones
(Image credit: Andy Jones)

What affect does power-to-weight ratio have when battling wind and hills?

As we saw earlier, shifting mass uphill means that you have to work against the force of gravity. This explains why power-to-weight ratio becomes especially important when climbing. However, absolute power is still important.

To illustrate this, let’s compare power requirements of a 70kg and 80kg rider riding a 6kg road bike up a hill of seven per cent gradient at 16kph (10mph) in still winds. Using data on rolling and aerodynamic resistance, we can calculate that an 80kg rider would have to maintain an average power output or around 298W, requiring a power-to-weight ratio of 3.73W/kg. The 70kg rider would only need to average 266W to ride up the same hill at the same speed on the same bike. However, although it’s 32W less power overall, this translates into a slightly higher power-to-weight ratio of 3.80W/kg.

Two riders on a 6kg road bike, travelling at 16kph up a 7 per cent gradient

  • 80kg rider - 298W (3.73W/kg)
  • 70kg rider - 266W (3.80W/kg)

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Why is this? In simple terms, although much of the riders’ power requirements are a function of body mass (because they’re climbing), there’s an extra, fixed amount of work that has to be done to push the air out of the way (i.e. overcoming aerodynamic resistance), which is the same for both riders. As speeds rise, the contribution from aerodynamic resistance becomes proportionately greater. This in turn begins to favour absolute power output over power-to-weight. Of course, it’s worth bearing in mind that, in a real-life scenario, the heavier rider is likely to be physically larger and have greater frontal surface area, increasing their aerodynamic resistance further (a discussion to be explored another time).

To illustrate this, let’s now suppose that the riders are travelling twice as fast (32kph) but the gradient is half as steep (3.5 per cent). The figures now become:

  • 80kg rider – 462W (5.77W/kg)
  • 70kg rider – 429W (6.12W/kg)

The rate of ascent overall is still the same and the 70kg rider still requires around 32W less power than the 80kg rider to maintain a speed of 32kph. However, both riders have had to find a massive 163W extra to overcome the increased aerodynamic resistance experienced at 32kph compared to the resistance at 16kph.

What does this mean in practice?

Essentially, the hillier the terrain, the more your power-to-weight ratio matters. The flatter the terrain, the less power-to-ratio matters and the more absolute power output matters (figure 1). We can draw another conclusion: when power-to-weight ratios are identical, the rider with the highest

absolute power will be faster. For example, if rider A weighs 80kg and can sustain 240W, while rider B weighs 70kg and can sustain 210W, they both have a power-to-weight ratio of 3W/kg. But A will be faster because he/she will have more power to overcome aerodynamic and frictional drag.

Figure 1: Terrain and absolute power versus power-to-weight ratio

Figure 1: Terrain and absolute power versus power-to-weight ratio

The flatter the terrain, the more important absolute power becomes.

The hillier the terrain, the more important power-to-weight becomes.

Testing your own power output

Calculating your own power-to-weight ratio requires only two measurements: your weight and your maximum sustainable power output. The first is easy to measure — just hop on some accurate bathroom scales. The second requires a power output measurement. To do this, you’ll need to use a bike with a reliable power meter fitted (SRM, Powertap, etc) or better still, a stationary bike with accurate power metering (e.g. WattBike) where you can pedal furiously without needing to slow down for bends, traffic, etc.

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To measure maximum sustainable aerobic power, ride gently for 10 minutes to make sure you’re thoroughly warmed up. Take a couple of minutes’ rest, then ride as hard as you possibly can for 20 minutes and record your average power output figure in watts. This is your 20-minute maximum sustainable power output. Your one-hour maximum sustainable power output will be five to 10 per cent lower (depending on fitness) than this figure, e.g. a 20-minute figure of 275W would equate to around 260W for an hour. Your five-minute maximum power will be around 10 per cent higher than the 20-minute figure, e.g. in this example, around 295W).

Sa Calobra crop

Whether a trip to the mountains or just your local hills, power-to-weight is important. Photo: Andy Jones

Practical tips for improving your power-to-weight ratio

We’ve seen that increasing power, reducing bodyweight or a combination of both can significantly improve your power-to-weight ratio. But how can you best achieve this? This will depend on your cycling background:

Relative beginners/novices

Simply riding more miles will boost your power-to-weight ratio. Putting in more miles will not only boost your level of aerobic fitness (i.e. your sustainable power output); you’ll almost certainly lose a bit of excess body fat in the process. For example, if you drop from 86 to 82kg and increase your 20-minute power output from 210 to 235W, your power-to-weight ratio increases from 2.4W/kg to a very respectable 2.9W/kg.

Fitter and more experienced riders

You need to be a bit more focused than simply adding more miles. Yes, more miles might result in reduced bodyweight, but add too much extra volume and you run the risk of fatigue and burnout. Moreover, an attempt to reduce weight when your body-fat levels are already quite low can lead to muscle mass loss as well as fat loss. Given power is generated within muscle tissue, you might end up reducing your weight but losing some power with it, resulting in minimal improvements in power-to-weight ratio. In fact, remembering that absolute power is still very important, you might be worse off overall.

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A better option is to include some specific training to boost maximal power output. This includes sessions such as intervals (long and shorter, more intense), hill repeats and some threshold rides. Because these sessions are quite demanding, make sure you build in sufficient recovery time into your weekly schedule — it’s during recovery that your muscles adapt and become more powerful.

Weight training

Another useful strategy, especially for more accomplished riders, is to perform some regular weight training. Studies have shown that performing heavy resistance training for the key cycling muscles (quadriceps, hamstrings, buttocks and calves) not only boosts muscle efficiency, it can help prevent the loss of muscle power during periods of high-volume training, or during periods of weight loss.

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Regardless of your riding ability, consuming a healthy diet with a minimum of sugary, fatty and processed foods will play a part in improving power-to-weight ratio. All other things being equal, higher intakes of sugar and sugary foods in particular have been unequivocally linked with higher levels of body fat (ref 1,2). Unlike muscle tissue, excess body fat blunts power-to-weight ratio and contributes nothing to power output. By the same token, a plentiful intake of dietary protein is recommended, especially after training. Protein is needed for recovery and repair after training, and studies show that higher intakes of protein can help prevent muscle mass loss when training volumes are high.

1. BMJ. 2012 Jan 15;346:e7492. doi: 10.1136/bmj.e7492.
2. Obes Rev. 2013 Aug;14(8):606-19.

Words by Andrew Hamilton
This article was originally printed in the March 26, 2015 issue of Cycling Weekly