If you’re into cycling, there’s a good chance you’ve heard the term “power output” in training or perhaps even looked to “save a few Watts” in the context of new cycling gear.
But what exactly is a “Watt” – and how is Watts calculated in cycling?
Simply put, a “Watt” (W) is a unit of power. On a scientific level, it’s equivalent to one joule per second – but we’ll explain more on that later.
Watts are most commonly calculated for cyclists through the use of a power meter, which measures torque and cadence to display power output.
As a PhD candidate in the field of physics who also happens to be cycling-mad, perhaps I’m biased in thinking that the scientific side of cycling is absolutely fascinating – though with “marginal gains” now a firmly accepted cyclist’s cliche, gaining a deeper understanding of one of the most important performance metrics in the sport has more value than ever.
In this article, I’ll be explaining what Watts means for cycling power, why it’s such an important element of power to understand, and how this knowledge can be implemented in your cycling training.
Let’s get started!
What Are Watts In Cycling?
A Watt (W) is a scientific unit of power.
The unit is named after legendary Scottish engineer and inventor James Watt. By revolutionizing the steam engine, he played a major role in the industrial revolution.
During the process of testing his new inventions, he developed the concept of “horsepower” – still used in the context of engines today – to compare the power output of a steam engine with that of a horse.
Using horses as a unit of measurement, however, is incredibly impractical. Once the SI unit for power was developed, it was named after the man who defined it: James Watt.
The Real Meaning of “Power”
Power as a concept refers to energy (measured in joules) per unit of time. In other words, power is the rate of energy transfer, not just the total amount of energy itself.
A Watt is equivalent to one joule per second.
If you’re struggling to make sense of the difference between energy and power, think of it like this: Cyclist A cycles gently for 100 seconds at an average power of 100 Watts, while Cyclist B sprints for just ten seconds, but at a power of 1000 Watts.
Both cyclists have produced the same total amount of energy – 10,000 joules or 10 kilojoules – but at vastly different power outputs.
How Is Watts Calculated In Cycling?
Although the definition of Watts is simple, it’s not quite so straightforward to calculate in practice – particularly in the context of cycling.
Without getting too deep into the physics jargon, the problem is that “energy” is an extremely diverse quantity that is calculated differently in different scenarios.
You’re not going to use the same equation to calculate a useful metric for a cyclist as you are for a wall plug, even though both are measured in Watts and both measure power.
In cycling, Watts can be calculated using a power meter, most commonly located in the pedals (though there are also crank, bottom bracket, and freehub-based models).
A cycling power meter is a device that measures the torque and cadence of the pedals to calculate and display your power in Watts.
Rotational “work” or “energy” is measured by multiplying together the torque and the angle through which the object is rotated.
Torque, for the science-savvy cyclist, is a measure of “rotational force”. In other words, it means the force you apply to the pedals, multiplied by the length of the cranks (longer cranks generate more torque due to the principle of mechanical leverage).
But we’re looking for the power, which is the rotational energy per second. So, we need to multiply torque by the rate of rotation of the object.
Luckily, the rate of rotation of the pedals is a very easily understood concept for a cyclist: pedaling cadence.
So, a power meter calculates your power output in Watts by multiplying the force you apply on the pedals by the length of your cranks and your pedaling cadence.
Note that a cycling power meter measures the rotational energy at the pedals (or another part of the bike), not at your legs.
This means that any of the energy generated by your legs that is lost due to an inefficient power transfer to the pedals is lost in the calculation of power in cycling.
This also offers us an intuitive explanation as to why, if you have both a freehub power meter and pedal power meters, the freehub’s power meter always gives a slightly lower reading. Some of the energy you put through the pedals is lost before reaching the wheel due to mechanical inefficiencies in the drivetrain.
How Does Power Output Translate To Cycling Performance?
So, now we have a detailed explanation of what power is and how it’s measured on a bike; how does it translate to cycling performance?
Well, first of all, it’s important to understand that if you sustain the same power output over a varied ride, your speed will vary significantly.
If you’re going uphill, for example, a significant amount of energy is used to overcome gravity, or if you’re on bumpy terrain, energy is wasted in the vertical vibrations of the frame.
Power-to-Weight Ratios (W/kg)
Arguably the single most important performance metric for a cyclist to understand is the power-to-weight ratio.
This is the power output of a rider divided by their body weight, and is measured in Watts per kilogram (W/kg).
The power-to-weight ratio is a vital factor in climbing performance. The steeper the gradient of the road, the more gravity comes into play, and the more decisive a rider’s W/kg ratio becomes.
The relationship between weight and power is much more important than either on their own: a cyclist weighing 50 kg and averaging 250 Watts (5 W/kg) will climb a hill much faster than a cyclist putting out 400 Watts but weighing 100 kg (4 W/kg).
For reference, Jonas Vingegaard sustained a superhuman 7.6 W/kg for 13 minutes during the legendary Stage 16 time trial at the 2023 Tour de France.
At their FTP (functional threshold power), which is the maximum average power a cyclist can sustain over an hour, a decent amateur might produce around 3 W/kg, while anything above 6 W/kg would be extremely surprising outside the professional peloton.
Given that major road cycling races such as the Tour de France are almost always won and lost in the high mountains, Watts per kilogram is the performance metric that attracts more attention than any other.
How Watts Relates To Cyclist Types and Roles
However, W/kg is only relevant in scenarios in which weight matters (in other words, races in which the winner is likely to be decided through climbing).
In track cycling, for example, weight is relatively unimportant, meaning W/kg as a concept is too.
But that doesn’t mean that Watts and power output don’t matter outside of climbing – quite the opposite.
Without having to give as much consideration to the weight of extra muscle bulk, cyclists can focus on being able to produce the highest possible power output, either in short bursts to win sprints or for long, sustained periods in the service of teammates (i.e. domestiques).
Sprinters, whether on the road or track, will generally be able to produce the highest peak power output of any cyclist type.
The great André Greipel, for example, recorded a power output of over 1900 Watts during a sprint and could sustain over 1000 W for 30 seconds – far greater than Tour de France winner Vingegaard could hope for, even with his 7.6 W/kg ratio.
However, they might not be able to sustain such power for long. A rouleur (a versatile racer optimized for road races with rolling hills), for example, would be able to sustain a slightly lower power output but for a much longer distance.
So, that’s why sprinters usually have behemoth legs and are generally stockier in build. They need just raw power to output such a huge peak Wattage.
Climbers, on the other hand, need to sustain high Watts for an extremely long time, particularly on long, grueling alpine climbs of the Tour de France or Giro d’Italia, but balanced against their weight to maintain a high power-to-weight ratio.
How Should Understanding Watts Impact Your Cycling Training?
Watts is arguably the most useful metric to guide your training.
Different riders in differing conditions, riding different bikes, will produce different speeds despite riding at the same power output.
For example, if you’re riding into a headwind, on a rough surface, or on a rusty and old bike, your speed isn’t going to be remotely comparable to that when you’re on a state-of-the-art brand new road bike on perfect asphalt with no wind resistance and low rolling resistance.
So, just using speed (or even heart rate) doesn’t give you a great gauge of your performance progress when comparing different rides or cyclists.
Power output, however, is much more objective because it’s measured at the point of contact rather than after all of the resistive forces have sapped your speed.
It allows more accurate calculation of fitness progress and training load, but it also provides a system through which you can devise your training plan.
This is through the use of power zones, which represent zones of your power output compared to your FTP.
One of the best ways to structure a training plan is to aim for different power zones in different rides in your plan.
For example, training in Zone 4 will help to increase your maximum power output and speed over a normal ride. Training in Zone 2 will help to increase your endurance and the distance you can cycle.
Whatever training plan you choose, you’ll generally be doing a mix of rides and workouts in which you aim for Zone 2 or Zone 4, and this will help you to increase your cycling performance across the board in anticipation of an event or race.
