In running, performance is naturally expressed in time. A “Sub 3” marathon (running 42.2 km in under 3 hours) is the universally recognised holy grail for amateur runners. Anyone who has run any race knows what it means and what it took to get there.
Cycling is different. Wind, drafting, and gradient distort time so heavily that it’s a poor measure of fitness. Instead, cyclists use power-to-weight ratio: how many watts you can sustain per kilogram of bodyweight.
The cultural equivalent of the Sub 3 marathon in cycling is 4 W/kg.
What 4 W/kg means
Functional Threshold Power (FTP) is the maximum average power you can sustain for one hour. Divide that by bodyweight in kilograms and you get W/kg.
- A 75 kg rider needs to hold 300 watts for an hour to hit 4 W/kg.
- A 65 kg rider needs to hold 260 watts.
That’s a brutal hour, regardless of body size.
Tiers
| Tier | Marathon equivalent | Cycling FTP (W/kg) |
|---|---|---|
| World-class pro | ~2:00–2:05 | 5.5–6.5 |
| Domestic / Cat 1 | 2:15–2:30 | 4.5–5.0 |
| Amateur holy grail | Sub 3 hours | 4.0 |
| Dedicated recreational | 3:30–4:00 | 2.5–3.2 |
Tour de France contenders sit around 6.0 W/kg for an hour and can push 6.5+ W/kg on critical climbs. Crossing 5 W/kg as an amateur generally requires winning the genetic lottery and training as a near full-time occupation.
Why 4 W/kg is hard
It’s not just a fitness number. It’s a lifestyle number, much like Sub 3:
- It requires grueling, structured interval training.
- Because it’s a ratio, it demands dietary discipline. You’re chasing both higher watts and lower body weight without sacrificing power.
- You cannot accidentally arrive at it. There is no version of “just ride your bike more” that gets you there.
Honorable mention: the sub-5-hour century
If you want a time-and-distance benchmark in cycling, the closest equivalent is the sub-5-hour century: 100 miles in under 5 hours. That requires a relentless 20+ mph average. It’s a worthy goal, but a rider can cheat the wind by drafting inside a group, so it doesn’t have the pure individual truth of 4 W/kg.
Translating from running fitness
A 3:09 marathon and a 40-minute 10K imply the cardiovascular engine of someone who should be sitting comfortably above 3.0 W/kg on the bike. Runners coming to cycling often see surprisingly low FTP numbers when they start, and the reason is mostly mechanical, not aerobic:
- Running uses elastic recoil — the Achilles tendons act as springs.
- Cycling is pure concentric muscle force — there’s no bounce, just pushing dead weight in circles.
For a strong runner transferring to cycling, the aerobic system is already built; only the cycling-specific muscular endurance is missing. That makes the early gains very fast — jumping from 2.2 W/kg to 3.0 W/kg can take only a few months of structured cycling-specific work (sweet spot, threshold intervals).
A warning about smart-bike wattage numbers
Most consumer smart bikes (Peloton-style, Egojin, and similar) do not actually measure power. They estimate it from flywheel speed and resistance level. This “virtual power” or “zPower” is notoriously inaccurate, often underestimating strong riders by 30–50% and capping out at higher efforts.
If the numbers from a smart bike look much lower than expected given the rider’s running performance, the bike is almost certainly lying. See Power Meter Options for ways to get a real measurement without buying a new trainer.