Without a power meter you can estimate; on the flat you cannot
Cycling power splits between three resistances: gravity, rolling and air. The proportion between them changes completely with the gradient, and from that comes the most useful rule of thumb in amateur cycling.
Above 5 % gradient, more than 85 % of the power is explained by weight, gravity and climbing speed alone: variables you know precisely. That is why estimating watts on a climb carries a typical error of 3-5 % even without knowing your aerodynamic coefficient. On the flat, by contrast, the air term grows with the cube of speed and depends on a CdA almost nobody has measured in a wind tunnel: there the estimate stops being reliable.
W/kg, and why it is not everything
Uphill, watts per kilogram rule, because you are fighting your own weight. On the flat and in time trials absolute power rules, because air resistance depends on your frontal area rather than your mass. An 80 kg rider at 320 W goes faster on the flat than a 60 kg rider at 280 W, even though on the climb exactly the opposite happens.
| Level | W/kg at 20 min | Approximate VAM |
|---|---|---|
| Recreational rider | 2.0 – 2.8 | 500 – 750 m/h |
| Trained amateur | 2.8 – 3.6 | 750 – 1,000 m/h |
| Competitive amateur | 3.6 – 4.5 | 1,000 – 1,250 m/h |
| Elite amateur | 4.5 – 5.3 | 1,250 – 1,500 m/h |
| Professional | > 5.5 | > 1,600 m/h |
VAM, the quick way to compare yourself
VAM is the metres of elevation you gain per hour. Its advantage is that it depends on neither distance nor wind, and it relates to watts per kilogram through an approximation that works surprisingly well:
Climb 1,000 metres of elevation in an hour on an average gradient of 8 % and you are pushing around 3.6 W/kg. It is the calculation everyone does in their head in the car park after a climb.