Power-to-weight ratio

Power-to-weight ratio (watts per kilogram) is the key metric in cycling performance, especially on climbs. It measures your sustainable power output relative to body mass:

Category	W/kg (FTP)
Untrained	~1.5 W/kg
Recreational cyclist	~2.5 W/kg
Trained amateur	~3.5 W/kg
Cat 1/2 racer	~4.5 W/kg
Elite / Pro	5.0–6.5+ W/kg

Losing 1 kg of body weight has the same effect on climbing as increasing FTP by ~3–4 watts at moderate fitness levels.

Functional Threshold Power (FTP)

FTP is the highest average power you can sustain for one hour. It's the cornerstone of cycling training and the basis for training zones. To test FTP:

Warm up for 15–20 minutes with progressive efforts.
Ride at maximum sustainable effort for 20 minutes on a flat route or trainer.
Multiply your average power by 0.95 to estimate FTP. Example: 250W avg × 0.95 = 238W FTP.

Retest FTP every 6–8 weeks to track fitness changes and adjust training zones accordingly.

Training zones

Zone	% of FTP	Description
Z1 Active Recovery	<55%	Very easy spin, flushing legs
Z2 Endurance	56–75%	Base-building pace, aerobic foundation
Z3 Tempo	76–90%	Comfortably hard, "sweet spot" training (88–93% FTP)
Z4 Threshold	91–105%	Sustained hard effort, right at FTP (~1-hour max)
Z5 VO2max	106–120%	5–8 minute intervals, very hard breathing
Z6 Anaerobic	121–150%	1–3 minute max efforts, lactate accumulation
Z7 Neuromuscular	>150%	All-out sprints, <30 seconds

Physics of cycling power

Power required to maintain a given speed is the sum of four resistance forces:

Aerodynamic drag: Proportional to velocity squared. CdA (drag coefficient × frontal area) is the key parameter. A road bike in hoods position: CdA ≈ 0.40 m². Drops: ≈ 0.32 m². Time trial position: ≈ 0.24 m².
Rolling resistance: Crr (coefficient of rolling resistance) × weight × g. Road tires: Crr ≈ 0.004–0.006. Increases with rough pavement.
Gravity: Weight × gradient (%) × g. On a 5% climb, gravity force is 5% of body + bike weight.
Drivetrain loss: ~2–3% for a clean chain. Neglected in most calculators.

At 20 km/h (12.4 mph), most power fights rolling resistance. At 40 km/h (24.9 mph), aero drag dominates. This is why aero gains matter more at higher speeds.