FTP (Functional Threshold Power) and CP (Critical Power) both answer the question "how hard can I go for a long time?" But they answer it differently - and using the right one matters for your training.
FTP: the practical standard
FTP, popularised by Dr. Andrew Coggan, is a single number: the highest power you can sustain for approximately 60 minutes. In practice, most people estimate it from a 20-minute test (95% of 20-min power) or a ramp test (75% of peak power).
FTP's strength is simplicity. One number. Multiply by percentages to get zones. Easy to track over time. The whole cycling world uses it.
FTP's weakness: it assumes a single threshold. Your sustainable power for 30 minutes, 60 minutes, and 90 minutes are different - FTP flattens them into one number. For riders with an anaerobic bias (strong short efforts, weaker sustained power), FTP overestimates sustainable power. For diesel engines (weak sprints, strong endurance), FTP underestimates it.
Critical Power: the mathematical model
Critical Power comes from a different tradition - the critical power model from exercise physiology. It uses two parameters:
- CP (Critical Power): the power you can sustain indefinitely in theory (in practice, ~30–60 minutes)
- W' (W-prime): your anaerobic work capacity - total work above CP you can do before exhaustion (measured in kilojoules)
These are estimated from two maximal efforts at different durations - typically 3 minutes and 12 minutes. CP ≈ your 12-minute best power (roughly). W' ≈ the extra work your 3-minute power produced above CP, scaled up.
The CP model is more accurate for predicting performance across time scales. Your 5-minute power, 20-minute power, and 60-minute power can all be estimated from CP + W'. FTP can't do that.
When to use which
Use FTP when:
- You're following a training plan that uses Coggan zones (TrainingPeaks, TrainerRoad, Zwift)
- You want a single number to track and share with your coach
- You don't have reliable maximal efforts at two different durations (required for CP)
Use CP when:
- You want to predict performance at durations you haven't tested
- You're analysing your power curve to find strengths and weaknesses
- You're self-coached and want deeper understanding of your physiology
The practical answer:
Use both. FTP for day-to-day zone-based training. CP to understand your power profile and track your W' (anaerobic capacity) - which FTP can't tell you anything about. Baseline computes both automatically from your ride data.