W' (pronounced "W prime") is your anaerobic work capacity—the total energy available for efforts above your threshold.
W' vs FTP: What's the Difference?
| Metric | What It Measures | Units |
|---|---|---|
| FTP/CP | Your sustainable threshold power | Watts |
| W' | Energy available above threshold | Kilojoules (kJ) |
FTP tells you how powerful your aerobic engine is.
W' tells you how much "boost" you have for hard efforts.
You need both to understand your complete performance profile.
Typical W' Values
| Level | Typical W' Range |
|---|---|
| Beginner | 10-15 kJ |
| Intermediate | 15-20 kJ |
| Advanced | 18-25 kJ |
| Elite | 20-30+ kJ |
W' varies based on genetics, training, and muscle fiber composition. "Punchy" riders often have higher W' relative to their FTP.
How W' Is Estimated
WattPlan estimates your W' using:
Method 1: Experience Level
If you're new, we use a conservative default based on your training level.
Method 2: Ride History
If you've imported rides with power data, we analyze your best efforts at different durations to calculate W' from your power-duration curve.
Method 3: CP Test (Most Accurate)
A dedicated test with 3-minute and 12-minute all-out efforts gives the most precise W' measurement.
The Math (Optional)
W' comes from the Critical Power model:
Work = CP × Time + W'By measuring your best power at multiple durations, we can solve for both CP and W'.
Example calculation:
- 3-minute best: 350W → Work = 63,000 J
- 12-minute best: 280W → Work = 201,600 J
From these two points, we derive:
- CP = 257W
- W' = 16,900 J
How to Improve Your W'
W' responds to high-intensity, short-duration training:
Effective Workouts
- 30-second sprints with full recovery
- 1-2 minute all-out efforts
- Hill sprints (15-60 seconds)
- Tabata intervals (20 sec on, 10 sec off)
Training Frequency
1-2 W'-focused sessions per week during build phases. More isn't better—these sessions are taxing.
Viewing Your W' in WattPlan
Performance Metrics
Your estimated W' and CP values are shown on the Performance Metrics page. You can also see your Power Profile spider chart showing your strengths across 5 durations, with automatic rider type classification.
Workout Player
The W'bal gauge shows real-time depletion and recovery with these features:
- TTE (Time to Exhaustion): When above threshold, shows how long until W' depletes at current power
- Recovery Timer (↑80%): When below threshold, shows time to recover to 80% at current power
- Ghost Arc: A 2-minute projection showing where your W'bal will be if you maintain current power (green = recovering, red = depleting)
Ride Review
The Advanced tab shows total W' expended and breakdown of hard efforts.
Stats
Track W' capacity trends over time to see if your anaerobic fitness is improving.
Common Questions
Can W' change?
Yes. W' responds to training, though it changes more slowly than FTP. High-intensity interval training can increase W' over 8-12 weeks.
Is higher W' always better?
Not necessarily. It depends on your goals. Time trialists benefit more from high CP. Criterium racers benefit more from high W'. Most riders benefit from developing both.
Why is my W' different from other platforms?
Different platforms use different estimation methods and models. Variations of 10-20% between platforms are normal.