Critical speed calculator: find your running threshold
Use your test runs to calculate your running threshold (critical speed) and your anaerobic distance capacity D′.
Enter your test runs
Enter your times for the three all-out runs. Critical speed and D′ are calculated from the three data points by linear regression.
With two runs the line passes exactly through both data points. That is quicker, but three runs give you a more reliable result.
Enter two to six all-out runs over different distances. Results are most reliable with run times between about 2 and 20 minutes.
Distance in metres, time as mm:ss (00:00), e.g. 1600 m in 05:40.
Result
How to do the test
- 1Warm up easy for 15–20 minutes, including a few short strides.
- 2Run each distance at an even pace and as fast as you can just about hold for that distance.
- 3Recover easily for at least 20–30 minutes between runs, or split the runs over two days.
- 4Ideally on a track or a flat, accurately measured route without wind.
What do the values mean?
d = CS · t + D′
The model describes the distance d you can run in time t as a straight line: d = CS · t + D′. The slope CS (critical speed) is the highest speed you can sustain over a longer period. That is your threshold. D′ is the extra distance you can cover above that speed before you have to slow down.
Critical speed FAQ
What is critical speed?
Critical speed (CS) is the fastest running speed you can sustain for a longer period in a stable metabolic state. It separates the heavy from the severe intensity domain and can be held for roughly 20 to 40 minutes, depending on your fitness. In training it serves as the threshold that guides your intensities.
How does critical speed differ from lactate threshold?
Both describe the transition to intensities you can’t sustain for long. Lactate threshold is measured in a lab from blood samples; critical speed comes from your run times alone. Critical speed is usually close to the maximal lactate steady state, often slightly above it.
What does D′ (D-prime) mean?
D′ is the extra distance you can run above your critical speed. This reserve is limited. Running fast uses it up, and it refills during easy phases. Runners with a lot of sprint and middle-distance strength usually have a larger D′.
Which distances work for the test?
The proven test uses 200 m, 1000 m and 3000 m. 1000 m and 3000 m is quicker. With custom distances, run times should be roughly between 2 and 20 minutes and clearly different, for example 1200 m and 3600 m.
Can I use race times?
Yes, if you ran the races flat out, they were only a few weeks apart and took place on flat, accurately measured courses. 1500 m, 3000 m and 5000 m on the track work well, for example. Enter them under “Custom distances”.
How often should I repeat the test?
Roughly every 6 to 8 weeks or at the start of a new training block. That way your threshold always matches your current form.