@eliteperformanceNI · free tool
Critical speed
calculator
Two time trials, one afternoon on a track. Out comes the one pace your physiology actually draws a line at — and every training zone anchored to it, instead of to a percentage of a max heart rate you probably guessed.
The test
Two all-out efforts, 30 minutes apart
Do both on the same track, same session, same conditions. Even pacing on each — going out too hard and dying wrecks the maths. The classic pairing is 1200 m and 3600 m, but any two efforts of clearly different durations work.
Your numbers
Enter the two trials
Distance against time
Your two trials sit on a straight line. Its steepness is your critical speed; where it crosses the vertical axis is D′.
Your zones
Anchored to LT and critical speed
Critical speed is the heavy/severe boundary — the same line the zone charts call LTP or VT2. The first threshold sits at roughly 80–84% of it, and where exactly depends on how well trained you are. Those two lines are physiology. The splits inside them are coaching judgement, and they're marked as such.
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If you've had it tested, enter it — it replaces the estimate entirely.
Above critical speed
How long can you hold it?
Above CS you're spending D′, and it's finite. The model gives you the exact time to exhaustion at any pace — which is how you set rep length instead of guessing it.
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Race predictions
What the line says you can run
Straight from the same equation: distance equals critical speed multiplied by time, plus D′. It's most trustworthy between about 2 and 20 minutes, so treat the long ones as optimistic.
| Distance | Predicted time | Confidence |
|---|
Take it with you
Get your zone sheet
One page with your critical speed, D′, both zone models, your interval paces and rep lengths. Save it to your phone or print it for the fridge.
Your sheet is generated in your browser — nothing is stored unless you tick the box.
Your sheet is ready
Opens your print dialogue — choose Save as PDF to keep it on your phone.
The maths
Nothing hidden
Two points, one line
Total distance covered in an all-out effort is a straight-line function of its duration: a fixed reserve, plus a sustainable speed multiplied by time.
With two trials, that solves directly. The slope between the points is critical speed; the intercept is D′.
D′ = d₁ − CS × t₁
What each number means
- Critical speed — the boundary between the heavy and severe intensity domains. Below it, oxygen uptake and lactate can settle. Above it, they climb until you stop.
- D′ — the finite distance you can cover above CS before that reserve is empty. Typically somewhere around 100–300 m in trained runners.
- The first threshold — estimated from your critical speed, using a review of 26 studies and 527 runners. Where it sits depends on training status: 80.6% of CS in the least trained group, 83.2% in the middle, 84.2% in the best trained. Across everyone it averaged 82.3%.
- Why the range matters — those are population averages with real spread, and the authors are explicit that LT can't be pinned down precisely from CS alone. The band shown next to your LT is the 95% confidence interval. If you've had lactate testing done, enter that number instead.
Monod & Scherrer (1965) · Hill (1993) · Jones & Vanhatalo (2017)
Kordi, Menzies & Galbraith (2019), Int J Sports Physiol Perform — two-trial model
Nixon, Kranen, Vanhatalo & Jones (2021), Eur J Appl Physiol — CS as maximal metabolic steady state
Hunter, Meyler, Maunder, Cox & Muniz-Pumares (2024), Int J Sports Physiol Perform — first threshold at 82.3% of CS
The next step
This gets you close.
Testing gets you the number.
Critical speed is measured — your first threshold here is estimated from it, and that estimate carries real spread between individuals. A lactate profile in the lab replaces the estimate with your own value, plus the heart rates and economy data no field test can give you.
Northern Ireland · Prof. Stephen Patterson