SplitLab

Critical Speed Calculator

8:41/mi

6.90mph

8:41 /mi · 6.90 mph · 3.09 m/s · 370 m · 405 yd D′ · 2-point line

Effort A

mi
First all-out race or time trial. Header US / Metric switches miles and kilometres. Default 5K.
Finish as h:mm:ss or mm:ss. Default 25:00.

Effort B

mi
Second all-out effort at a different distance. Default 10K.
Finish as h:mm:ss or mm:ss. Default 52:00.

Effort C (optional)

mi
Optional third all-out effort. Uses least squares with A and B. Leave blank for the two-point line.
Optional. Pair with distance C.
CS pace
8:41/mi

5:24 /km

CS speed
6.90mph

11.11 km/h · 3.09 m/s

D′
370m

405 yd

Fit
2point

exact line

At CS

5K

distance ÷ CS

27:00

10K

distance ÷ CS

54:00

Half

D′ not applied

1:53:56

Marathon

D′ not applied

3:47:51

Finish times are distance ÷ CS. D′ is not added for 5K through marathon — that reserve is for short all-out work, not a half or marathon prediction.

Hughson–Jones / Jones field estimate from recent all-out efforts. Not lab VO₂max, not VDOT, not a training plan. Even pace on the pace calculator. Riegel on the race time predictor.

Pace · Predictor · 5K · 10K · Age grade.

Worked examples

  • 5K 25:00 · 10K 52:00

    CS 8:41 /mi · 5:24 /km · 6.90 mph · D′ 370 m

  • Same 5K even pace, not CS

    25:00 5K is 8:03 /mi — the race speed, not the slope

  • At CS, D′ not applied

    5K 27:00 · 10K 54:00 · half 1:53:56 · marathon 3:47:51

Formulas

Hughson–Jones / Jones critical-speed model. Distance against time is a straight line. The slope is CS; the intercept is D′. Two efforts give the exact line; a third uses ordinary least squares. This page does not clone VDOT, McMillan, or Jack Daniels tables.

  • D = CS · t + D′
  • two-point CS = (D₂ − D₁) / (t₂ − t₁)
  • D′ = D₁ − CS · t₁
  • pace sec/mile = 1609.344 / CS
  • long-race time at CS = distance / CS (D′ not applied)

Hughson–Jones / Jones linear critical-speed model. Field estimate from recent all-out efforts — not lab VO₂max, not a training plan. Even-pace math on the pace calculator. Race equivalence on the race time predictor. 5K · 10K · age grade · mile pace.

FAQ

What is critical speed?

Critical speed (CS) is the slope of distance versus time from recent all-out races or time trials: D = CS · t + D′. CS is a field estimate of sustainable aerobic speed. D′ is anaerobic distance capacity in metres. Default 5K in 25:00 and 10K in 52:00 is 8:41/mi (5:24/km). There is no Calculate button; CS updates live.

Why isn’t CS the same as 5K pace?

A 25:00 5K is 8:03/mi even pace (about 3.33 m/s). Critical speed from that 5K plus a 52:00 10K is slower — 8:41/mi — because the model is the slope between the two efforts, not the shorter race’s speed. Hughson–Jones / Jones, not VDOT.

Do I need a third race?

Two different distances are enough for an exact two-point line. A third all-out effort uses ordinary least squares and usually steadies the fit. Same distance twice, non-positive times, or a pair that yields CS ≤ 0 are rejected.

Why aren’t 5K / 10K / half / marathon times D′ plus CS?

Those rows are distance ÷ CS only. D′ is a short-event reserve; applying it to a half or marathon overstates how long you can hold CS. Treat long-race times as a CS-pace sketch, not a predictor. Riegel lives on the race time predictor.

Is this VDOT or a lab VO₂max?

No. This page does not clone VDOT, McMillan, or Jack Daniels tables. It is not a laboratory VO₂max, not a training plan, and not medical advice. Use recent all-out efforts; easy runs will flatten CS.