Critical Speed & D′ Calculator
Enter two or more recent maximal efforts to fit your critical speed (the pace you can hold aerobically) and D′ (your finite reserve above it). The two numbers behind every running power-duration curve.
The critical speed model says the distance you can cover in time t follows a straight line: D = CS · t + D′. Plotting your efforts as speed against 1/time, the intercept is your critical speed (the highest pace your aerobic system can sustain) and the slope is D′ (a fixed distance “battery” you can spend above CS). We fit that line by least squares.
Use efforts that were genuinely maximal and reasonably spread in duration — for example a 1500 m or mile paired with a 5K or 10K. Two all-out efforts of similar length give an unstable fit. CS sits close to, but usually a touch above, your lactate-threshold pace.
A fit from two or three efforts is only as good as those efforts. One pacing mistake or an off day skews CS and D′ noticeably, and the model can’t tell a maximal effort from a controlled one — it trusts whatever you enter.
Vertical fits your critical speed from your full training history with Bayesian shrinkage, weighting each effort by how maximal it really was, and updates it after every run — so your zones and race predictions track the engine you actually have.
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