Pace Calculator — Running, Min/Km, and Race Time

The calculator handles three problems with the same underlying maths. Pick which one you're solving with the tab selector, enter the two known quantities, and read off the third. Internally all results come from one identity:

time = pace × distance

With pace expressed in seconds per kilometre and distance in kilometres, every other quantity is a rearrangement:

• pace = time ÷ distance — primary pace mode.
• distance = time ÷ pace — useful when you know how long you want to run and at what pace.
• speed (km/h) = distance ÷ (time / 3600) — straight unit conversion.

To convert between kilometres and miles, the calculator uses the NIST-exact factor 1 mi = 1.609344 km. That single constant avoids the rounding errors you get when tools chain approximate conversions (1 mi ≈ 1.61 km is enough to drift a marathon prediction by several seconds).

The race-time prediction table uses the Riegel endurance formula published by Pete Riegel in the 1981 American Scientist paper “Athletic Records and Human Endurance”:

T₂ = T₁ × (D₂ ÷ D₁) ^ 1.06

where T₁ is a known time at distance D₁, and T₂ is the predicted time at the new distance D₂. The 1.06 exponent encodes empirically observed endurance decay — doubling the distance more than doubles the time, but only by about 6%. The formula holds to within ±5% for trained runners on comparable terrain, between efforts of roughly 3.5 minutes and 4 hours. Outside that range (very short sprints, ultras above ~50 km), it tends to under-predict because muscular and metabolic limits change.

As a cross-check the calculator computes pace twice — directly as time ÷ distance, and indirectly via speed (distance ÷ time, then inverted to pace). Both paths agree to floating-point precision; the “verified” tick next to the pace tile is the live confirmation.

• NIST Special Publication 811 (2008 ed.), Appendix B — 1 mile = 1609.344 m exactly
• Riegel, P. S. (1981). Athletic Records and Human Endurance. American Scientist 69(3): 285–290
• World Athletics Competition Rules (Book C, C2.1 § 240) — road race distances

Formulas and standard distances were last cross-checked against the sources above on 2026-05-11. The page is reviewed annually or whenever World Athletics revises a road-race standard.