PaceCalc is a free, single-purpose website for runners: a five-mode pace calculator that converts between the numbers the sport actually uses — min/km, min/mile, km/h, mph, finish times and race predictions. Everything computes in your browser the moment you type; nothing you enter leaves the page.

What each mode does

• Pace — distance plus time becomes min/km, min/mile, km/h, mph and a 400-meter lap split, plus a table of what that pace finishes at the standard race distances.
• Finish time — a pace and a race distance become a predicted finish and a complete per-km or per-mile splits table.
• Race prediction — the Riegel formula, T2 = T1 × (D2/D1)^1.06, translates a result at one distance to another, shown with an honest accuracy window that widens as the gap grows.
• Speed → pace — treadmill and bike-computer units converted both directions.
• Training zones — a recent race result becomes six pace zones (easy through repetition) as percentage bands of a Riegel-equivalent 10K anchor.

The honesty rules we build to

The site’s point of view is that running numbers should show their work:

• Constants are stated. The mile is 1.609344 km, the half marathon 21.0975 km, the marathon 42.195 km — no rounding between your input and the answer.
• Named formulas, not magic. Riegel’s equation and its 1977 origin are on the page, as is the caveat that the exponent is empirical — a fit to race data, not physiology.
• Uncertainty is displayed, not hidden. Predictions come with a window and the reasons it can miss; zones are labeled heuristics.
• “Good” is treated honestly. Our pace benchmarks use level tables and field medians rather than flattering generalities.

Who maintains it

PaceCalc is built and maintained by a small independent team that makes focused utility sites. We check the arithmetic against worked examples — a 50:00 10K really is 5:00/km, 8:02.8/mile and 12 km/h — and we keep the edge cases (empty fields, partial last splits, custom distances) explicit rather than smoothed over.

Something look off? Corrections get priority — write to us.

PaceCalc is an educational tool, not a coach or medical resource. See the disclaimer.