British running has a unit problem. Race distances are metric, watches default to kilometres, club runners talk in miles, and treadmills do whatever they like. Converting 7:30 per mile into pace per kilometre in your head, mid-run, is nobody's idea of a good time.
PaceCalc started as a tool to stop doing that arithmetic manually.
What the tools do
The core calculator solves for pace, distance or time given the other two, in either unit. On top of that sit distance-specific calculators for 5K, 10K, the mile, the half marathon and the marathon, each showing split times and the pace needed for common target finishes.
There is also a race time predictor and a set of training pace zones derived from a recent result.
The formulas we use
Race predictions use the Riegel formula, which estimates a time at one distance from a known time at another using an endurance exponent. It has been used by coaches for decades and is reliable within a reasonable range of distances.
It is less reliable at the extremes. Predicting a marathon from a 5K assumes an endurance base the 5K result cannot demonstrate, and the prediction will usually be optimistic. We say so on the pages where it matters rather than presenting every projection as equally trustworthy.
Training zones are calculated as percentages of threshold pace: easy and recovery running well below it, tempo work close to it, intervals above.
Who publishes this site
PaceCalc is built and maintained by CJ Software Ltd, a UK software company that develops a small family of free, single-purpose calculators. We are a software business, not a coaching service, and we say so plainly because it matters when you decide how much weight to give a number on a screen.
The site is funded by advertising. Advertisers have no influence over the calculations, the guidance, or what we choose to write about.
Accuracy and common sense
A pace calculator does arithmetic perfectly and knows nothing about you. It cannot see hills, heat, wind, how you slept, whether the course measures long, or how your training has actually gone. Race day pacing should always be run on effort as well as on numbers.
Nothing here is coaching advice, and it is not a training plan. If you are returning from injury or new to structured training, a coach or physiotherapist is worth more than any calculator. If you find a calculation that looks wrong, please tell us and we will check it.