Pace Calculator

How the pace calculator works

Pace, time and distance are tied together by one relationship: pace is the time it takes to cover one unit of distance. Know any two and the third follows. Choose what you want to find:

• Pace from a distance and a time, such as a race result or a training run. You also get your average speed and the equivalent pace in the other unit.
• Finish time from a distance and a target pace, for planning a race.
• Distance from a time and a pace, for example how far you cover in a 45-minute run at your easy pace.

Pick a standard race or enter your own distance. Race distances are fixed in kilometers: 5 km, 10 km, the half marathon at 21.0975 km and the marathon at 42.195 km, which World Athletics gives as 26 miles 385 yards. Miles are converted with the exact international definition, 1 mile = 1.609344 km.

Below the result, the split table shows the time for every kilometer or mile at an even pace and the elapsed time at each marker, with a shorter final split when the distance isn’t a whole number of units. The last table estimates what the same effort might mean over other race distances.

Pace, speed and race-time formulas

Pace and speed are reciprocals: 5:00 per km is 12 km/h, because 60 minutes ÷ 5 minutes = 12. To change units, multiply a per-km pace by 1.609344 to get the per-mile pace, and divide a per-mile pace by 1.609344 for the per-km pace.

The race estimates use the endurance formula published by Peter Riegel in 1981:

T₁, D₁

a time you have run and its distance

T₂

the estimated time for a new distance D₂

1.06

the fatigue exponent: pace slows slightly as the distance grows

An exponent of exactly 1 would mean holding the same pace at any distance. At 1.06, doubling the distance multiplies the time by 2^1.06 ≈ 2.085, so the pace per unit is about 4% slower.

Worked example

You run a 10K in 50 minutes flat:

1. Time: 50 × 60 = 3,000 seconds.
2. Pace per km: 3,000 ÷ 10 = 300 seconds = 5:00 per km.
3. In miles: 10 km ÷ 1.609344 = 6.2137 mi, and 3,000 ÷ 6.2137 = 482.8 seconds, which rounds to 8:03 per mile.
4. Speed: 10 km ÷ 0.8333 h = 12 km/h, or 6.2137 mi ÷ 0.8333 h = 7.46 mph.
5. Mile splits: each full mile takes 8:03, so you pass mile 6 at 48:17; the last 0.21 mi takes 1:43 and you finish at 50:00.

Riegel’s formula turns the same 10K into estimates of 23:59 for a 5K (3,000 × 0.5^1.06 = 1,438.9 s), 1:50:19 for a half marathon and 3:50:01 for a marathon.

Planning the other way round: a half marathon at 6:00 per km takes 21.0975 × 360 = 7,595.1 seconds, a finish time of 2:06:35.

Using the estimates sensibly

Riegel fitted his formula to world-record performances, so it describes what a well-trained runner can do at each distance. For many recreational runners, the longer predictions are optimistic: a fast 5K says little about the endurance a marathon needs, and a prediction made from a much shorter race carries the most uncertainty. A recent result over a distance close to your target is the better guide.

• Even pace is a model. Real races have hills, turns, water stations and crowded starts. Use the splits as checkpoints, not as a rule for every kilometer.
• GPS watches measure a little long or short. Race courses are measured along the shortest route a runner could take, and weaving around other runners adds distance, so your watch’s pace can differ from the official one.
• Conditions matter. Heat, humidity, altitude and wind all slow the same effort.