4:58 min/km
Divide total run time by distance to get pace in minutes and seconds per mile or per kilometer. Solve for pace, time, or distance from any two of the three.
pace = time / distance
Predict a race time at a target distance from a known race result at another distance. The 1.06 exponent reflects the typical fade trained runners see as race distance grows.
T2 = T1 × (D2 / D1) ^ 1.06
Running pace is simply total time divided by distance, with the result reported in minutes and seconds per mile or per kilometer. The calculator solves for any one of pace, time, or distance from the other two, so you can plan a training run, back into a goal time, or work out how far you ran given a watch time and a known route pace. The race-time predictor uses Pete Riegel's formula T2 = T1 × (D2 / D1) ^ 1.06, which assumes the same runner under the same conditions slows by a predictable factor as race distance grows. A 20:00 5K, plugged into Riegel, predicts roughly a 3:11:49 marathon — within striking distance of the Galloway and Pfitzinger benchmarks long-distance coaches publish.
How fast is a 5-mile run in 40 minutes flat, and what marathon time would Riegel's formula predict from a 20:00 5K?
The Riegel result is a model, not a guarantee — heat, hills, hydration, and how well the runner has trained for the longer distance all push the actual race time up or down.
Pace is the inverse of speed: a faster runner has a smaller pace number (fewer minutes per mile) but a larger speed number (more miles per hour). Converting between min/mile and min/km uses the fixed ratio 1 mile = 1.609344 km, so 8:00 min/mile corresponds to about 4:58 min/km. The Riegel exponent of 1.06 means that doubling the distance multiplies the time by about 2.085, not 2.0 — runners slow down a little as distance grows. The model breaks down at the extremes: it under-predicts very short sprint times (where anaerobic limits dominate) and over-predicts very long ultra times (where fueling and pacing strategy dominate over raw aerobic fitness).
A widely cited rule of thumb is that a sub-30-minute 5K (roughly 9:40 min/mile or 6:00 min/km) is a solid recreational benchmark, sub-25 (8:00 min/mile, 5:00 min/km) is competitive for an age-group runner, and sub-20 (6:26 min/mile, 4:00 min/km) is approaching the level of regional competitive runners. The best benchmark, though, is your own previous race — a five percent improvement is a meaningful jump regardless of starting point.
Divide your total time by the distance you ran: pace = time / distance. For example, a 5-mile run in 40 minutes is 40 / 5 = 8 minutes per mile, written as an 8:00 min/mile pace. The calculator handles the unit conversion automatically and shows the equivalent min/km value side by side.
Riegel's formula is T2 = T1 × (D2 / D1) ^ 1.06, where T1 and D1 are a known race time and distance and T2 is the predicted time for a new target distance D2. The 1.06 exponent is the average rate at which trained runners slow down as race distance grows, derived by Pete Riegel from a large database of race results. A 20-minute 5K predicts roughly a 3:11:49 marathon, in the same neighborhood as the long-running Galloway and Pfitzinger benchmarks coaches use.
Pace and speed are reciprocals: speed (mph) = 60 / pace (min/mile), and speed (km/h) = 60 / pace (min/km). An 8:00 min/mile pace is 60 / 8 = 7.5 mph, and a 5:00 min/km pace is 60 / 5 = 12 km/h. The calculator reports pace because pace is what runners track in races and on watches — speed is more common for cycling.
Aerobic energy systems have a limit, and the longer the race the closer that limit gets. Riegel's 1.06 exponent says that doubling the distance multiplies the time by about 2.085, not 2.0 — so a runner who covers 5 km in 20 minutes won't quite cover 10 km in 40, but in about 41:43. The fade compounds over longer races: a 5K time predicts a marathon roughly 9.4× as long, not 8.44× as the distance ratio alone would suggest.
Plug your 5K time into the Riegel formula with D1 = 5 km and D2 = 42.195 km. The predictor multiplies your 5K time by (42.195 / 5) ^ 1.06 ≈ 9.591. A 20-minute 5K predicts about 3:11:49; a 25-minute 5K predicts about 3:59:47. The prediction assumes you have trained adequately for the marathon — without long-run mileage in your legs, the actual race time will be significantly slower.
Average pace is total time divided by total distance — the single number this calculator returns. Split pace is the pace for one segment of the run (a mile, a kilometer, a lap), and the splits often vary significantly inside a single average. A runner with an 8:00 average mile pace might be running 7:30s for the first half and 8:30s for the second; this calculator gives you the average, not the splits.
The pace = time / distance math is identical for any sport, so the pace, time, and distance solvers work for cycling and swimming too — just enter the distance in the appropriate units. The Riegel predictor, however, is calibrated for running and is not reliable for predicting cycling or swimming times across distances; those sports have different fade curves driven by drafting, aerodynamics, and stroke mechanics.
Reference: Riegel exponent of 1.06 from Peter Riegel, "Athletic Records and Human Endurance," American Scientist 69:285–290 (1981). Standard race distances are the IAAF / World Athletics certified distances: 5 km, 10 km, half marathon (21.0975 km), marathon (42.195 km).
Pace is the simple ratio of time to distance, and the three pace solvers all rearrange the same equation:
Where:
For race-time prediction, the calculator uses Pete Riegel's formula, an empirical model for how a trained runner's race time scales with race distance:
Where:
Pace from Time and Distance
A weekend runner finishes a 5-mile training run with a watch time of 40:00 flat and wants to know the per-mile pace.
the run was at an 8:00 min/mile pace, or about 4:58 min/km — a steady aerobic effort for most recreational runners.
This is the calculator's default example because it lands on a clean round-number pace and exercises both units side by side.
Time from Pace and Distance
A runner planning a half marathon wants to know the projected finish time at a goal pace of 8:00 per mile.
the half marathon takes about 1:44:53 at an 8:00 min/mile pace.
Riegel Race Prediction
A runner with a recent 20:00 5K wants a goal time for an upcoming marathon, assuming they have done the long-run training.
Riegel's formula predicts a 3:11:49 marathon from a 20:00 5K — in the same neighborhood as the published Galloway and Pfitzinger benchmarks.
The prediction assumes adequate long-run training; without the mileage in your legs, the actual race time will be significantly slower.