Running Pace Calculator - Pace, Speed & Race Predictions

Distance and time in, pace and race predictions out

You ran 10 km in 52 minutes. That is a pace of 5:12 min/km and a speed of 11.5 km/h. But the more interesting number is what comes next: the Riegel formula predicts your half marathon at 1:55:08 and your marathon at 4:01:37. One run feeds four predictions. The formula works because human endurance follows a power-law curve - as distance doubles, pace slows by a predictable percentage (exponent 1.06). Not perfect for everyone, but remarkably reliable across recreational and competitive runners.

Parkrun regular
Ran 5 km in 24:15. Pace: 4:51 min/km | Speed: 12.4 km/h
Predicted 10K: 50:33 | Half marathon: 1:51:59 | Marathon: 3:53:41

First-time half marathoner
Ran 10 km in 1:05:00. Pace: 6:30 min/km | Speed: 9.2 km/h
Predicted half marathon: 2:23:59 | Marathon: 5:01:14

Sub-3 marathon chaser
Ran 21.1 km in 1:25:00. Pace: 4:02 min/km | Speed: 14.9 km/h
Predicted marathon: 2:58:45 - sub-3 is within reach

Casual jogger
Ran 3 km in 22:30. Pace: 7:30 min/km | Speed: 8.0 km/h
Predicted 5K: 38:47 | 10K: 1:20:49

Speed demon at a track meet
Ran 1.5 km in 5:00. Pace: 3:20 min/km | Speed: 18.0 km/h
Predicted 5K: 18:00 | 10K: 37:33 | Half: 1:23:08

How to use this calculator - step by step

Distance - enter the distance you ran in kilometers. Common race distances: 5K, 10K, half marathon (21.1 km), marathon (42.2 km). You can also enter any training run distance (e.g. 7.5 km). For miles, multiply by 1.609 first.
Time (minutes) - full minutes of your run. If your GPS watch shows 48:22, enter 48 here.
Time (seconds) - the remaining seconds (0-59). For 48:22, enter 22. Leave at 0 if you only know full minutes.
Read your results - you get your pace (min/km), speed (km/h), predicted race times for four standard distances using the Riegel formula, and a pace classification from elite to beginner.

Pace classification

Pace (min/km)	Speed (km/h)	Level	Typical context
Under 4:00	15.0+	Elite / professional	Sub-2:49 marathon, national-level
4:00 - 5:00	12.0-15.0	Advanced	Competitive age-group racer
5:00 - 6:00	10.0-12.0	Intermediate	Amateur marathon, regular 10K
6:00 - 7:00	8.6-10.0	Recreational	Beginner marathon, long runs
7:00 - 8:00	7.5-8.6	Easy jog	Zone 2 base building, LSD
Over 8:00	Under 7.5	Walk/jog	Warm-up, recovery, beginners

How the Riegel formula works

Peter Riegel published his race-time prediction formula in 1981. The idea is straightforward: T2 = T1 x (D2 / D1)^1.06, where T1 is your known time for distance D1, and T2 is the predicted time for distance D2. The exponent 1.06 captures the fact that you slow down as distance increases - not linearly, but as a power law. Doubling the distance adds roughly 4-6% to your per-km pace.

The formula is most accurate when predicting from a distance close to your target. A 10K time predicts a half marathon better than a 1 km sprint predicts a marathon. It also assumes similar training for both distances - a pure sprinter's 1500 m time will overpredict their marathon potential.

Sample race time predictions from a 5K result

5K time	Pace	10K	Half marathon	Marathon
20:00	4:00	41:40	1:32:13	3:12:59
25:00	5:00	52:05	1:55:17	4:01:14
30:00	6:00	1:02:30	2:18:20	4:49:29
35:00	7:00	1:12:55	2:41:24	5:37:44
40:00	8:00	1:23:20	3:04:27	6:25:59

Predictions use the Riegel formula (exponent 1.06). Actual race times depend on training, nutrition, weather and course profile.

FAQ

How accurate is the Riegel formula?

For runners who train consistently at multiple distances, predictions are typically within 2-5% of actual race times. Accuracy drops when predicting across very different distances (1500 m to marathon) or for undertrained runners (someone who only runs 5K predicting a marathon without long-run training). The formula assumes proportional training - if you have never run more than 10 km, the marathon prediction is aspirational, not guaranteed.

Should I use my race time or training time?

Use your best recent race time (within the last 8-12 weeks) for the most accurate predictions. Training runs are typically 10-20% slower than race effort due to lower motivation, no taper and no adrenaline. If you only have training data, subtract about 5-10% from your time mentally, or enter a recent hard tempo run / time trial as input.

What pace should I run my long training runs at?

Most coaches recommend long runs at 60-90 seconds slower than your marathon race pace (Zone 2, conversational effort). If your predicted marathon pace is 5:30 min/km, your long runs should be around 6:30-7:00 min/km. Running long runs too fast is the most common training mistake - it causes fatigue without building aerobic base proportionally. Use the Training Heart Rate Calculator to stay in Zone 2.

How does elevation affect pace?

A rough rule of thumb: add 12-15 seconds per km for every 100 m of cumulative elevation gain per km. A hilly 10K course with 200 m total climb will feel about 25-30 seconds/km harder than a flat course. Altitude also matters: above 1500 m, VO2max drops by about 3-5% per 1000 m. If you trained at sea level and race in Denver (1600 m), expect 5-8% slower times until acclimatized.

What is negative splitting and should I do it?

Negative splitting means running the second half of a race faster than the first. Most marathon world records were set with negative or even splits. The physiological reason: starting conservatively prevents early lactate accumulation and glycogen depletion. For a marathon, aim for the first half 30-60 seconds slower than target pace, then gradually accelerate. For shorter races (5K-10K), even splits work well since glycogen is not a limiting factor.

Can I convert min/km to min/mile?

Multiply your min/km pace by 1.609. A pace of 5:00 min/km equals approximately 8:03 min/mile. A 6:00 min/km is about 9:39 min/mile. Quick reference: 4:00 min/km = 6:26 min/mile, 5:00 = 8:03, 6:00 = 9:39, 7:00 = 11:16, 8:00 = 12:52.

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