How to Calculate Running Pace: Min/Km, Min/Mile and Race Times

Pace Is Speed Turned Upside Down

Cars measure speed: distance divided by time, giving kilometres per hour. Runners measure pace: time divided by distance, giving minutes per kilometre. They describe the same run, but they are mathematical opposites — and that inversion catches people out constantly.

The practical consequence is that a bigger pace number means you are going slower. Dropping from 6:00/km to 5:00/km is a full minute faster per kilometre, even though the number went down. Runners talk in pace because it answers the question that actually matters mid-race: how long will the next kilometre take?

The Pace Formula

Pace = Total Time ÷ Distance

Work in seconds, then convert back at the end — that one habit avoids most pace errors.

Convert your time to seconds. 25 minutes = 25 × 60 = 1,500 seconds.
Divide by the distance. 1,500 ÷ 5 km = 300 seconds per km.
Convert back to minutes and seconds. 300 ÷ 60 = 5:00 min/km.

That same 5 km run works out at 8:03 min/mile, because 5 km is 3.107 miles and 1,500 ÷ 3.107 = 483 seconds.

The Decimal Trap

This is the single most common pace mistake, and it costs people their goal times.

If a calculator gives you a pace of 5.5 minutes per kilometre, that is 5:30, not 5:50. The number after the decimal point is a fraction of a minute, not a count of seconds. Half a minute is 30 seconds.

To convert a decimal pace into minutes and seconds, multiply the part after the point by 60:

5.25 min/km → 0.25 × 60 = 15 → 5:15
5.5 min/km → 0.5 × 60 = 30 → 5:30
5.75 min/km → 0.75 × 60 = 45 → 5:45
6.8 min/km → 0.8 × 60 = 48 → 6:48

Converting Between Min/Km and Min/Mile

Because a mile is longer than a kilometre, it takes more time to cover — so your min/mile figure is always the bigger number.

Min/km to min/mile: multiply by 1.609
Min/mile to min/km: divide by 1.609

Convert to seconds first, or the decimal trap will bite. A 5:00/km pace is 300 seconds; 300 × 1.609 = 483 seconds; 483 ÷ 60 = 8.05, which is 8:03 min/mile.

The Pace Triangle: Any Two Give You the Third

Pace, time, and distance are locked together. Know two and the third follows:

Pace = Time ÷ Distance — how fast was that run?
Time = Pace × Distance — what will this race take at my planned pace?
Distance = Time ÷ Pace — how far can I get in 40 minutes?

Example: You plan to run a half marathon at 5:30/km. Time = 330 seconds × 21.0975 km = 6,962 seconds = 1 hour 56 minutes.

Goal Race Paces

Most runners work backwards from a target time. Here is the pace each popular goal actually requires:

Sub-25 minute 5K: 5:00 min/km (8:03 min/mile)
Sub-50 minute 10K: 5:00 min/km (8:03 min/mile)
Sub-2 hour half marathon: 5:41 min/km (9:09 min/mile)
Sub-90 minute half marathon: 4:16 min/km (6:52 min/mile)
Sub-4 hour marathon: 5:41 min/km (9:09 min/mile)
Sub-3:30 marathon: 4:59 min/km (8:01 min/mile)

Two of those pairings are worth committing to memory. A sub-2 hour half and a sub-4 hour marathon need exactly the same pace — the marathon simply demands you hold it twice as long. Likewise, sub-25 for 5K and sub-50 for 10K are both 5:00/km.

Predicting a Race Time From a Shorter One

The best-known method is Riegel's formula, published by Pete Riegel in 1977:

T2 = T1 × (D2 ÷ D1)^1.06

The exponent above 1 reflects the fact that you cannot hold your 5K pace for a marathon. Starting from a 25-minute 5K, it predicts:

10K: 52:07
Half marathon: 1:55:00
Marathon: 3:59:47

One caveat matters enormously: the formula assumes you have trained for the longer distance. A runner with a sharp 5K and no long runs behind them will not deliver that marathon time, because the wall is a fuelling and endurance problem rather than a speed one. Treat it as a ceiling on a well-trained day, not a promise.

Why Your Watch Says the Race Was Too Long

Cross the line at a marathon and your watch will usually read closer to 42.5 km than the official 42.195. You have not been cheated, and the course is not mismeasured. Two things cause it:

Courses are measured along the shortest legal route — the tangents, cutting each corner tightly. Nobody runs that line in a crowded field, and every metre of weaving adds up.
GPS drift. Buildings, trees, and tunnels scatter the signal, and your watch fills in the gaps by estimating.

The practical fix is to race on average pace rather than the instantaneous reading, which lurches around by fifteen seconds or more. Better still, use split times at the mile or kilometre markers, which are on the official course and cannot drift.

Negative Splits Beat Heroic Starts

The most reliable way to hit a target is to run the second half slightly faster than the first — a negative split. Adrenaline makes the opening kilometres feel effortless, which is exactly why so many runners bank time early and pay it back with interest over the closing stretch.

For a sub-4 hour marathon at 5:41/km, a sensible plan is around 5:45/km through halfway and 5:37/km after it. The average lands where it needs to, and the difficult miles come when you have something in reserve.

Conclusion

Pace is time divided by distance, decimal minutes are not seconds, and any two of pace, time, and distance give you the third. That is genuinely all the arithmetic that racing requires. Put your distance and target time into a Pace Calculator to get min/km and min/mile side by side, then build your splits from there — and go out slightly slower than feels right.

Try our Pace/Speed Calculator

Related Calculators: Step-to-Distance Calculator