software-architecture.aiWork out running pace: which pace for which finish time, with splits, prediction and training zones
Running Pace Calculator
Drag the pace band and see live what your pace means for finish time, splits and training.
Pace
Time
calculatedLap (400 m)
2:12
km/h
10.9
At this pace you'd run
5 km 27:30
10 km 55:00
Marathon 3:52:04
All input stays in your browser, no data is transmitted. All results are carefully calculated guide values without guarantee: no coaching or medical advice, race pacing and training remain your own responsibility.
5:30 min/km, 1:56:02 for 21.1 km.
Splits & race tactics
Even pacing is good; a negative split (faster second half) is often better. Drag the slider and watch your kilometre times redistribute, the finish time stays exactly the same.
1st half
58:19
Ø 5:32 min/km
2nd half
57:44
Ø 5:28 min/km
| km | Split | min/km | Total |
|---|---|---|---|
| 5 | 27:43 | 5:33 | 27:43 |
| 10 | 27:35 | 5:31 | 55:18 |
| 15 | 27:27 | 5:29 | 1:22:44 |
| 20 | 27:19 | 5:28 | 1:50:03 |
| 21.1 | 5:59 | 5:27 | 1:56:02 |
By the way: this tool was built in one afternoon. What that can mean for ideas of your own is explained further down.
Race predictor
Your time, projected onto other distances. Not at the same pace, but with the Riegel formula: for every doubling of distance, runners typically slow a little (exponent 1.06). The comparison shows what difference that makes.
5 km
25:13
5:03 min/km
10 km
52:35
5:16 min/km
Marathon
4:01:56
5:44 min/km
Honestly: the formula assumes you're as well trained for the target distance as for the source distance. A 5 km race says little about a marathon without long runs.
Training zones
From your time you can estimate your threshold pace, the pace you could hold for about an hour in a race. Five training zones follow from that as guide values.
Estimated threshold pace 5:18 min/km
Easy (base runs)
Relaxed base, you can still hold a conversation
6:42 – 8:02 min/km
Steady
Brisk but controlled
5:53 – 6:37 min/km
Threshold
About the pace you could hold for an hour
5:12 – 5:31 min/km
Intervals
Hard, short efforts with recovery
4:44 – 5:03 min/km
Repetitions
Very short, very fast
4:14 – 4:41 min/km
Guide values, not coaching advice. Daily form, terrain and weather shift everything, and if you have health concerns, check your training with a doctor.
Half marathon pace: the sub-2-hours example
The 2-hour mark is many runners' first big half marathon goal. Behind it sits a pace of 5:41 min/km over 21.0975 km, about 2:16 per 400 m lap on a track. The calculator shows the kilometre splits and, via the negative-split slider, what a controlled start with a faster second half looks like.
Marathon pace: staying under 4 hours
A marathon under 4 hours takes the same 5:41 min/km as a half under 2 hours, just over the full 42.195 km. That is exactly why the splits matter: paced evenly, the mark means about 28:26 for every 5 km checkpoint. Go out too fast and you pay double later.
10K under 50 minutes, 5K under 25
Both classics sit on the same pace: 5:00 min/km, i.e. 12 km/h. The difference shows in the race prediction: run 5K in 25:00 and the Riegel formula predicts not 50:00 for 10K but roughly 52:07, because pace typically drops a little with distance. That exact difference between linear and realistic is a toggle in the calculator.
Negative split: the second half wins the race
A negative split means: faster at the back than at the front. The slider models this as a steady build from start to finish pace and redistributes your target time exactly, so you see immediately how much calmer the first kilometres may be without risking the goal. A plan you can note down as splits before the race.
Training zones: what your race time means for training
From a race time you can estimate your threshold pace, the pace you could hold for about an hour. From that the calculator derives five zones, from easy runs to repetitions, each as a pace range. The derivation is laid open instead of a black box, and the zones are guide values, not coaching advice.
All values are guide values without guarantee and replace neither training planning nor medical advice.
Frequently asked questions
- What does pace mean in running?
- Pace is the time per kilometre, given in minutes and seconds, for example 5:30 min/km. It is the inverse of speed: the smaller the number, the faster you run. 6:00 min/km equals exactly 10 km/h.
- How do I work out my pace?
- Divide time by distance: 50 minutes for 10 km gives 5:00 min/km. The calculator handles the seconds arithmetic and works live in all three directions, any two values give you the third.
- What pace do I need for a half marathon under 2 hours?
- Faster than 5:41 min/km over 21.0975 km. A small curiosity: a marathon under 4 hours needs exactly the same pace, just held twice as long.
- How do I convert min/km to km/h?
- Divide 60 by the pace in minutes: 5:00 min/km is 60 ÷ 5 = 12 km/h. For odd paces the calculator does it for you, the min/km, min/mi and km/h switch sits right above the pace band.
- What is a negative split?
- Running the second half of a race faster than the first. Many personal bests are run this way, because starting too fast gets expensive over the final kilometres. The slider in the calculator redistributes your target time accordingly, the total stays exactly the same.
- How reliable is the race prediction?
- It uses the Riegel formula with the common fatigue exponent of 1.06, a well-established guide value, not a guarantee. It assumes comparable training for the target distance: a strong 5K time does not replace long runs for a marathon.
Built at the workbench
This was the small part.
This calculator came together in one afternoon: calculation logic with over 30 tests, the pace band, two languages. That is exactly what this collection is about: good small software is no longer a cost question. If your business is missing a tool that should feel like this, have a look at how I work.