Half Marathon Pace Calculator
Plan a half marathon by its checkpoints, not by its average — 5 km splits, an even-split target and a negative-split alternative, from a goal time you actually intend to run.
Half Marathon Pace Calculator
Results recalculate instantly on every keystroke. Nothing you type is transmitted.
What this result does not account for
- Converts a goal you supply — it does not assess whether that goal matches your fitness.
- Assumes a flat, accurately measured course.
- GPS watches typically record 1–2 per cent long, so on-watch pace will read slower than plan.
- No adjustment for heat, wind or altitude.
In short: A 1:50:00 half marathon requires 5:13 per kilometre, or 8:23 per mile, passing 5 km at 26:04, 10 km at 52:08 and 15 km at 1:18:12. The half is the best single predictor of marathon performance, and unlike the marathon it is short enough that Riegel's formula remains well calibrated.
Formula
pace = goal time ÷ 21.0975 km
negative split: first half × 1.01, second half × 0.99
checkpoint(d) = pace1 × d for d ≤ 10.55 km
Split factors are applied so that the two halves always sum back to the goal time; a 2 per cent negative split is modelled as +1 per cent on the first half and −1 per cent on the second, not as a 2 per cent cut to the finish.
Worked Example
- Convert the goal time to seconds and divide by 21.0975 km for average pace.
- Apply the strategy factors to obtain first-half and second-half paces.
- Accumulate each 5 km checkpoint using the pace applying at that point on the course.
- Repeat at 3-mile intervals for runners racing in imperial units.
- Project to the marathon using both Riegel and the empirical 2.14 multiplier.
A 1:50:00 goal → 6,600 s ÷ 21.0975 km = 5:13 per kilometre, passing 5 km at 26:04, 10 km at 52:08, 15 km at 1:18:12 and 20 km at 1:44:17.
Strengths & Limits Of This Model
Where this engine is strong
- Checkpoints rather than a single average, which is what you can actually use mid-race
- Quantifies the cost of a fast start instead of merely warning against it
- Carries the empirical marathon multiplier rather than the optimistic one
Where it stops
- Fixed to the half marathon distance
- Cannot judge whether your goal time is achievable
Practical Use Cases
Writing a race-day pace band
Produces the 5 km checkpoints that go on your wrist.
Choosing a pacing strategy
Compares even, negative and positive splits on the same goal time.
Costing a fast opening 5 km
Quantifies what the remaining 16.1 km must average to rescue the goal.
Racing a course marked in miles
Prints mile checkpoints alongside kilometre ones.
Projecting the marathon
The full treatment is on the Marathon Pace Calculator.
Methodology & Editorial Standards
The engine implements the standard published formula for this calculation. Inputs are validated for domain and sign before evaluation, and any undefined case returns an em-dash rather than a spurious value. Checkpoint times are accumulated segment by segment using the pace applying at each point on the course rather than by scaling the average, so negative and positive split plans produce correct intermediate times. Split factors are constrained to sum back to the entered goal.
Computation runs in IEEE-754 double precision at full internal precision; rounding to two decimal places occurs strictly at the display layer, so no cumulative drift enters the result. All monetary outputs use accounting presentation — grouped thousands, two decimals, negatives in parentheses — so figures can be transcribed directly into a model or working paper. Division-by-zero and out-of-domain inputs return an em-dash rather than a misleading number.
This engine was reconciled against an independent reference implementation and hand-verified for the worked example above before release. Our full five-stage review process is published on the About Us page.
Disclaimer. This calculator is provided for informational and modelling purposes only and does not constitute financial, tax, legal, medical, or engineering advice. Verify all figures with a qualified professional before acting on them.
Half Marathon Pace Calculator — 20 Expert FAQs
20 analyst-written answers to the questions practitioners actually ask — optimised for voice and answer-engine retrieval.
What pace do I need for a 1:50 half marathon?
5:13 per kilometre, or 8:23 per mile. That puts you through 5 km at 26:04, 10 km at 52:08, 15 km at 1:18:12 and 20 km at 1:44:17. Writing those four checkpoints on your wrist is more useful in the race than knowing the average.
Should I run even splits or negative splits?
Negative splitting is associated with better outcomes and is how almost every distance world record has been run. Even splits are a sound default and simpler to execute. What you should avoid is the accidental positive split, which is what happens when the first 5 km feel easy and you take the bait.
How long is a half marathon exactly?
21.0975 kilometres, or 13.1094 miles — precisely half the marathon distance of 42.195 km. Most race material rounds it to 21.1 km or 13.1 miles, a difference of about 2.5 metres, which is irrelevant to pacing but worth knowing if you are comparing GPS distance to the official figure.
Do I need to take gels during a half marathon?
It depends on your finishing time. Below roughly 75 minutes most runners gain little from carbohydrate during the race. Beyond about 90 minutes it becomes worthwhile, at 30 to 60 g per hour. The half sits directly on that boundary, which is why you will find confident advice in both directions.
How much does a fast first 5 km actually cost?
More than it gains. Running the first 5 km ten seconds per kilometre too fast banks 50 seconds, but requires the remaining 16.1 km to be run about 3 seconds per kilometre slower than target to still hit the goal — and that is the optimistic case, assuming the early effort costs you nothing physiologically. It usually costs considerably more.
Is the half marathon a good predictor of marathon time?
It is the best single-race predictor available. Multiply by about 2.14 for a realistic estimate rather than the 2.0849 that Riegel's formula implies. Riegel is well calibrated up to the half and becomes optimistic beyond it.
Why does an even effort not produce even splits?
Because courses are not flat. Holding constant pace up a hill requires a surge in effort that you pay for later, so experienced runners hold effort roughly constant and accept that the pace fluctuates. On an undulating course, even splits and even effort are different strategies.
What is a good half marathon time?
It depends entirely on age, sex, training history and goals, and comparisons to strangers are rarely useful. A more meaningful benchmark is your own trend across several races on comparable courses. Sub-2:00 and sub-1:30 are the round-number milestones most recreational runners aim at.
How do I use a pace band?
Write the cumulative time for each checkpoint, not the split for each segment, because mental arithmetic degrades sharply when you are tired. Check at each marker, and correct gently — a 10-second deficit at 10 km should be recovered over the next several kilometres, not in one.
Should I adjust my pace for heat?
Yes, and before the start rather than at 15 km. Performance degrades measurably above roughly 10 to 12°C and the effect compounds through the race as core temperature rises. Adjusting the goal by a modest margin on a hot day is far cheaper than blowing up in the closing kilometres.
What is a positive split and why is it modelled here?
It means running the first half faster than the second. It is included so you can see its cost explicitly rather than discovering it during the race. Very few runners plan a positive split; most simply execute one by accident.
Why is my GPS distance longer than 21.0975 km?
Courses are measured along the shortest legal route, which nobody runs. Add weaving through crowds, wide turns and GPS drift of 1 to 2 per cent, and 21.3 to 21.5 km on the watch is normal. Your average pace calculated on that longer distance will look slower than the official figure.
How many weeks of training does a half marathon need?
For a runner with a modest base, typically 10 to 14 weeks, with the long run building gradually toward 16 to 18 km. The distance is forgiving enough that most reasonably active people can complete one, and demanding enough that racing it well takes structured preparation.
Can I use this calculator for a 10 mile race?
Not directly — it is fixed at 21.0975 km. Use the Pace Calculator for arbitrary distances, or the Split Time Calculator for checkpoints at any distance you specify.
Does the pace band change on a hilly course?
It should. Fixed checkpoint times assume constant pace, which is the wrong instruction on rolling terrain. On a hilly course, plan by effort and treat the checkpoints as a rough guide rather than a contract.
What happens if I miss an early checkpoint?
If you are behind by a few seconds, do nothing — it will come back if the effort is right. If you are more than about 30 seconds down at 10 km, the honest move is usually to adjust the goal rather than to chase, because chasing at halfway rarely ends well.
Is this half marathon pace calculator free to use?
Yes. It is free, requires no account, and has no usage limits. ApexConverter is funded by contextual advertising, never by selling user data.
Is my data sent to a server?
No. The engine runs as Vanilla JavaScript inside your browser under our Zero-Server Client-Side Execution model. Your figures are computed locally and are never transmitted, logged, or stored.
How accurate is this calculator?
It applies the standard closed-form formula in IEEE-754 double precision, rounding only at the display layer. The engine is reconciled against an independent reference implementation before release.
Does it work on mobile?
Yes. The interface is mobile-first with numeric keypad hints and is tested down to a 320-pixel viewport with no horizontal scrolling.