Half Marathon Pace Explained: The Maths Behind 13.1 Miles, Even-Effort Splits, and How to Pace Sub-2:00

What is half marathon pace?

Half marathon pace is the average speed you need to hold to cover 21.0975 km — 13.1 miles — in a given target time. It is the number a training block is built around: the pace you rehearse in tempo runs, the figure on the pacing group’s bib, and the line your watch beeps against on race morning. The maths behind it is a single division. Everything hard about the half marathon lives in whether you can actually hold that number for 13.1 miles without slowing down.

The distance is not arbitrary. A half marathon is exactly half the marathon distance of 42.195 km fixed by World Athletics in Technical Rule TR4.3 (Distances of Road Races), so it is 21.0975 km on every certified course in the world. Using the NIST exact factor of 1 international mile = 1.609344 km (NIST SP 811:2008, Appendix B.8), that works out to 13.10938 miles — the familiar “13.1”. The half marathon pace calculator does the division and prints the split times you should hit at 5K, 10K, halfway, 15K, 10 miles, 20K and the finish.

The half sits in a useful spot. It is long enough that pacing discipline matters — go out too hard and the last 5K punishes you — but short enough that most trained runners can hold close to an even effort the whole way, which is rarely true of the full marathon. That makes it the distance where a clean pace plan pays off most reliably.

How half marathon pace is calculated

Required pace per kilometre is your target finish time in seconds divided by 21.0975. Required pace per mile is the same target time divided by 13.10938. Average speed in km/h is 21.0975 divided by the target time expressed in hours; multiply by 0.6213712 for mph. That is the entire formula — there is no hidden physiology in the arithmetic, only in your legs.

Worked out for the three benchmark targets most runners chase:

• Sub-2:00 half (2:00:00). 7,200 s ÷ 21.0975 = 341.27 s/km, which rounds to 5:41 per km. Per mile: 7,200 ÷ 13.10938 = 549.22 s/mile = 9:09 per mile. Average speed 10.55 km/h, 6.55 mph.
• Sub-1:45 half (1:45:00). 6,300 s ÷ 21.0975 = 298.61 s/km = 4:59 per km or 8:01 per mile. Average speed 12.06 km/h, 7.49 mph.
• Sub-1:30 half (1:30:00). 5,400 s ÷ 21.0975 = 255.96 s/km = 4:16 per km or 6:52 per mile. Average speed 14.07 km/h, 8.74 mph.

A neat consequence of the distance being exactly half the marathon: for any given per-km pace, a marathon takes exactly twice the time of a half. A 1:45:00 half and a 3:30:00 marathon are both run at 4:59 per km. That symmetry is why the half marathon pace calculator and the marathon pace calculator share the same underlying arithmetic — only the distance constant changes.

Worked example: pacing a sub-2:00 half

Target 2:00:00, the single most popular half marathon goal. The required pace is 5:41 per km, or 9:09 per mile. The even-effort splits — the clock time you should read at each marker if you run a perfectly constant pace — look like this:

• 5K — 28:26
• 10K — 56:53
• Halfway (10.54875 km) — 1:00:00
• 15K — 1:25:19
• 10 miles (16.09344 km) — 1:31:32
• 20K — 1:53:45
• Finish (21.0975 km) — 2:00:00

Two adjustments experienced runners make on top of the raw even-effort line. First, carry a small cushion: aim for a projected 1:58:45 (about 5:38 per km) so a slight fade over the final 5K still lands you under two hours. Second, run the first 5K a touch slower than goal pace, not faster. The 28:26 split is a ceiling, not a target — 28:40 to 28:50 through the opening 5K is closer to optimal, because the energetic cost of going out too hard grows nonlinearly and is almost never recovered. Feed those numbers into the half marathon pace calculator and it will print the whole table for your own target so you can load it onto your watch.

Even, even-effort and negative splits

Three phrases get used interchangeably and they are not the same thing.

Even splits

Each half of the race takes the same clock time — the 1:00:00 / 1:00:00 half marathon. Clean on paper, harder in practice, because heat builds up and glycogen depletes over the back half, so matching your first-half clock time usually requires a little more effort the second time round.

Even-effort splits

What the calculator prints. Every kilometre takes the same number of seconds. This is what the division gives you, and it is the reference line to check yourself against — not a promise of what your body will do.

Negative splits

Second half faster than the first. This is the pattern associated with the best-judged races: start conservatively, conserve fuel and muscle, then spend what is left over the final few kilometres. The catch is that large data sets show most recreational runners do the opposite. A big analysis of split-time data across marathon and half-marathon fields (Ngétich et al., pacing across performance levels) found positive pacing — each segment slower than the last — in every age group and both sexes, though the half marathon held a more even pace than the marathon and, unlike the marathon, showed no end spurt. The takeaway is not “always negative-split” so much as “stop accidentally positive-splitting by 5–8 % because you started too fast”.

Factors that change realistic pace

Heat and humidity

Warm, humid conditions slow endurance running measurably — distance-running performance degrades once wet-bulb globe temperature climbs above roughly 10–15 °C, and the effect compounds over 13.1 miles. A 1:45 attempt in cool 10 °C air is realistically a minute or two slower in 22 °C sun. Adjust the target downwards before the start rather than fighting goal pace into a wall of heat at 15K.

Course and elevation

A net-uphill or lumpy course costs time roughly in proportion to the total climb — a rough guide is 15–30 seconds per 100 m of ascent at half marathon effort. Flat, point-to-point courses like Valencia or the Great North Run’s fast stretches are where personal bests happen; trail and hill halves are their own event. Check the elevation profile before you commit to a goal time built on flat-road training paces.

Fuel and hydration

A half marathon sits right at the edge of what most runners can complete on stored glycogen alone. Under about 90 minutes you can often get away with water and a good breakfast; beyond that, a gel or two from around 45 minutes in helps hold pace through the closing 5K. Dehydration of even 2 % of body mass measurably slows endurance pace, so a fluid plan matters on warm days. The calorie calculator and TDEE calculator cover daily training energy, which is a separate question from race-day fuelling.

Training relative to goal

The least glamorous factor and the one that overrides the rest. Half marathon pace is a derived number; whether you can hold it is decided by the previous 10–12 weeks of mileage, tempo work and long runs. A goal pace your longest tempo runs cannot already touch, with a little in reserve, is a goal pace that becomes a positive split on the day. Pick the target your training supports, then use the race time predictor to sanity-check it against a recent race result.

How to use the splits

• Load the split table onto your watch, don’t do mental maths on the road. Dividing pace by distance at 12K with a high heart rate is unreliable. Pre-programme the even-effort splits from the half marathon pace calculator and let the watch alert you at each marker.
• Treat the 5K split as a ceiling. Running the first 5K 5–10 seconds per km slower than even-effort pace is the single best predictor of holding pace to the finish. Time banked early is almost always paid back with interest.
• Re-assess at halfway. If you are on the even-effort half-split and still feel within yourself, the second half is yours to race. If you are 30 seconds up and breathing hard, ease off — the negative split is gone and it is now about damage limitation.
• Race the last 5K. A well-paced half marathon really starts at about 16K. If you saved fuel and legs, this is where you make time; if you spent them early, this is where the clock slips regardless of what the splits say.
• Pick one pace unit and commit. Half-converting per-km splits into per-mile in your head wastes attention. Choose km or mile before race day and set watch and brain to match.

Common mistakes

Treating the even-effort splits as a race plan. They are the reference line, not the strategy. Most runners are best served by a slightly slower first 5K and a push over the final 5K — the splits tell you where the line is, not which side of it to run.

Confusing per-km and per-mile splits. A 5:00 per km pace is 8:03 per mile; a 5:00 per mile pace is 3:06 per km — utterly different targets. Always double-check which unit your GPS watch is showing. The speed converter and running pace calculator both handle the conversion if you want to check.

Choosing a goal the training does not support. A sub-1:45 attempt off sub-1:50 fitness usually becomes a 1:52. The honest goal time is the one your best recent long tempo run already hints at, with energy to spare.

Ignoring the forecast. Going for goal pace in 24 °C is going for goal pace in a different race. Trim the target on warm, humid mornings before the gun instead of blowing up at 15K.

When to seek a coach

The arithmetic of half marathon pace is universal — every runner divides target time by 21.0975 km the same way. The strategy is not. A coach earns their fee when you have plateaued across two or more training cycles, when you are chasing a time-qualifying standard and sitting within a couple of percent of it, when you are rebuilding from injury and need pace triangulated against load, or when you are an older runner whose body responds differently to identical training. The half marathon pace calculator gives you the numbers for free; a coach helps you decide which of them to actually aim at.

Frequently asked questions

What pace do I need for a sub-2 hour half marathon?

5 minutes 41 seconds per kilometre, or 9 minutes 9 seconds per mile, held for the full 13.1 miles. Sub-2:00 is the most popular half marathon target and roughly the median for trained recreational runners. To carry a cushion, aim for about 5:38 per km (a 1:58:45 projection) so a small fade in the last 5K still keeps you under two hours.

What is the pace for a sub-1:45 or sub-1:30 half?

Sub-1:45:00 needs 4:59 per km (8:01 per mile); sub-1:30:00 needs 4:16 per km (6:52 per mile). Sub-1:45 is a common strong-club-runner benchmark and sub-1:30 — the “sub-90” line — is a target many experienced amateurs chase for years.

What is a good half marathon time?

It depends on age, sex and training, but useful reference points for recreational runners are: simply finishing for a first-timer, sub-2:00 for a committed amateur, sub-1:45 for a strong club runner and sub-1:30 for an experienced competitive amateur. Average finish times sit around 2:00 for men and 2:10 for women. For scale, the world records are Jacob Kiplimo’s 56:42 (Barcelona, 2025, the first half under 57 minutes) and Letesenbet Gidey’s 1:02:52 (Valencia, 2021) — roughly 2:41 and 2:59 per km.

How does half marathon pace relate to marathon pace?

For the same per-km pace, a marathon takes exactly twice the time of a half, because it is exactly twice the distance — so a 1:45 half and a 3:30 marathon share 4:59 per km. In practice nobody holds half marathon pace for a full marathon; a common rule of thumb is that realistic marathon pace is about 8–10 % slower per km than half marathon pace.

How do I convert pace per kilometre to pace per mile?

Multiply by 1.609344, the NIST exact conversion factor. A 5:00 per km pace is 5 × 1.609344 = 8.047 minutes per mile, or 8:03. Going the other way, divide by 1.609344. A quick mental rule: pace per mile is about 60 % larger than pace per km, because a mile is about 60 % longer than a kilometre.

Should I run even or negative splits?

Aim to avoid a positive split first. Negative splits are associated with better races, but a well-judged even effort works well for most runners over 13.1 miles. The common error is a fast first 5K driven by adrenaline and a crowded start — run the opening 5K a few seconds per km slower than goal pace, settle in through 15K, then push the final 6K if the legs are there.

Does the calculator account for hills, heat or wind?

No. It returns the mathematical pace required to average 21.0975 km in your target time, assuming even effort. Adjust the target downwards for hot, hilly or windy days before entering it.

Related calculators

• Half Marathon Pace Calculator — the parent tool: target time in, required pace and even-effort splits out.
• Marathon Pace Calculator — the same maths for the full 42.195 km, with 5K splits.
• Running Pace Calculator — pace, speed and finish-time projections for any distance and time pair.
• Marathon Time Predictor — project a longer-race time from a recent shorter result to sanity-check your goal.
• Speed Converter — convert between km/h, mph, m/s and knots.
• Calorie Calculator — daily calorie targets for a training block: cut, maintain or bulk.