Pace Calculator

Pace, time, and distance are related by a single formula: time = pace × distance. Given any two, the third follows — pace = time ÷ distance, or distance = time ÷ pace. This calculator solves for whichever one you're missing.

The splits table shows your cumulative time at key checkpoints (every km or mile for short runs, every 5 or 10 for longer distances) assuming an even pace throughout — useful for planning intermediate checkpoints during a race or workout.

Worked examples:

• 5K in 25:00 → pace 5:00/km
• 10:00/mi pace for a half marathon (13.11 mi) → time 2:11:06
• 4:00:00 marathon (26.22 mi) → pace 9:09/mi, or 5:41/km
• 3:30:00 marathon → 4:59/km · 4:30:00 marathon → 6:24/km

The splits table is a plan, not a prediction

Every row assumes you hold one pace from the gun to the line. Almost nobody does, and the table is more useful once you know how real races deviate from it.

Running the second half slower costs more than it feels like it should. Take that 4:00:00 marathon at 5:41/km. Run the first half exactly on plan and the second half just 10% slower — which is what fading feels like from the inside, not blowing up — and you finish in 4:12:00. Twelve minutes, from a slowdown you would describe afterwards as "I held on reasonably well".

So the standard advice is to plan a negative split: start marginally slower than target and finish marginally faster. Going out 1% slow and coming back 1% fast still totals 4:00:00 — 2:01:12 then 1:58:48 — but it is a far more reliable way to arrive there, because the energy you spend being early is spent at the point where you have the least to gain and the most to lose. Runners who bank time almost never keep it.

Your watch will read long, and the splits table won't

This one catches people out every single race. Certified courses are measured along the shortest legal route — the racing line through every bend, tangent to tangent. You will not run that line: you drift wide at corners, weave around slower runners, cross to the water tables and back. GPS error adds a little more on top. The result is that a watch typically reads about 1–2% long over a marathon.

If you hold 5:41/km on the watch and the watch ends up reading 42.62 km (1% long), you finish in 4:02:24. At 1.5% long it is 4:03:36, and at 2% it is 4:04:48. You will have run every kilometre exactly on plan and still missed your target by up to five minutes.

Two things follow. Pace off the course markers, not the watch's distance — take your split at each official kilometre or mile sign and compare it to the table above. And if you are chasing a specific time, plan a few seconds per kilometre faster than the arithmetic says, to buy back the distance you are actually going to cover. Run the tangents where the course allows it; that is where the difference is smallest.

Predicting one distance from another

Pace does not carry across distances — it degrades as the distance grows, and by a fairly predictable amount. Riegel's formula is the usual estimate: t₂ = t₁ × (d₂ ÷ d₁)^1.06. From a 25:00 5K it predicts about 52:07 for 10K, 1:55:00 for a half, and 3:59:47 for a marathon. From a 30:00 5K: 1:02:33, 2:18:00 and 4:47:44.

The useful part is the pace gap it implies. That 25:00 5K is 5:00/km, but the predicted marathon pace is 5:41/km — 41 seconds per kilometre slower. Runners who pick a marathon target by rounding down from their 5K pace are choosing a time they were never able to run, and the splits table will confirm their plan without complaint.

Treat the prediction as a ceiling that assumes appropriate training for that distance. It is reliable between 5K and the half; for the marathon it over-predicts anyone who has not done the long runs, because the last 10 km is limited by fuel and durability rather than by the speed the formula is extrapolating. Predicting from a half — 1:50:00 gives 3:49:21, a 5:13/km half turning into 5:26/km — is more trustworthy than predicting from a 5K, because the two distances share more of the same limits.

The conditions the arithmetic ignores

Heat is the big one, and it is not linear. Above roughly 12–15°C most runners start losing time, and the loss accelerates: a warm, humid day can cost a marathon runner several minutes even with sensible pacing. Humidity matters as much as temperature, because it is what stops sweat evaporating. On a hot day the correct move is to adjust the target before the start rather than to fight the plan for 30 km and pay for it later.

Hills do not net out. Climbing costs more than the matching descent gives back — and a steep descent has its own cost, in quad damage you will feel at 30 km. Pace by effort on rolling terrain and let the splits drift; use the table for the overall target rather than for individual segments.

Instantaneous GPS pace is noise. The "current pace" field jumps several seconds per kilometre from stride to stride, and chasing it produces exactly the surging that wastes energy. Use a lap or average pace field, or run by the markers.

The wall is fuel, not weakness. Around 30–32 km — 2:50:38 into that 4:00:00 plan — a runner who has not taken carbohydrate on board runs out of stored glycogen and the pace collapses regardless of fitness or resolve. It is an inputs problem with an inputs solution: start fuelling early and on a schedule, well before you feel you need it.