PACER
01 · PACE / TIME / DISTANCE — EDIT ANY TWO

RUN THE
NUMBERS.

DISTANCE PRESET
DISTANCE
km
= 6.21 mi
TIME
h:m:s
0.83 hours
PACE· OUTPUT
5:00
/km
8:03 /mi · 12.0 km/h
Computed · TIME ÷ DISTANCE
SPEED12.00 km/h
EQUIV8:03 /mi
1KM SPLIT5:00
02PACE ↔ SPEED
PACE
/km
SPEED
km/h
03SPLITS — 10 KM AT 5:00/km
SPLIT BY
#SPLITPACECUM
015:005:00 /km5:00
025:005:00 /km10:00
035:005:00 /km15:00
045:005:00 /km20:00
055:005:00 /km25:00
065:005:00 /km30:00
075:005:00 /km35:00
085:005:00 /km40:00
095:005:00 /km45:00
105:005:00 /km50:00
04RACE TIMES — FROM A KNOWN RESULT

Enter a recent race you ran. We project to other distances.

SEED DISTANCE
km
SEED TIME
h:m:s
SEED DISTANCE
FATIGUEk = 1.06–1.12
DISTANCEPREDICTEDPACE
1 mi5:573:42 /km
5KSEED20:004:00 /km
10K41:594:12 /km
Half1:34:424:29 /km
Marathon3:28:544:57 /km
50K4:23:395:16 /km
PACER · MADE FOR RUNNERS · NO TRACKING · OPEN-SOURCE MATHv1.0 — RIEGEL · OPENFREEMAP · MINETTI GRADE COST

How to use the half marathon pace calculator

Type your goal half-marathon time and the calculator returns the exact per-km or per-mile pace you need to hit. Pin DISTANCE instead and the half is fixed at 21.0975 km — edit time or pace and watch the other update live.

Most half-marathon PRs come from a negative split: ease into the first 8–10 km, push the back half. Pick NEG in the Splits section to see the per-km plan. The drift slider tunes how aggressive the split is.

Benchmarks: a sub-2:00 half needs 5:41 /km (9:09 /mi). 1:45 needs 4:58 /km. 1:30 needs 4:16 /km. Sub-1:20 needs 3:47 /km.

Half marathon pace chart

FINISH TIME PACE /KM PACE /MI
1:15 3:33 5:43
1:20 3:47 6:06
1:30 4:16 6:52
1:40 4:44 7:38
1:45 4:58 8:00
1:50 5:13 8:24
2:00 5:41 9:09
2:15 6:24 10:18
2:30 7:06 11:26
Half marathon FAQ
What's a negative split and why does it matter?
A negative split means running the second half of a race faster than the first half. Research consistently shows it produces faster overall times than positive or even splits, because you stay aerobic longer and have energy when others fade. Most world records — including the marathon — were set with negative splits. Pick "NEG" in the Splits section to plan one.
How do I convert pace to speed?
Pace and speed are reciprocals. Speed (km/h) = 60 ÷ pace (min/km). So 5:00 min/km = 12 km/h. Going the other way, pace (min/km) = 60 ÷ speed (km/h). The PACE ↔ SPEED converter on this page does the math for both units (km/h and mph) live as you type.
How do I convert min/km to min/mile?
A mile is 1.609344 km, so min/mile = min/km × 1.609344. 5:00 /km = 8:03 /mi. 4:00 /km = 6:26 /mi. 6:00 /km = 9:39 /mi. Toggle KM ↔ MI at the top of the calculator to see your pace in both units instantly.
How accurate is the race time predictor?
It uses the Riegel formula T₂ = T₁ × (D₂ / D₁)^k with the Vickers-Vertosick (2016) distance-dependent exponent — k=1.06 short, climbing through 1.08 (half), 1.10 (marathon), 1.12 (ultras). That fixes Pete Riegel's original fixed-k=1.06 which underestimates marathon fade. Accurate to within ~2% for trained runners on flat courses; less accurate for first-time marathoners or hilly courses (in which case nudge k toward 1.10–1.15 manually). The connected coach goes further — see /coach.
What does PIN do?
PIN tells the calculator which field is the output. The other two are inputs. The pinned field updates live as you edit the others. By default PACE is pinned — so you set distance and time, and pace is computed. Pin DISTANCE to plan "how far can I run in 45:00 at 5:00/km?". Pin TIME to plan "what time do I run 10K in at 4:30/km?".
Can I share my calculation with someone?
Yes. Every change updates the URL with your full state — distance, time, pace, units, split mode, predictor seed, everything. Copy the URL or hit SHARE to grab a clipboard-ready link. Sending it to a friend opens the calculator with exactly the same numbers you saw.