Which two inputs do I provide?

Any two of pace, time and distance — the third is computed. Most runners enter the distance of their last race plus the finish time, then read off the pace; or enter a target pace plus a race distance to see the finish time they're chasing.

How accurate is the 5K-to-marathon prediction?

The Riegel formula (Peter Riegel, 1981) is accurate within roughly ±3% for trained runners who do the mileage the longer race actually demands. If you predict a marathon time from a 5K but only run 30 km/week, reality will be slower — Riegel assumes appropriate endurance training, not just raw speed.

Why the 1.06 exponent?

1.06 is Riegel's empirically fitted fatigue factor across thousands of race results. It says doubling the distance costs you a bit more than double the time. Lower exponents (1.04) flatter the longer distance, higher (1.08) penalise it more; 1.06 is the industry standard and what almost every race-predictor app uses under the hood.

Do you support imperial (miles)?

Yes. Switch the unit toggle to mi and pace becomes min/mi, distance becomes miles. The Riegel predictor is unit-agnostic — it works on any distance pair, so switching units doesn't change the predicted times.

What is the Maffetone 180-age zone?

Phil Maffetone's MAF method caps aerobic training at heart rate = 180 - age (with adjustments for injury or beginner status). It's a low-intensity ceiling for base-building — not your race heart rate. We show it as a reference; your actual zones depend on a lactate test or field-tested max HR.

Why doesn't my watch's predicted marathon match this?

Garmin/Coros race predictors use VO2max plus a fatigue model trained on your wearable data — they downweight a one-off fast 5K if your weekly load is low. Riegel only sees the single race you typed in. If your watch says slower, trust the watch — it has more context.

Is this 100% offline?

Yes. All math runs in your browser; nothing is uploaded. You can use it on airplane mode at a race start line.

Reverse-engineering the splits for a sub-2:00 half marathon

You signed up for a half and want to break 2 hours. Type 21.0975 km and a finish time of 01:59:00, and the calculator hands you 5:38/km (or 9:04/mi). Now you know the exact number to hold on the watch at km 3 and at km 18, not a vague feel. Tape it to your hand.

Sanity-checking a 5K PR against a goal marathon

You just ran 22:30 for 5K and dream of a 3:30 marathon. Predict from the 5K and Riegel projects roughly 3:42, not 3:30. That 12-minute gap tells you the goal needs more long-run endurance, not more speedwork. Better to learn it now than at km 32 on race day.

Converting a treadmill speed to outdoor pace

The treadmill shows 12.0 km/h and you have no idea what that means outdoors. Enter 1 km and a time of 00:05:00 and read 5:00/km. Switch the toggle to miles and it becomes 8:03/mi. Now your gym sessions and your park runs speak the same language and you can pace both.

Pacing a relay or group long run to a fixed arrival time

Your run club leaves at 7:00 and wants coffee by 8:15. With 12 km to cover in 75 minutes, enter the distance and time to get 6:15/km. Share the result link in the group chat so everyone arrives together and nobody gets dropped or stands shivering outside the cafe.

Running Pace Calculator — Pace, Time, Distance & Race Predictor

Running pace calculator — convert between pace / time / distance, predict 5K/10K/half/full marathon from one race.

Runs locally
Category Calculator
Best for Getting a realistic range before a purchase, plan, workout, or schedule decision.

Solve for

Enter any two — the third is computed.

Pace (min/km)

min

sec

Finish time

0:55:00

Distance (km)

Goal-pace table — target finish time → pace per km / mi

Race	Target finish time	Pace (min/km)	Pace (min/mi)
5K (5 km)	0:25:00	5:00	8:03
10K (10 km)	0:55:00	5:30	8:51
Half (21.0975 km)	2:00:00	5:41	9:09
Full (42.195 km)	4:30:00	6:24	10:18

Maffetone 180-age aerobic ceiling

Age

150bpm

Low-intensity training ceiling for base building — not your race heart rate.

What this tool does

Free online running pace calculator. Enter any two of pace (min/km or min/mi), finish time (HH:MM:SS) and distance (km or miles) and we compute the third. The race-time predictor uses the Riegel formula (T2 = T1 × (D2/D1)^1.06) — the same model used by Runner's World, McMillan and the IAAF age-grade tables — to project a 5K performance to 10K, half marathon and full marathon (and vice versa). A goal-pace table converts target 5K / 10K / half / full marathon finish times directly to the kilometre and mile splits you need on the watch. A Maffetone 180-age aerobic heart-rate zone is included as a low-intensity reference. 100% client-side: nothing leaves the browser.

Tool details

Input: Text + Numbers; The page exposes text boxes, numeric controls, file pickers, or structured inputs depending on the tool.
Output: Live result; The result area focuses on usable output, with copy, download, or preview actions when supported.
Privacy: Browser-side processing; The main tool logic does not call an external API, so inputs normally stay in the current tab.
Save / share: No account required; Open the page and use it; whether results survive refresh depends on the tool.
Performance budget: Initial JS <= 18 KB; No WASM budget is declared, keeping the tool quick to open on mobile.
Best fit: Calculator · Content Creator; Category and role tags drive related tools, internal links, and quick fit checks.

How to use

1. Input

Paste or drop your content into the tool panel.
2. Process

Click the button. All processing is local in your browser.
3. Copy / Download

Copy the result or download to disk in one click.

How Running Pace Calculator fits into your work

Use it for fast estimates, comparisons, and planning numbers before you make the final call.

Calculation jobs

Getting a realistic range before a purchase, plan, workout, or schedule decision.
Comparing scenarios by changing one input at a time.
Turning rough assumptions into a number you can discuss.

Calculation checks

Double-check units, dates, rates, and rounding assumptions.
Treat health, finance, tax, and legal outputs as planning aids, not professional advice.
Save the inputs that produced an important result so you can reproduce it later.

Good next steps

These links move the current task into a more complete workflow.

Real-world use cases

Reverse-engineering the splits for a sub-2:00 half marathon
You signed up for a half and want to break 2 hours. Type 21.0975 km and a finish time of 01:59:00, and the calculator hands you 5:38/km (or 9:04/mi). Now you know the exact number to hold on the watch at km 3 and at km 18, not a vague feel. Tape it to your hand.
Sanity-checking a 5K PR against a goal marathon
You just ran 22:30 for 5K and dream of a 3:30 marathon. Predict from the 5K and Riegel projects roughly 3:42, not 3:30. That 12-minute gap tells you the goal needs more long-run endurance, not more speedwork. Better to learn it now than at km 32 on race day.
Converting a treadmill speed to outdoor pace
The treadmill shows 12.0 km/h and you have no idea what that means outdoors. Enter 1 km and a time of 00:05:00 and read 5:00/km. Switch the toggle to miles and it becomes 8:03/mi. Now your gym sessions and your park runs speak the same language and you can pace both.
Pacing a relay or group long run to a fixed arrival time
Your run club leaves at 7:00 and wants coffee by 8:15. With 12 km to cover in 75 minutes, enter the distance and time to get 6:15/km. Share the result link in the group chat so everyone arrives together and nobody gets dropped or stands shivering outside the cafe.

Common pitfalls

Predicting a marathon from a 5K without the mileage to back it up — a 20:00 5K projects a 3:18 marathon, but only if you actually run 60+ km/week; on 25 km/week, expect 20+ minutes slower.
Mixing units mid-calculation, e.g. entering distance in km but reading pace as if it were min/mi. Check the toggle, because 5:00/km and 5:00/mi are 36 seconds apart per kilometre.
Treating the Maffetone 180-age number as a race heart rate. It is a low-intensity base-building ceiling; trying to race a marathon at 180-age will leave you walking the back half.

Privacy

Everything runs in your browser. The paces, times and distances you type are never sent to a server, logged, or stored. When you share a result link, your inputs are encoded into the URL so the recipient sees the same splits — so only put a result link somewhere public if you are fine with those numbers being visible. Close the tab and nothing remains.

FAQ

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Running Pace Calculator — Pace, Time, Distance & Race Predictor

What this tool does

Tool details

How to use

1. Input

2. Process

3. Copy / Download

How Running Pace Calculator fits into your work

Calculation jobs

Calculation checks

Good next steps

Real-world use cases

Reverse-engineering the splits for a sub-2:00 half marathon

Sanity-checking a 5K PR against a goal marathon

Converting a treadmill speed to outdoor pace

Pacing a relay or group long run to a fixed arrival time

Common pitfalls

Privacy

FAQ

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