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.

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: 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.

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

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