Running Pace Calculator

About this tool

The Running Pace Calculator solves for pace, finish time, or distance when the other two values are known. Enter any two of the three values and the third is calculated instantly. The tool also includes a race time predictor: enter a recent race result for one distance and it predicts finish times for other common race distances (5K, 10K, half marathon, marathon) using the Riegel formula. Additional features include split pace tables for even splits and a pace-to-speed converter (min/km ↔ min/mile ↔ km/h ↔ mph). Distances can be entered as a preset (5K, 10K, marathon) or as a custom value in km or miles. The Riegel formula's exponent (1.06) reflects the empirical observation that as race distance doubles, time more than doubles by a small consistent factor — this is the universal pattern of fatigue across endurance distances.

Why use it

• Solves for pace, time, or distance from any two inputs.

• Riegel race predictor converts one race result to predictions for other distances.

• Generates even split tables for race pacing strategy.

• Supports both metric and imperial units.

• Replaces the manual long division most runners do mid-run when checking pace at split markers.

• Privacy-first: your race times and target paces are never sent to any server.

How to use

1. Enter any two of the three fields: Distance, Time, or Pace.
2. The third field is calculated automatically.
3. Switch between metric (km) and imperial (miles) using the unit toggle.
4. Use the Race Predictor tab to predict other race distances from a known result.
5. Use the metric/imperial toggle once at the start of a session — switching mid-calculation can cause confusion between km and mi values.
6. After predicting a race time, generate splits at 1 km or 1 mi intervals to use as a wrist-tape pacing guide on race day.

When it helps

• Planning training runs at a target pace.
• Predicting marathon finish time from a recent half marathon result.
• Calculating whether a target race time is achievable at a given pace.
• Generating per-km or per-mile splits for a pacing strategy.
• Tapering before a goal race and verifying that the chosen target pace matches your fitness.
• Pacing a friend through a race and needing accurate per-km splits to call out.

Examples

Tips

• Run your easy days 30-60 seconds per mile slower than predicted race pace — most age-group runners run their easy days too fast.
• Use the Riegel predictor only with reference races within 2× the target distance — predicting marathon time from a 5K dramatically overestimates speed endurance.
• Build pace memory by running the first kilometer of every workout at exact target pace for the day — this trains pacing intuition.
• Adjust target pace upward by 1-2% per 10°C above 20°C — heat dramatically reduces sustainable pace, especially after the 60-minute mark.
• Check splits every 1-2 km in a race rather than every km — this prevents over-correcting on a single slow split caused by elevation or wind.
• If your goal is a sub-marathon time, generate a training plan using McMillan or Daniels VDOT, not just race-pace splits — easy/threshold/interval paces all matter.

Frequently Asked Questions

Glossary

Pace

Time per unit distance; expressed as min:sec/km or min:sec/mile. A 5:00/km pace means each kilometer takes 5 minutes.

Speed

Distance per unit time; expressed as km/h or mph. The reciprocal of pace.

Riegel formula

T2 = T1 × (D2/D1)^1.06. Predicts time over a new distance from a reference race time at another distance.

VDOT

Jack Daniels' performance metric that combines VO₂ max and running economy; a more accurate cross-distance predictor than Riegel.

Negative split

Running the second half of a race faster than the first. Optimal pacing strategy for most distances.

Marathon (42.195 km)

Standardized long-distance race of 26.2 miles or 42.195 km, originating from the 1908 London Olympics.

Threshold pace

Pace sustainable for ~60 minutes; corresponds to lactate threshold and roughly 15K race pace for trained runners.