Pace Calculator
Calculate your running pace, speed, and predict race times
How It Works
Overview
A pace calculator converts a distance and a finish time into a steady pace per kilometer or per mile, plus your average speed. Knowing pace is fundamental for runners — it lets you set realistic race goals, plan training intervals, and figure out how to evenly distribute effort across an event so you don't blow up in the final miles.
This tool also predicts equivalent race times across the standard distances (5K, 10K, half marathon, marathon) using the Riegel formula, which accounts for the natural slowdown that occurs as distance grows. The same engine elite coaches use for goal-setting works just as well for a beginner running their first 5K.
The Formula
Pace is simply total time divided by total distance.
Race-time prediction uses Riegel's endurance formula, where:
- T1 = your known time at distance D1
- D1 = the distance you actually ran
- D2 = the target race distance
- T2 = predicted time at the target distance
- 1.06 = the fatigue exponent — slightly above 1 because pace slows as distance grows
An exponent of exactly 1.0 would mean perfectly linear scaling (impossible — no one can hold sprint pace for a marathon). 1.06 is empirically calibrated from tens of thousands of race results.
Worked Example
Suppose you ran 5 km in 25 minutes flat:
- Pace per km: 25:00 / 5 = 5:00/km
- Pace per mile: 5:00 × 1.609 = 8:03/mile
- Average speed: 5 / (25/60) = 12.0 km/h
Riegel predictions from this 5K:
- 10K: 25:00 × (10/5)^1.06 = ~52:08
- Half marathon (21.0975 km): ~1:55:30
- Marathon (42.195 km): ~4:00:30
The marathon prediction is optimistic for someone whose longest run is 5K — it assumes equal endurance training. Most runners need to add 5–15 minutes to a Riegel marathon estimate from a 5K.
When to Use This
- Setting a race goal pace — work backward from a target finish time to know exactly how fast each kilometer should feel.
- Planning a long run — easy-run pace is typically 60–90 seconds slower per km than 5K pace.
- Interval workouts — calculate the per-lap (400m) split for a target 5K pace: pace_per_km / 2.5.
- Treadmill conversion — most treadmills show speed in km/h or mph. Convert your outdoor pace to a treadmill speed setting.
- Negative-split planning — break a race in half and target the second half 5–10 seconds/km faster than the first.
Common Mistakes to Avoid
- Using Riegel from a 5K to predict a marathon. The further the extrapolation, the worse the estimate. Use the longest race you've actually run.
- Forgetting GPS overshoots. A "5K" on your watch is often 5.05–5.10 km because of zigzag tracking. Use the actual measured race distance.
- Mixing up min/km and min/mile. A 5:00 split is fast in min/km (12 km/h) but only moderate in min/mile (~12 mph).
- Going out at predicted pace from rep 1. Always start a race 3–5 seconds/km slower than goal pace and let it settle.
- Ignoring elevation and heat. Add roughly 12–20 seconds per km for every 100 m of climb, and 10–20 seconds per km in hot weather (above 25°C).
Frequently Asked Questions
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