Health⏱ 5 min read
How to Calculate Target Race Pace from Training Runs
Your easy run and threshold run times can predict your race pace with surprising accuracy. Here is the formula for 5K, 10K, half marathon, and marathon from any training metric.
Race pace prediction prevents the most common racing mistake: starting too fast. A prediction based on recent training is more reliable than gut feeling, and the maths behind it is surprisingly simple.
The Riegel Formula (Race Equivalency)
Peter Riegel's 1977 formula predicts performance at one distance
from a known performance at another distance:
T2 = T1 x (D2 / D1)^1.06
T1 = known time, D1 = known distance
T2 = predicted time, D2 = target distance
Example: recent 5K in 22:00 (1,320 seconds)
Predict 10K:
T2 = 1320 x (10 / 5)^1.06
= 1320 x 2^1.06
= 1320 x 2.083
= 2,750 seconds = 45:50
Predict half marathon:
T2 = 1320 x (21.1 / 5)^1.06
= 1320 x 4.22^1.06
= 1320 x 4.488
= 5,924 seconds = 1:38:44
Quick Reference Equivalency Table
5K Time10K PredictedHalf MarathonMarathon
20:0041:341:31:373:09:45
22:0045:441:40:503:28:44
25:0051:571:54:463:57:19
28:0058:102:08:434:25:54
30:001:02:222:17:374:44:28
Adjusting Predictions for Real Conditions
The Riegel formula assumes flat course, good conditions, rested state.
Apply these adjustments:
Course elevation: add 60-90 seconds per 100m of elevation gain per km
Warm/humid conditions: add 5-20 seconds/km above 20°C
Wind: headwind can cost 5-15 seconds/km; tailwind helps less
First marathon vs predicted: add 3-7 minutes (the wall effect)
Overracing (tired from recent races): predictions unreliable
Heart rate pacing as a check:
Aerobic threshold HR (LT1): sustainable marathon pace HR
For most runners: 80-85% of max HR is marathon effort
If predicted pace feels too easy or hard at this HR: adjust the pace
Calculating Pace Per Mile or Per Kilometre
From predicted time, calculate required pace:
Pace per km = Total seconds / Distance in km
Half marathon in 1:40:50 = 6,050 seconds over 21.1 km:
Pace = 6,050 / 21.1 = 286.7 seconds/km = 4:47/km
Pace per mile: multiply km pace by 1.60934
4:47/km x 1.60934 = 7:42/mile
GPS watch tip: set target pace brackets at ±5 seconds/km.
If you finish even splits, you've run a well-paced race.
Most amateur runners lose 3-8% of pace in the second half -- this is avoidable.