Race Equivalency
Race equivalency is one of running's most useful — and most misunderstood — tools. It translates your performance at one distance into predicted potential at another, based on aerobic capacity. Used correctly, it reveals what you're capable of. Used naively, it sets expectations that distance-specific training must earn.
Quick answer: Equivalency predicts your aerobic potential at an unfamiliar distance. It is most accurate for similar distances (10K ↔ half marathon) and least accurate for the marathon, which requires glycogen-specific adaptations that raw aerobic capacity does not capture. Use it as a ceiling estimate — meeting it requires distance-specific training.
Distance conversion tables
5K → Marathon Equivalency
Given your 5K time, what is your equivalent marathon potential? 12 time points from 16:00 to 35:00.
10K → Half Marathon Equivalency
Given your 10K time, what is your equivalent half marathon potential? The most physiologically accurate distance-pair for equivalency.
How race equivalency is calculated
The calculation uses the Daniels/Gilbert oxygen cost equation — a physics-based formula published in 1979 that describes the relationship between running velocity and oxygen consumption. From your race time and distance, it computes your Running Performance Index (RPI) — a single number representing your current aerobic capacity.
Given that RPI, the formula uses a binary search to find the time at any target distance that would require the same aerobic capacity to sustain. This is the “equivalent” time — the performance that aerobic fitness alone would predict.
The prediction is accurate when the two distances make similar demands. When they diverge — especially for the marathon — the prediction represents potential, not guarantee.
Common questions
What is race equivalency?
Race equivalency translates your performance at one distance into a predicted time at another, using the Daniels/Gilbert oxygen cost equation. The calculation finds the Running Performance Index (RPI) — a measure of aerobic capacity — from your race result, then determines what time at a different distance would require the same RPI. It is the most rigorous publicly available method for cross-distance performance prediction.
When should I trust race equivalency — and when should I not?
Equivalency is most accurate between physiologically similar distances. 10K → half marathon predictions are highly reliable because both events demand sustained effort near lactate threshold. 5K → marathon predictions are less reliable because the marathon requires specific training adaptations — long runs, glycogen management, pacing experience — that pure aerobic fitness does not capture. A 20:00 5K runner may have the aerobic capacity for a 3:11 marathon, but without marathon-specific training, the actual performance will fall short. Trust equivalency as a potential ceiling, not a race-day prediction.
Why does my marathon time underperform my 5K equivalency?
Almost always: insufficient marathon-specific preparation. The marathon punishes runners whose training is not distance-specific, even when aerobic capacity is high. The key elements equivalency does not measure: long-run adaptation (training the body to sustain 3+ hours of effort), glycogen economy (efficient fat metabolism during extended running), and pacing discipline (the ability to run the first half at effort that feels "too easy"). Runners who do the specific training — 18-22 mile long runs, marathon-pace segments, consistent high volume — tend to match their equivalency predictions closely.
How is race equivalency different from WMA age-grading?
They measure different things. Race equivalency converts performance between distances using aerobic capacity (RPI). WMA age-grading compares your performance to the world-record standard for your age and sex — it tells you how your time ranks on a 0–100% scale relative to the best possible performance for your demographic. Equivalency is about what you could run at a different distance; age-grading is about how good your time is relative to your age group.
Equivalency predicts potential — StrideIQ tracks actuals
Race equivalency formulas treat all runners with the same RPI as identical. StrideIQ tracks how your body specifically responds to different training stimuli — which sessions produce your sharpest fitness gains, how your aerobic ceiling shifts across a training cycle, and what conditions produce your best race-day readiness. The formula gives you the population prediction. Your training data gives you the individual truth.