Enter a recent race time and pick the race you want a prediction for.
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.
Given your 5K time, what is your equivalent marathon potential? 12 time points from 16:00 to 35:00.
Given your 10K time, what is your equivalent half marathon potential? The most physiologically accurate distance-pair for equivalency.
The calculation uses StrideIQ's RPI equation, which 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.
Race equivalency translates your performance at one distance into a predicted time at another, using StrideIQ’s RPI 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.
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.
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.
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.