Yasso 800s Calculator (Marathon Workout & Prediction)
Turn a marathon goal time into Yasso 800 interval times — or your 800 m repeat average into a marathon prediction — with 400 m splits, pace per km and mile and the full session time.
How it works
Marathon time in hours:minutes = 800 m repeat time in minutes:seconds · e.g. 3 h 30 min ⇄ 3:30 per 800 m · pace per km = repeat seconds ÷ 0.8 · recovery jog = same time as each repeat.
Bart Yasso of Runner’s World noticed that runners who could average a time of X minutes Y seconds for ten 800 m repeats — with equal-time recovery jogs — tended to run a marathon of X hours Y minutes. The workout is usually built from four or five repeats up to ten, finishing about 2–3 weeks before race day. It is a fitness check, not a guarantee: the prediction assumes you have also done the long runs and weekly mileage that marathon endurance requires, so many coaches treat it as optimistic for runners with low mileage.
Worked example
For a 3 h 30 min marathon goal, run each 800 m in 3:30 (1:45 per 400 m, a pace of 4:23 /km or 7:02 /mi), then jog 3:30 to recover. Ten repeats with their jogs take 1 h 10 min. Race pace for the marathon itself is a much easier 4:59 /km.
Sources: Runner’s World — Yasso 800s · Yasso 800 — workout description
Frequently asked questions
How do Yasso 800s work?
Convert your marathon goal from hours:minutes to minutes:seconds and run 800 m repeats at that time. A 3:30 marathon goal means 800s in 3:30, with a 3:30 recovery jog between them, building up to 10 repeats.
Are Yasso 800s accurate?
They are a decent fitness check for runners with solid long-run training, but many runners find the prediction optimistic, because 800s test speed more than endurance. Use them alongside long runs and a half-marathon race result.
How often should I do Yasso 800s?
Once a week or every other week during the build, starting with 4–5 repeats and adding one each session until you reach 10, about two to three weeks before the marathon.
Can I do Yasso 800s on a treadmill?
Yes. Set the speed to match the repeat pace (for 3:30 per 800 m that is 13.7 km/h or 8.5 mph) and use a slow jog for the recovery.
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This calculator is for educational purposes. Double-check important results before acting on them.