Last updated: 2026-06-30

Yasso 800s Marathon Time Predictor

The Yasso 800s are a marathon-training workout that doubles as a finish-time predictor. The idea, popularised by Bart Yasso of Runner's World, is that the average time a runner can hold for a set of 800 m track repeats — read in minutes and seconds — matches the marathon they are ready to run, read in hours and minutes. This calculator takes the average 800 m repeat time and returns the predicted marathon time and the even pace that goes with it.

How the prediction works

Write the average 800 m repeat as $m$ minutes and $s$ seconds, so its total length is $R = 60m + s$ seconds. The Yasso heuristic re-reads the same digits one unit larger: the $m$ becomes hours and the $s$ becomes minutes, giving a marathon time of $3600m + 60s$ seconds.

Substituting $s = R - 60m$ shows the minute/second split cancels:

So the predicted marathon time in seconds is simply the average repeat time in seconds multiplied by 60. The marathon pace follows by dividing that time over the 42.195 km race distance.

Worked example

Suppose the average 800 m repeat is 3:30, which is $R = 210$ seconds:

The same digits, 3:30, now read as hours and minutes. Spread over 42.195 km, a 3:30:00 marathon is a pace of about 12,600 / 42.195 ≈ 299 s/km, or roughly 4:59 per kilometre (about 8:01 per mile). A runner averaging 3:00 repeats projects to a 3:00:00 marathon, and 4:00 repeats to a 4:00:00 marathon.

The workout itself

The classic build is a set of 800 m repeats at the target time with a recovery jog that takes about as long as the repeat. Runners typically start with a handful of repeats and add one each week, aiming to complete ten 800s at the goal time in a single session three to four weeks before race day. The point is even pacing across all ten — finishing the last repeat at the same time as the first is the signal that the goal is realistic. Running the early repeats much faster defeats the purpose.

How much to trust it

The Yasso 800s are a rule of thumb, not a physiological model, and they assume the endurance to sustain marathon distance is already in place. In practice the prediction often lands within a few minutes of a runner's actual marathon, but it tends to run optimistic for those with a thin long-run base or in hot, humid conditions, and it can be conservative for very experienced marathoners. It is most useful as one checkpoint among several: compare it against a power-law estimate from a recent race with Race Time Predictor or an equivalent-performance table from VDOT Calculator, and against the pace you can hold on long runs. A large disagreement between your race-based and workout-based predictions usually points to a gap between speed and endurance worth addressing in training. To convert the predicted finish into split targets, see Running Pace Calculator.