If you've run one honest race — even a single hard 5K — you already know your potential at every other distance. Your 10K. Your half. Your first marathon, before you've ever run one. It's sitting there in that one result, and there's a 50-year-old piece of math that reads it out. That's the whole point of this page: give you the number, and show you how it works.
The method is called extrapolation, and it's not a hack or a hype. It's validated, published science — the same math coaches have quietly used for decades to answer the only question that matters when you plan a race: what can I actually run?
Now search "how to run faster" and see what you get instead. A hundred videos about how to hold your mouth, when to breathe, which ₱12,000 shoe unlocks your inner Kipchoge. Almost none of it mentions the one thing that actually tells you what you're capable of. That knowledge got buried under content. So let me hand it to you straight — the way it was handed to me, back when this was just how running was done.
The one idea that changes everything
Your pace slows as the distance grows. Obvious, right? But how much it slows is the whole game — and it's not what most people assume.
Beginners think prediction is simple multiplication. "I ran a mile in 6:00, so a 5K is just 5 × 6:00 = about 18:38." Wrong. If you run a mile in 6:00, your 5K potential is closer to 20:40. Two full minutes slower than the naive math says. Why? Because fatigue compounds. The longer you go, the more each kilometer costs you.
Prediction isn't your short time multiplied out. It's your short time run through the real curve of human fatigue.
In 1977, an engineer and runner named Pete Riegel put a number on that curve. It's held up for almost half a century, across millions of race results, and it's still the backbone of nearly every serious calculator online — including the free one on this page.
You don't need to do this by hand — that's what the tool is for. But you should understand what's happening, because understanding it is what separates a runner with a plan from a runner with a wish.
What this looks like in real life
Say you just ran a genuine, all-out 5K in 22:00. Not a chill parkrun with your friends — a real effort, lungs burning at the line. Here's what that single result says about you:
| 1 Mile | ~6:33 |
| 5K — your result | 22:00 |
| 10K | ~45:45 |
| Half Marathon | ~1:41 |
| Marathon | ~3:32 |
Look at that marathon number. You've maybe never run more than 10K in your life, and the math already knows you have a 3:32 marathon in you — if you train for it. That's not a fantasy. That's your engine, measured. What's missing isn't talent. It's the specific work to unlock it.
Why this matters more for your generation, not less
Here's the part I actually care about. Young runners today have tools my generation would have killed for — GPS watches, instant data, every race result in your pocket. And yet most of you are training blind, chasing paces that are either too soft to improve you or too ambitious to survive.
Extrapolation fixes that. Once you know your potential at every distance, you know:
What goal is actually realistic. Not the one your friend hyped you into. The one your current engine supports.
How to pace your race. If the math says your honest 10K potential is 45:45, going out at 42:00 pace isn't brave — it's a blow-up waiting to happen at kilometer 7.
How to train at the right speeds. This is the big one, and it's where prediction becomes coaching. Your potential number unlocks your training paces — how fast your easy days should actually be (slower than you think), how hard your intervals need to bite, where your tempo lives. Guess these wrong and you spend months running medium-hard all the time, which is the surest way to never get faster.
Running the same medium pace every day is the national sport of runners who don't improve. Prediction is how you escape it.
How to get a number you can trust
The math is only as honest as what you feed it. Three rules:
1. Use a real effort, not a training run
Training runs are 8–15% slower than race effort. Feed the tool a jog and it'll hand you back a pessimistic, useless prediction. Use a genuine race, or a solo time trial where you finished with nothing left.
2. Use something recent
A result from four to six weeks ago reflects who you are now. A PR from three years and one pandemic ago does not.
3. Predict across a sane range
Predicting your half from your 10K? Rock solid, within a couple percent. Predicting your marathon from a single mile? Treat it as a rough ceiling, not gospel — that's an 8× leap and the endurance demands change. The closer the distances, the tighter the truth.
Try it on yourself
Enter one honest race result and see your potential across every distance from the mile to the marathon — plus the training paces a coach would build your week around. Free. Built on the real science, not the noise.
Open the Fisher Race Predictor →The bottom line
You don't need a breathing trick. You don't need a $250 shoe. You need to know what you're actually capable of, and then do the specific, unglamorous work to reach it. That's the whole secret. It was never hidden — it just got drowned out.
Run one honest race. Feed it the math. Find out who you actually are as a runner. Then go train like you mean it.
— Monte, the old guy who still shows up to the track