Built by a runner. For every runner.
The story behind the site
I grew up a swimmer and a track athlete. Speed was always the goal — the clock was the opponent, and every practice was another argument with it. When competitive swimming ended, so did most of the structured training. For years, running was something other people did.
Then, in my forties, I started running again. Not seriously at first — just enough to stay in shape, to stay sane. But something clicked. The same relationship I'd had with the clock in the pool came back, except now the distances were longer and the math was more interesting.
The clock was always the opponent. Running just gave me a longer track.
By my fifties I was running marathons. Then ultras. At 58, I finished a 100-mile ultramarathon — something my younger, chlorine-soaked self would have found completely absurd. But that's what running does. It keeps raising the ceiling.
Along the way I became obsessed with the numbers. Pace per mile, predicted finish times, training load, heart rate zones, VO2 max estimates — the math behind running performance is genuinely fascinating, and most of the tools available to calculate it were either clunky, ad-saturated, or cost money for no reason.
So I built my own. calculaterun.com started as a personal tool — a place to run the formulas I was already doing by hand. It grew from there into 34 free calculators covering pace, race prediction, training zones, fueling, and more.
How the calculators work
Most of the race prediction tools are built on the Riegel formula, first published by Peter Riegel in a 1977 issue of American Scientist. The formula predicts finish times across distances based on a known performance, using a fatigue coefficient that reflects how much harder longer races are relative to shorter ones. You can read a full breakdown in How the Riegel Formula Predicts Your Race Times.
It is a model, not a guarantee. Your training, race day conditions, nutrition, course elevation, and a hundred other variables affect the outcome. The calculators give you a starting point — a number to train toward, argue with, and eventually beat.
Everything runs in your browser. Nothing you type into a calculator is stored or transmitted anywhere.
The tools
Pace Calculator
Time, distance, and pace — solve for any one.
Race Predictor
Predict finish time at any distance using Riegel.
VO2 Max Estimator
Estimate aerobic capacity from race performance.
Training Paces
Easy, tempo, interval, and race paces from one effort.
Heart Rate Zones
Calculate your 5 training zones from max HR.
All 34 Calculators →
View the full suite of free running tools.
What's next
The site keeps growing. New calculators get added regularly, and the goal is always the same: free, fast tools that help real runners train smarter. If there's a calculation you'd like to see here that isn't already, reach out.
You can also follow the training, race analysis, and running math on the Running by the Numbers YouTube channel — where the same obsession with pace, data, and performance plays out in video form.
Contact
Questions, calculator suggestions, or just want to talk running? Email: info@calculaterun.com