Almost nothing in this category is a law of nature. Most of these numbers come from an equation somebody fitted to a sample of people, and the equations disagree with each other: three of them estimate a one rep max and part company by several kilos on the same set, two estimate maximum heart rate and differ by fifteen beats a minute at ninety, and the exponent behind every race time predictor is a population average that modern refits spread from 1.04 to 1.12. So these tools show the competing versions side by side and print how far apart they are. That gap is the honest width of the answer, and a calculator that returns one figure cannot tell you about it. Two of the tools are the exception and say so: pace is a division, and bicycle gearing is geometry, so both are exact once you know the distance or the wheel — what makes them worth a page is the definitions around them, from the 42.195 km World Athletics fixed in 1921 to the rim size that is the only honest measurement on a tyre. What none of these tools do is tell you what to train, what to eat, or what your body can take. That is medicine, it is not what this is, and the pages stop at the point where the source stops.
Tools in this category
One Rep Max Calculator
Estimate a one rep max from a set you actually did, with the three published equations side by side and how far apart they are.
Running Pace Calculator
Turn a distance and a finish time into a pace per kilometre and per mile, a speed, and what the same pace would give you over a marathon.
Race Time Predictor
Turn one finish time into predicted 5 km, 10 km, half marathon and marathon times, with the fatigue exponent shown and the marathon caveat stated.
Heart Rate Zone Calculator
Estimate maximum heart rate from age with two published equations side by side, and read the moderate and vigorous bands off whichever you pick.
Bike Gear Calculator
Turn a chainring and sprocket into a gear ratio, gear inches, metres of development and a gain ratio that takes your crank length into account.
Suggest a tool
Tell us what is missing and where its numbers should come from.
What should it convert, calculate or generate, and where would the authoritative numbers come from? A link to the standard or publication that defines them is the most useful thing you can send.
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How these tools are built
Every tool in this category runs entirely in your browser: there is no upload, no account and no request to a server once the page has loaded. The numbers behind them are transcribed from the organisation that defines them — NIST for units, the W3C for web formats, the WHO for health measures — and each page names its source next to the formula. A tool whose reference values stop reproducing fails the build and does not ship, which is the whole reason the catalogue grows slowly.