About Swim Pace Calculator

I'm Ian Welkins, an independent web developer, and I build small calculator sites that each do one job properly. This one exists because of a frustration I kept running into: I wanted a straight answer to a simple question — what pace am I actually swimming? — and the tools I found kept burying it. General swimming calculators tend to be one crowded page that tries to do everything at once: a pace box squeezed between a calorie guesser, a lap counter and a BMI widget, none of them explained, none of them sourced. The big generic calculator sites are worse. Pace gets a single text box, a formula nobody shows you, and a result you are expected to trust because a large website printed it.

That is backwards, because pace is the number swimmers actually train by. Coaches write sets in pace. Training zones are built around a threshold pace. Race plans, split sheets, send-offs, cut-off checks — all of it is pace, expressed different ways. A swimmer checking whether last month's work made them faster is comparing paces. A triathlete working out whether they will beat a swim cut-off is doing pace arithmetic. If one number carries that much of the sport, it deserves a site of its own, where every variation of the question gets a proper tool, the formula is printed on the page, and the sources are named so you can check them yourself.

So that is what I built: a site that does pace and the jobs immediately around it — splits, course conversions, threshold pace, race-day estimates — and nothing else. No accounts, no sign-up, no app to install, and no general-purpose clutter competing for the page.

The site has ten calculators and a reference chart, and every one of them runs entirely in your browser. Nothing you type is sent to a server, stored, or logged — the arithmetic happens on your own device, which is also why the tools work instantly and keep working on a poolside phone with a weak signal.

The homepage swim pace calculator is the centre of the site: give it a time and a distance and it returns your pace per 100, your speed, your send-offs and a split plan — or work backwards from a target pace to a finish time. Around it sit the tools for the follow-up questions pace always raises. The swim split calculator breaks a goal time into per-length splits. The swim conversion calculator moves times between yards and metres courses. The CSS swim calculator finds your Critical Swim Speed — your threshold pace — from two time trials. There are also tools for predicted finish times, laps and distances, calories burned, open-water estimates and Ironman swim pacing against the official cut-offs, plus the printable swim pace chart for the wall of the pool office. Each tool page carries its own method, worked examples, reference tables and limitations, because a number without its working is only half an answer.

Every calculator on this site starts from a published source, never from another calculator site's output. The course conversion factors come from the conversion method published by USA Swimming and implemented in Hy-Tek's meet software — the system American teams actually use for meet entries — with the NCAA championship factors shown alongside where the two systems differ. Calorie figures use the stroke and effort values from the 2024 Compendium of Physical Activities, the standard research reference for exercise energy costs. The CSS calculator implements the threshold method from Wakayoshi and colleagues' published research on critical swimming velocity. Ironman pacing is built from the swim distances and cut-off times in IRONMAN's own athlete guides, and the pool-course definitions and competition framework follow World Aquatics, the sport's international governing body. The full source list, with what each source is used for, is on the Methodology page.

Checking is built into how the site is made, not added afterwards. Every page shows its formula in plain sight. Every worked example printed on a page is produced by running the example through the site's own code, and that code is covered by automated tests that re-run the published examples and known reference values — so if a change ever broke a formula, the examples would fail before the page shipped. Where a figure is an estimate rather than a measurement — open-water adjustments are the obvious case — the page says so, shows the adjustment separately, and states the range honestly instead of hiding it inside one confident-looking number.

I use AI tools as assistants when I build these sites — for drafting text, scaffolding code and working through layouts faster than I could alone. I would rather tell you that plainly than pretend otherwise. But no formula, conversion factor or figure reaches this site on an AI tool's say-so. Every one of them is checked by me, by hand, against the official or published source named on the page before it ships, and the worked examples are verified against the site's own tested code. The sources are listed precisely so that you do not have to take my word for any of it — you can open the same documents and check. If something here is wrong, it is my error, and the section below is how it gets fixed.

I would rather fix a wrong number than defend it. If you find a figure on this site that disagrees with a current published source, a worked example that does not reproduce, or a formula that looks off, please tell me through the report-an-error page — say which page, what you entered and what you expected, and I will check it against the source myself. Confirmed errors are corrected in the underlying data, the affected pages are re-checked, and the correction is dated. The standards the site is held to, including how corrections are handled, are set out in the editorial policy. For anything else, email me at ianwelkins@gmail.com.