About FitCalc

Last updated: 16 September 2026

Why this site exists

Almost every fitness calculator on the web has the same two problems. The first is that it won't tell you what it's actually doing — you type in your weight, a number appears, and there is no indication of which equation produced it or how much you should trust it. The second is that getting the number often costs you an email address, an app download, or a free trial that quietly becomes a subscription.

FitCalc was built to be the opposite of both. Every tool runs entirely in your browser, needs no sign-up, and stores nothing. Every calculator names the formula behind it, and every formula has a guide explaining where it came from, what it's good at, and — more importantly — where it stops being reliable. If a number is a rough estimate, the site says so rather than presenting it with false precision.

It's run by one person in the UK, not a media company or a supplement brand. There is no editorial board and no pretence of one. What there is instead is a deliberately narrow scope: FitCalc covers a small number of well-established calculations properly, rather than a large number of them badly.

How the formulas were chosen

The selection rule was simple and applied to every calculator: the equation had to be published in peer-reviewed literature, widely used in clinical or sports-science practice, and validated against measured data by someone other than its original authors. Nothing on this site is invented, tuned by hand, or adapted from another website. Where two credible formulas compete, the site uses the one with better validation in the general adult population, even when a rival is more famous.

That rule ruled some popular options out. Harris–Benedict, for instance, is the equation most people have heard of, but it dates from 1919, was revised in 1984, and consistently overestimates resting metabolic rate — particularly in people carrying more body fat. Mifflin–St Jeor replaced it for good reasons, so that's what FitCalc uses.

What's behind each calculator

How the calculators were verified

Publishing the right equation isn't the same as implementing it correctly, so each calculator was checked three ways before it went live:

• Against worked examples from the source. Where the original paper or a standard textbook gives a worked example, the calculator was fed those exact inputs and had to reproduce the published output.
• Against independent reference implementations. Each tool was cross-checked against at least two established calculators — typically an NHS, academic, or professional-body tool — using the same inputs. Discrepancies beyond normal rounding were treated as bugs and traced back to the source equation.
• At the edges. Very short and very tall heights, very low and very high weights, one rep and fifteen reps, zero-hour and multi-hour race times. This is where unit-conversion bugs and silently wrong assumptions surface, and it's the check that caught the most problems.

The metric and imperial modes are also checked against each other: entering the same person in both units has to produce the same result, because conversion errors are by far the most common defect in calculators of this kind.

Where these numbers stop being reliable

Every formula here is a population-level model. It describes the average of a large group of people well, and any individual within that group only approximately. Being honest about that is part of the point of the site, so it's worth being specific:

• BMR and TDEE estimates typically land within about 10% of measured values for most adults — which on a 2,500 kcal maintenance figure is a spread of roughly 250 kcal. The activity multiplier adds considerably more error than the equation itself.
• The Navy body fat method is generally within 3–4 percentage points of a DEXA scan, and is most useful for tracking change over time rather than as a single absolute figure.
• BMI cannot distinguish muscle from fat and routinely misclassifies muscular people, and it was never designed to assess individuals at all.
• 1RM estimates drift upward as the rep count rises; above about ten reps they say more about your endurance than your maximum strength.
• Race pace predictions assume you've trained for the target distance. Riegel's model will happily convert a good 5K into a marathon time you are not prepared to run.

None of these tools account for medical conditions, medication, pregnancy, disordered eating, or anything else a formula can't see. They are not diagnostic, and they should never be the reason someone changes how they eat or train while managing a health condition.

Editorial approach

The guides are written to explain a calculation honestly, including the parts that make it look worse. If a method has a well-known weakness — and most of them do — the guide says so rather than burying it. Content is non-prescriptive by design: the aim is to explain what a number means and how it's normally used, not to tell anyone what to do with their body.

Guides are reviewed when the underlying guidance changes, and the date on each page reflects the last substantive update rather than a cosmetic edit. If something here is wrong, out of date, or unclear, telling me about it is genuinely useful — corrections get made and credited.

How FitCalc is funded

The site is free and intended to stay that way. It's supported in two ways, both disclosed openly:

• Advertising. FitCalc displays ads served by Google AdSense. Ad placement is deliberately kept away from calculator inputs and results so it never interferes with using a tool.
• Affiliate links. Some guides link to relevant equipment on Amazon. As an Amazon Associate, FitCalc earns from qualifying purchases at no extra cost to you. These links are labelled wherever they appear, and every gear recommendation carries a visible disclosure.

What funding does not do is decide what the guides say. No brand has paid for a mention, a recommendation, or a placement anywhere on this site, and nothing here is sponsored content. The equipment suggestions exist because the guide's topic genuinely calls for a tool — a kitchen scale for calorie accuracy, a tape measure for circumference methods — not the other way round. Full detail is in the privacy policy.

Get in touch

Corrections, questions, and bug reports are all welcome, and factual corrections get priority. The contact address is hello@fitcalc.uk, and there's more detail on what's useful to include on the contact page.

FitCalc provides estimates for general information and planning only. It is not medical advice and is not a substitute for consultation with a doctor, registered dietitian, or qualified coach. If you are managing a health condition, follow the guidance you have been given over any online calculator.