Range & Reason publishes free health and fitness calculators. There are no accounts, no saved results, and no data sent to a server — every calculation runs in JavaScript on the page in front of you, and nothing you type leaves your device.
Why the calculators look the way they do
Most health calculators return a number and stop. That is the least useful part of the answer. A BMI of 27 means something quite different for a powerlifter than for a sedentary office worker, and a bare figure hides that entirely.
So every result here is shown on a scale, with the bands marked and your position on it visible. The band is the answer. The number is just where the needle happens to land.
Where the equations come from
Every formula used on this site is a published one, and every page names the equation it uses so you can look it up yourself.
| Calculator | Basis |
|---|---|
| BMI | Quetelet index with World Health Organization adult cutoffs |
| Daily calories | Mifflin-St Jeor equation with standard activity multipliers |
| Sleep | 90-minute average sleep cycle; age bands from published adult sleep duration recommendations |
| Water intake | Approximately 33 ml per kg bodyweight, adjusted for exercise and climate |
| Heart rate zones | Karvonen heart rate reserve method; HR-max estimated by the HUNT, Tanaka or classic age formulas |
Stating the limits is the point
Every one of those equations has known failure modes, and each page says so plainly rather than burying it beneath the tool. BMI cannot distinguish muscle from fat. Calorie prediction equations miss individual resting metabolism by roughly ten per cent in either direction. Sleep cycles vary by half an hour between people. Age-based heart rate formulas carry a standard deviation of ten to twelve beats.
This is not a disclaimer bolted on at the end. It is most of the value. Someone who knows their BMI is 27 and understands why that might mean very little for them is better informed than someone who knows only the number.
How content is written and reviewed
- Every calculator page states which equation it uses and what that equation cannot see.
- Pages are reviewed when the underlying guidance changes — for example, when a health body revises its cutoffs or recommendations.
- Where evidence is contested or thin, the page says so rather than picking a side and presenting it as settled.
- Corrections are made on the page itself. If you spot an error, please tell us — errors in health content matter more than most, and we would rather hear about them.
This is not medical advice
Range & Reason publishes general health information. It cannot account for your medical history, your medications, or any condition you have. Nothing here is a diagnosis, and nothing here should replace a conversation with a qualified clinician.
Please speak to a doctor before acting on anything you read here if you are pregnant or breastfeeding, managing a chronic condition, taking medication that affects fluid balance or metabolism, under 18, or have any history of disordered eating.
How the site is funded
Range & Reason is free to use and funded by advertising. Ads are labelled and are placed below or beside the content, never crowding the calculator itself.
Advertisers have no involvement in what the pages say. No calculator recommends a product, and no result has ever been shaped by a commercial relationship. If that ever changes — through an affiliate arrangement or sponsorship — it will be disclosed on the page it affects.
Contact
Corrections, questions and feedback: hello@rangeandreason.online, or see the contact page.
Last updated 30 August 2026.