These tools produce estimates from published population formulas. They are not medical advice, they cannot diagnose anything, and they are not a substitute for talking to a clinician. That caveat is not boilerplate — the limitations below are specific and they matter for how much weight you should give any number on this page.
BMI is the clearest example. It was devised by Adolphe Quetelet in the 1830s as a way of describing populations, and it does exactly one thing: divides weight by height squared. It has no way to distinguish muscle from fat, no way to account for where fat is distributed, and it systematically misclassifies muscular people as overweight and people with low muscle mass as healthy. It remains useful as a rough population-level screen and a starting point for a conversation. It is a poor description of any particular individual.
Energy expenditure estimates carry a similar band of uncertainty. The Mifflin-St Jeor equation is the best-validated of the common basal metabolic rate formulas, and it still has a standard error of roughly 10% for an individual — meaning a calculated 2,000 kcal maintenance figure could plausibly be 1,800 or 2,200 for you specifically. Activity multipliers add more uncertainty, because self-reported activity level is notoriously optimistic. Treat the output as a starting point to test against real-world results over two or three weeks, not as a measured value.
The US Navy circumference method used by the body fat calculator typically lands within three to four percentage points of a DEXA scan, with the error running in a consistent direction for a given body shape. Its practical strength is that it is repeatable and free, so the trend across months is far more informative than any single reading.
For cycle and pregnancy estimates, Naegele's rule assumes a regular 28-day cycle with ovulation on day 14 — a pattern that a minority of people actually have. Roughly 4% of births occur on the estimated due date, with the great majority falling in a two-week window around it. Ovulation predictions from cycle history are estimates of a likely window, not a measurement, and should not be relied on as contraception.
Everything you enter here — cycle dates, weights, measurements, habits — is stored in your browser's local storage on your device and is never transmitted. Given what has happened to reproductive health data held by app companies, that architectural choice is the most important feature on this page.
How much to trust each estimate
The method behind each tool, and its realistic margin:
| Tool | Method | Typical margin | Main limitation |
|---|---|---|---|
| BMI Calculator | Weight ÷ height² | Exact arithmetic | Cannot distinguish muscle from fat |
| TDEE Calculator | Mifflin-St Jeor + activity factor | ±10% or more | Self-reported activity is unreliable |
| Body Fat Calculator | US Navy circumference method | ±3–4 points vs DEXA | Sensitive to how you measure |
| Sleep Calculator | 90-minute cycle model | Cycles vary 70–120 min | Cycle length is individual and shifts nightly |
| Due Date Calculator | Naegele's rule (LMP + 280 days) | Most births within ±2 weeks | Assumes a 28-day cycle |
| Period & Ovulation Tracker | Cycle-history averaging | Wide for irregular cycles | Not contraception |