Guides

Every calculator on this site returns a number in a fraction of a second, and none of them explain in that moment what the number actually is — which published equation produced it, what population it was fitted to, how wide the band around it runs, or which readers it quietly fails. These guides do that part.

They exist because a formula's output and a formula's meaning are different things. Every method implemented here — Mifflin-St Jeor, the WHO BMI bands, the US Navy circumference method, Karvonen, Naegele's rule, the classic one-rep-max equations, the NASEM protein and water reference values — is a published population estimate applied to you, rather than a measurement of you. An equation fitted to a study population describes that population's average; you are one person, and the distance between the two is not a flaw in the arithmetic. It is what the arithmetic is.

So each guide takes one tool and answers three questions in order: where the number comes from, how far it can be trusted, and how it is commonly misread. Derivations are written out. Limitations get more room than capabilities. Where the honest answer is that a reference value exists rather than a formula, the guide says so plainly instead of dressing the value up as a calculation.

For the same ground as condensed reference — formula by formula, with the error bands and the accepted input ranges the engine enforces — see the methodology page.

Pregnancy and cycle dating

Two tools built on calendar arithmetic, where the arithmetic is simple and the caveats are the substance. Both guides state plainly what the calculation can and cannot observe.

How Pregnancy Due Dates Are Calculated

Naegele's rule written out as the arithmetic it is — the last menstrual period plus 280 days — alongside the second dating path, which counts 266 days from a known conception date. The difference between those two figures is the clearest statement of why gestational age is counted from the LMP rather than from conception, and the guide uses it that way before moving on to cycle length, trimester boundaries as the tool implements them, and the fact that a clinician routinely revises the estimate — commonly after an ultrasound measurement. A due date is an estimate, and no calculator here replaces prenatal care. It sits behind the Due Date Calculator.

Understanding the Fertile Window and Cycle Estimates

The calendar method, described exactly as implemented: ovulation placed one 14-day luteal phase before the next expected period, with a window running from 5 days before that point through 1 day after it. The guide explains why the model anchors to the luteal phase rather than to a fixed cycle day, what it means that the estimate observes nothing at all about an individual body, and why a projection built from cycle arithmetic is neither contraception nor a fertility treatment or diagnosis. The tool it describes is the Ovulation Calculator.

Body measurements and daily energy

Two calculations that are easy to read as more precise than they are.

What BMI Does — and Does Not — Measure

BMI is kilograms divided by height in metres squared, and the imperial form — 703 × pounds ÷ inches squared — is the same ratio with the unit conversion folded into a single constant; the guide works that algebra through so the two stop looking like rival formulas. From there it covers the WHO adult cut-offs, why a screening ratio is not a body-fat measure, and the documented blind spots: muscular readers, older adults carrying little muscle, pregnancy, and children and teens, for whom the adult bands do not apply at all. Read it alongside the BMI Calculator.

Calorie Needs: BMR, TDEE, and Why Targets Drift

The chain that runs from Mifflin-St Jeor to a daily target, and every point along it where precision leaks away: the roughly ±10% band around measured resting expenditure, the activity multiplier ladder from 1.2 to 1.9, and the awkward fact that choosing your own rung is a self-assessment most people get wrong in the optimistic direction. The guide makes the case for treating the output as the opening move in a two-to-three-week experiment against an actual weight trend, and explains the intake floors the engine flags rather than hides. It covers the BMR, TDEE, and Calorie calculators together, because they are one chain.

Nutrition reference values

Some questions have a published reference value attached to them and no formula at all. Presenting such a value as though it were a calculated requirement is the mistake this category invites, so one guide covers both tools where it applies.

Protein and Water: What the Reference Values Actually Say

The heart of this one is a distinction that is easy to lose: the RDA of 0.8 g/kg/day is a minimum that prevents deficiency in nearly all healthy adults, and emphatically not an optimum — which is why the tool reports the higher athlete and muscle-building ranges beside it with their position-stand sources named. Water gets the same treatment in reverse: there is no official per-kg requirement, so the guide explains what the NASEM Adequate Intake reference values are, where the beverage share comes from, and why the exercise adjustment is labeled a rule of thumb — alongside the two things a hydration page has to say: thirst is a reasonable guide for most healthy adults, and overdrinking during long exercise can cause dangerous exercise-associated hyponatremia. It stands behind the Protein Intake Calculator and the Water Intake Calculator.

Training and recovery

Three guides about numbers that get treated as hard edges — a zone boundary, a maximum lift, a wake time — when each one is a convention with a spread around it.

Heart-Rate Zones: 220 − Age, Karvonen, and Zone 2

Why 220 − age is a population convention rather than a measurement, and what a spread of ±10–15 bpm does to anyone treating a zone boundary as exact. The guide derives the Karvonen heart-rate-reserve version — (max − rest) × intensity + rest — shows why it places the same intensity percentages at higher heart rates than percent-of-max does, and works through the 60–70% zone 2 band the two methods disagree about. It also covers the cases where formula zones mislead, including heart-rate-affecting medication such as beta blockers. Behind the Target Heart Rate Calculator and the Zone 2 Calculator.

One-Rep Max Estimates and Training Loads

All four prediction equations written out with their attributions — Epley, Brzycki, Lombardi, and O'Conner — and the structural reason they disagree: they are four different fits to the same rep-and-load relationship, which is why the tool reports the range instead of choosing a winner. The guide covers why prediction is most accurate from low-rep sets, why the tool flags results computed from more than 10 reps, why the Brzycki denominator has an unreachable asymptote, and why the percentage table underneath is labeled as the programming convention it is. Paired with the One-Rep Max Calculator.

Sleep Cycles and Choosing a Wake Time

The scheduler works in 90-minute cycles, but real cycle length varies across roughly 70–110 minutes — which makes the output a planning heuristic, not a prediction of where in a cycle you will be at a given moment. The guide is candid about that gap, then explains what the fall-asleep buffer is doing in the arithmetic and why the tool offers a short ladder of cycle counts instead of naming one correct wake time. It accompanies the Sleep Calculator.

Where to go next

Every guide names the tool it describes and links to it. The full calculator directory is on the home page, and the condensed formula-by-formula reference — provenance, error bands, accepted input ranges, and how the implementations are tested — is on the methodology page. If a result ever looks wrong, the contact page explains what to send so it can be reproduced.

Not medical advice: these guides provide general educational information about published population formulas. They cannot account for your individual health context and must not be used to diagnose or treat any condition — consult a qualified healthcare professional. See the methodology page.