Tip, BMI, percent — not a tax engine

Age calculatorHow old is this birth date today, plus the next birthday. It is Gregorian calendar subtraction, not proof of age for a door, a form, or a clinic.
BMI calculatorBMI is weight divided by height squared. This tab shows the number and a standard adult category. It is a screening statistic, not a diagnosis, and it misleads for many athletes, older adults, and pregnancy.
Body fat calculatorA tape-measure formula (Navy method) plus older ‘ideal weight’ equations. The output is an estimate. It is not a scan, not a diagnosis, and not a target we recommend you chase.
CalculatorA pocket expression box for the catalog. Specialized money, unit, and grade jobs have their own pages on purpose.
Calorie / TDEE calculatorEstimate basal metabolic rate and a TDEE-like total from age, size, sex, and an activity factor. It is a formula, not a metabolism test, and not advice to eat a specific deficit.
Celsius to FahrenheitThe pair people type into search. Same arithmetic as a textbook: multiply and add. Other unit families live on the general converter.
Compound interest calculatorA = P(1 + r/n)^(nt) with the numbers you type. Useful for intuition. Useless as a promise of future returns.
Date difference calculatorAny two calendar dates. Age wraps the same math as ‘from birthday to today.’ Hours calculator is punch-clock time.
GPA calculatorYou choose the points (4.0, plus/minus, a +1.0 AP bump). Admissions offices frequently rebuild GPA from the transcript with their own rules.
Grade calculatorOne course. You type the weights from the syllabus. The GPA page is many courses times credits.
Hours calculatorHow long from 09:15 to 17:45, plus duration add and decimal hours for a timesheet. This is not a paycheck calculator and not an overtime-law engine.
Mortgage calculatorMonthly payment math for a fixed rate you type: annuity (equal payment) or straight (equal principal). It is not a lender lock, not PMI/escrow, and not a city housing-policy tool.
Paycheck calculatorTurn hourly into annual (or back) using hours and weeks you enter, then apply a single tax %. It is envelope math, not a 2026 Form W-4 engine and not a filing tool.
Percentage calculatorThe three homework shapes: part from whole, whole from part, and ‘up 12%, down 12% is not back to start.’ Tips and pay have their own pages.
Due date calculatorNaegele’s rule from last menstrual period or a conception date you type. It is a calendar estimate. Clinical dating, especially ultrasound, can disagree — and that clinical date is the one that matters.
Tip calculatorWhat should each person Venmo if the table wants 18–22% on the pre-tax or post-tax amount you choose. Not a payroll product.

Arithmetic with a named job. Policy engines and clinics own the official answer. Age and date-difference stay on their own URLs.

Calculator.net and OmniCalculator own encyclopedia SERPs. CDC, NIH, and the IRS Withholding Estimator own trust for health and pay. This hub exists so each ToolAtlas page states what the engine actually does and points at the sibling instead of pretending one box does tax, BMI, and GPA.

Everyday vs YMYL

Everyday: percentage, tip (US restaurant custom, not a tax engine), age, date difference, hours, the pocket calculator.
School: GPA is the scale you type. Colleges often recalculate from the transcript. Grade calculator is weighted categories for one class, not a transcript GPA.
Money (formula only): paycheck is a tax % you enter — not Pub 15-T. Mortgage is annuity vs straight plus your rate. Compound interest is education math, not an investment pitch.
Health (formula only): BMI, body fat, calories/TDEE, due date. Link CDC. Not a diagnosis. Gestational age is not the birthday age page.

Do not mix these

Age is birthday → today. Date difference is any pair of dates (weekdays if you need them). Celsius↔Fahrenheit is a supporting URL; the unit converter is the broader pillar.

Honesty that belongs on this hub

Paycheck is a tax percentage you type, not the IRS Withholding Estimator and not Publication 15-T. Mortgage is amortization math, not a lender lock. Health pages link CDC-style explainers and refuse a diagnosis. Tip is US restaurant custom, not a sales-tax holiday engine.