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Calculators

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Dates, ages, and durations

Date arithmetic is where mental maths fails most reliably, because months have different lengths and leap years break any rule you invent. Four separate questions come up often enough to deserve their own tools: how old something is right now, how far apart two dates are, what date falls a given interval from another, and how long a span of hours and minutes adds up to.

Exact age matters for eligibility thresholds, visa and pension rules, and forms that want years and months rather than a birth date. Counting between two dates is the one that appears in notice periods, project timelines, and working out how long a document has been valid.

Adding or subtracting an interval is the forward-looking version — a deadline thirty working days out, a renewal eighteen months ahead, a review date a fortnight after a start date. Time duration handles the smaller scale, summing hours and minutes for timesheets, billable work, and travel legs.

Calculate age Days between dates Time duration

Percentages and borrowing

Percentage questions look identical and are not. What is eighteen percent of a bill, what percentage one figure is of another, and how much a value rose or fell between two readings are three different formulas, and mixing them up is the most common arithmetic error in everyday finance.

Percentage change deserves particular care because it is not symmetrical. A price that falls by fifty percent and then rises by fifty percent does not return to where it started, which is exactly the confusion that discount-then-markup pricing relies on.

Loan calculation is the compounding version of the same idea. Seeing the monthly payment is useful, but the amortization table is where the real information sits: early payments are mostly interest, and the total interest paid over the full term is usually a far larger number than borrowers expect from the headline rate. Comparing two terms in that table is the fastest way to see what a longer loan actually costs.

Calculate a percentage Percentage change Loan payments

Measurements, statistics, and number properties

Body mass index is a screening ratio of weight to height, available in metric or US units. It is a population-level indicator rather than a diagnosis, and it does not distinguish muscle from fat, so treat a result as a prompt to ask a clinician rather than an answer in itself.

The statistics calculator returns mean, median, and mode together, which is more informative than any one of them alone: when the mean and median diverge sharply, the dataset is skewed, and reporting only the average would mislead. Fractions are handled exactly rather than as rounded decimals, which is what you want for recipe scaling and measurements in inches.

Aspect ratio simplification sits between the calculators and the image tools — it works out the dimensions to scale an image to without distorting it. Prime checking and Fibonacci sequences round out the set for coursework and puzzles. It is not the only calculator that pairs with something else here, so the complete list is worth a scan even when you arrived for a single number.

Calculate BMI Mean, median, and mode Aspect ratio See the complete tool list

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