18 Free Calculators and Converters for the Numbers You Actually Deal With

Everybody needs to split a bill, work out whether a discount is genuinely a discount, and convert a temperature or a file size without thinking about it. Everybody also, at least once, gets this wrong in public.

This is a collection of eighteen calculators and converters covering money, dates, health, and the unit conversions that show up in work and travel. All of them run in the browser: nothing is saved, so the numbers you type are nobody's business but yours.

The most common real-world math is a percentage of something. Sales tax, a tip, a discount stack, a commission, a growth rate. The formula is simple, and yet the reason people get it wrong is almost always the same: they apply a percentage to the wrong base.

The loan calculator deserves a specific note. The number that matters is not the monthly payment — it is the total interest paid over the life of the loan, because that is the real cost of borrowing. Two loans with identical monthly payments and different terms can differ by tens of thousands in total interest.

Date arithmetic is where most people silently get it wrong, because human months have different lengths and the answer changes depending on whether you mean "calendar months" or "30 days".

The distinction that trips people up: the number of days between March 1 and April 1 is 31, but the number of *months* is one. If you are calculating a notice period or a subscription term, state which unit you mean, in writing.

BMI is a population-level screening statistic. It says nothing about muscle mass, body composition, or any individual's health, and it is a poor proxy for fitness in athletes, in older adults, and across different ethnic groups. Use it as one data point among several, or as a way to track a change over time — not as a verdict. If you are making decisions that affect your health, the right tool is a clinician, not a browser tab.

Unit conversion is divided into two kinds: conversions you must get exactly right, and conversions you need roughly. These tools handle both, and the work is in remembering which is which.

A calculator only helps if you know which number you are asking it about. Three habits make the difference.

Are my numbers stored anywhere? No. The calculations happen in your browser and nothing is transmitted or retained.

Do these work on a phone? Yes, and the calculators are built for one-handed use.

Which conversion standard do you use for data storage? Decimal (1 KB = 1000 bytes) by default, because that is how storage is sold. Operating systems report binary, which is why a drive appears to shrink — the tool explains the difference rather than silently picking a side.

Can I embed these on my own site? No. Bookmark them instead; the cost of that is zero and the accuracy problem is yours to own.

Almost every percentage error a person makes in daily life is one of the same four mistakes. Naming them makes them avoidable.

If you only take one habit from this whole section, take this one: before quoting a percentage to anyone, restate what it is a percentage *of*. Half the arguments you will ever have about a number are actually arguments about the denominator.

There is a point where precision stops being useful. A monthly payment to two decimal places is meaningful. A projected total cost to eight decimal places is a way of avoiding the actual decision, which is whether the loan fits inside what you can afford without stress.

The useful test is the sensitivity question: does the decision change if this input moves by 10%? If not, stop refining it and use the extra time on the variable that does move the answer — usually the term rather than the rate, or the list price rather than the discount.

And one boundary worth stating plainly: these calculators are arithmetic, not advice. A savings target, a retirement date and a debt payoff plan are all decisions about risk, behaviour and time, and a browser tab cannot weigh any of those. It can only tell you what the numbers imply once you have decided what you are assuming. That distinction is the difference between a calculator being useful and being responsible.

Bookmark the money and date calculators properly, treat BMI as one signal rather than a verdict, remember that stacked percentages multiply, and check a unit conversion whenever a number is going into anything contractual. That covers almost everything.