The online calculator for everything

An online calculator is a calculator that runs entirely in your browser. There is nothing to download, no app to install and no account to create: you open the page and you can calculate immediately. Everything you type is computed on your own device, so your calculation never leaves your computer or phone.

The big advantage over a physical calculator is that you never have to hunt for the right device. Whether you are on a laptop, looking at your phone or using a tablet, the same calculator is available. And unlike a pocket calculator, you can read back your calculation, correct it and reuse it.

Online Rekenmachine goes further than a single calculator. Besides the standard calculator at the top of this page you will find specialised calculators for VAT, percentages, mortgages, BMI and more. Each one shows not just the answer but the formula behind it and an explanation of every step.

This is arguably the most-asked question about calculators, and the answer explains a difference that surprises many people. Some simple calculators work strictly left to right: they do 2 + 2 = 4 first and multiply that by 2, giving 8. Mathematically, that is wrong.

The correct order of operations dictates that multiplication comes before addition. So you calculate 2 × 2 = 4 first and add 2, giving 6. The calculator on this page always follows mathematical order of operations and therefore returns 6.

The full order is: first whatever is inside parentheses, then powers and roots, then multiplication and division left to right, and finally addition and subtraction left to right. In doubt? Put parentheses around the part that must come first. Parentheses are never wrong and make your intent unambiguous.

One pitfall deserves separate attention: a minus sign in front of a power. The expression −2² means −(2²) and is therefore −4, while (−2)² equals 4. In the first case the minus is not part of the base. Confusing this produces a systematically wrong sign in physics and maths problems.

Percentages are the most common calculation in everyday life, and simultaneously the source of most arithmetic errors. A percentage is nothing more than a part per hundred: 21% means 21 out of every 100. To find 21% of an amount you multiply that amount by 0.21.

The classic mistake is in working backwards. If you have an amount including 21% VAT and want to know the amount excluding VAT, you do not subtract 21% from the gross figure. You divide by 1.21. For € 121 including VAT, dividing by 1.21 gives exactly € 100, whereas subtracting 21% lands on € 95.59 — almost five euros too low.

The same principle applies to discounts. A € 80 jumper with 30% off costs € 80 × 0.70 = € 56. And beware of consecutive percentages: a 50% rise followed by a 50% fall does not return you to the starting amount but to 75% of it. Percentage changes do not stack like ordinary numbers.

Strict rules apply in Dutch secondary education. In central exams you may only use an approved physical calculator, and it often has to be in what is called exam mode. In that mode the memory is locked so you cannot bring in formulas or notes. An online calculator is not permitted during such exams.

That does not make an online calculator less useful — just useful at a different moment. For homework, for checking your own work and for understanding a problem, a calculator that shows the intermediate steps is more valuable than a device that only displays a number. Use the calculators here to grasp the material, and practise separately with the device you will use in the exam.

Which device you need varies by level and subject. For pre-vocational education a simple or scientific calculator usually suffices, while havo and vwo maths often prescribe a graphing calculator. Your school states each year which models are allowed; that list is authoritative, not a shop’s advice.

A scientific calculator handles functions beyond the four basic operations: sine, cosine and tangent, logarithms, powers, roots and factorials. You use it when you want a number as the answer — calculating an angle, looking up a logarithm or determining a square.

A graphing calculator draws the shape of a function. Instead of one answer you see the whole curve: where it crosses the x-axis, where the vertex lies, whether there is an asymptote. That makes it indispensable once you need to analyse functions rather than just substitute values.

A practical difference that is often forgotten: trigonometric functions can work in degrees or radians. sin(30) is 0.5 in degrees but roughly −0.988 in radians. If your calculator is in the wrong mode, every answer involving an angle will be wrong. Always check that setting before you start.

Calculations in the browser use the same number format as virtually all software: double-precision floating point. That is accurate to roughly fifteen significant digits, more than enough for money, measurements and schoolwork.

One quirk is worth mentioning, because it occurs on every calculator: 0.1 + 0.2 does not give exactly 0.3 but 0.30000000000000004. That is because 0.1 has no exact representation in binary, just as 1/3 runs on forever in decimals. For practical use the difference is negligible; round to two decimals and you will never see it.

More important than the last decimal is that the answer addresses the right question. Our financial and health calculators therefore always state the formula used. A mortgage calculation, for example, shows the gross payment; what you actually keep depends on your personal circumstances. Treat the result as a well-founded indication and consult an adviser for major decisions.

Financial calculations almost always revolve around the same principle: an amount, a percentage and a period. Once you recognise that pattern, mortgages, interest, savings and salary suddenly become variants of the same sum. That is why every financial calculator here shows the formula rather than just an answer.

For a mortgage it works like this: the loan amount, the annual rate and the term together determine your monthly payment. With an annuity mortgage that payment stays constant, while the interest share falls and the repayment share rises. That explains why you seem to repay so little in the early years — you are mostly paying interest.

For entrepreneurs and freelancers, VAT is the calculation that most often goes wrong, particularly when working backwards from a gross amount. Profit margin causes confusion too: a 25% margin on the selling price is not the same as a 25% mark-up on the purchase price. Work both out before setting a price.

Not every calculation is about money. The BMI calculator gives a quick indication of whether your weight suits your height, with the caveat that it is a blunt measure: for very muscular people the result reads too high, because muscle weighs more than fat. Children also use separate growth charts rather than the fixed adult thresholds.

Everyday calculations look simple but go wrong surprisingly often. Splitting a bill with a tip, stacking a discount on an offer, or converting between kilometres and miles: each is a sum where one wrong order produces a noticeably different amount. Here too, seeing the intermediate step matters more than the number itself.

In construction and around the house it usually comes down to area and volume. How many litres of paint you need, how many tiles fit a room, or what solar panels yield: all calculations where you first determine the area and then multiply by a consumption per square metre. Always allow a margin for cutting waste.