Percentage Calculator

Four tabs, four questions

The buttons along the top switch between the four things people usually mean when they say they need a percent calculator. What % answers 'is what percent of' — put 25 in the first box and 100 in the second and you get 25%. % of works the other way round: 20 and 150 gives 30, because 20% of 150 is 30. Increase and Decrease both take a percentage first and the original value second, so 15 and 100 under Increase gives 115, and 25 and 100 under Decrease gives 75.

There is no calculate button. The answer in the green box updates as soon as both numbers are valid, and it stays as a dash until they are. Switching tabs clears both boxes, which saves you deleting the old figures by hand but does mean you cannot leave a number sitting there while you change question.

Both inputs are ordinary number fields, so decimals and negative numbers are fine. On a phone they stack vertically and bring up the number keypad rather than the full keyboard.

How to calculate a percentage by hand

Every one of the four calculations comes from the same idea: a percentage is just a fraction out of a hundred. To find what percentage one number is of another, divide the part by the whole and multiply by 100. So 45 out of 180 is 45 divided by 180, which is 0.25, then times 100, which is 25%.

Going the other way, to find a percentage of a number, turn the percent into a decimal by dividing by 100, then multiply. 15% of 200 becomes 0.15 times 200, which is 30. An increase multiplies by 1 plus that decimal — a 15% rise on 200 is 200 times 1.15 — and a decrease multiplies by 1 minus it, so 25% off 200 is 200 times 0.75.

It is worth working one through on paper alongside the tool the first few times, especially with a child who is learning this. The calculator is much more useful for checking an answer than for producing one you do not understand.

Working out a percentage change between two numbers

There is no dedicated percentage change tab, but the What % tab does the job in two steps. Subtract the old figure from the new one to get the difference, then put that difference in the first box and the original figure in the second. A class that went from 20 pupils to 26 has a difference of 6, and 6 as a percentage of 20 is 30%, so that is a 30% increase.

The order matters, and it is the most common mistake people make. The percentage change is always measured against the starting value, not the ending one. Going from 20 to 26 is a 30% rise, but going back from 26 to 20 is a 23.08% fall, because the 6 is now being compared against 26.

Everyday uses at home and in the classroom

Marking is the obvious one. A score of 34 out of 40 goes straight into the What % tab and comes back as 85%. Sale shopping is the next: a 30% discount on a 45 pound coat means 30 and 45 in the Decrease tab, which returns 31.50 as the price you actually pay. Tips, VAT, a 20% deposit, a pay rise, a recipe scaled by a percentage and interest on a small savings account all fit one of the four tabs without rearranging anything.

Percentages start properly in Year 6 and Grade 5 or 6, usually with simple ones such as 50%, 25% and 10% that children can find by halving and dividing. Increase and decrease questions tend to arrive a year or two later. If you are setting practice questions, the calculator is a quick way to generate an answer key before you write the worksheet.

Everything runs in the browser. Nothing is sent anywhere, there is no sign-in, and the numbers you type disappear when you close the tab.

Reading the result

Answers are rounded to two decimal places. The What % tab adds a percent sign; the other three give a plain number, because the answer is a value rather than a percentage. So 1 out of 3 shows as 33.33% rather than a repeating decimal, and small rounding differences can appear if you chain several calculations together.

If the result shows a dash, one of the boxes is empty or does not hold a number. The other case is a zero in the Total box on the What % tab — nothing can be a percentage of zero, so the tool declines to answer rather than showing an error.