Calculators: free online tools

5 free tools, processed locally in your browser.

Five calculations with no file to drop in and no data sent anywhere: everything resolves inside the page, on every keystroke. The question here is therefore not confidentiality but which formula is applied, because two calculators that both handle percentages do not necessarily follow the same convention, and the result shows it.

Three of them belong to finance — VAT, compound interest, mortgage — one to general arithmetic and one to health. You choose by the question you are asking: converting a price between net and gross, projecting savings, working out a monthly payment, applying a discount, or placing a weight-to-height ratio on a scale.

Tools in the Calculators category

VAT or percentages: the classic mistake

Taking twenty per cent off a tax-inclusive price does not give you back the price before tax. An item at 120 including a 20 per cent tax is 100 before tax, not 96: you divide by 1.20, you do not multiply by 0.80. The VAT calculator applies the right operation in both directions, since the selector states whether the amount you entered is net or gross, and it displays all three values together.

The rates offered cover the four French rates, 20 per cent standard, 10 intermediate, 5.5 reduced and 2.1 super-reduced, to which Belgium, Switzerland, Mauritius and a free field are added. That free field is what makes the tool usable anywhere: VAT, GST or a local sales tax applies exactly as entered, the calculator holding no opinion about which rate your jurisdiction expects. The percentage calculator covers what VAT does not: a share of a total, a discount, a change between two values, each with its own card.

Those three cards are worth reading before typing. The first works out what X per cent of Y comes to, the second what proportion X represents of a total Y, the third the variation from a starting value to an arrival value, signed so that a rise and a discount are told apart at a glance. All the fields accept a comma or a point as the decimal separator, and the result updates as you type, with no button to press.

Saving and borrowing: two conventions

Both calculators rest on compound interest, but they do not convert the annual rate the same way. For savings, the monthly rate is the actuarial equivalent, the twelfth root of one plus the annual rate, so that twelve months of compounding return exactly the headline yield. For the loan, the monthly rate is the annual rate divided by twelve, the proportional method in use in French mortgage lending.

The savings projection adds the deposit at the end of each month, after interest has been applied, and separates the total paid in from the interest earned in its results, which is the only honest measure of the snowball effect. The term is capped at sixty years, and the yearly table is sampled so that it stays readable over long periods.

What the loan calculation includes and leaves out

The monthly payment follows the standard constant-amortisation formula, to which borrower’s insurance is added, calculated here as an annual percentage of the initial capital spread over twelve months. That is the most widespread convention in group policies, but some insurance is charged on the outstanding balance instead: the premium then falls over time, and the real total will be lower than the figure displayed.

The total cost adds up the interest and the insurance over the whole term. It includes neither arrangement fees, nor the guarantee or mortgage registration, nor notary fees, and it is therefore not an APR in the regulatory sense — only the lending institution calculates that officially. The amortisation table, expandable year by year, shows the capital and interest share of each instalment and exports to CSV for further work in a spreadsheet.

BMI: what the index measures

Body mass index divides weight in kilograms by the square of height in metres. The calculator displays the value obtained, the reading against the World Health Organization thresholds — underweight below 18.5, normal range up to 25, overweight up to 30, then obesity — and the weight range corresponding to a BMI of 18.5 to 24.9 at your height.

The index remains a population-level statistic. It does not distinguish muscle mass from fat mass, ignores age, sex and where tissue sits, and means nothing for a child, a pregnant woman or a heavily muscled athlete. It is no substitute for medical advice.

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