Calc tools
10 tools that run in your browser. Nothing to install, nothing uploaded.
Everyday calculators — percentages, unit conversion, data sizes — that compute in your browser with no tracking. Each is careful about the detail people get wrong: percent change is measured against the starting value (so a rise and the matching fall are different percentages), temperature conversion uses proper formulas rather than a bare ratio, and data sizes are shown in both decimal and binary because that is why a "1 TB" drive reports as 931 GB.
- Break-Even CalculatorFind how many units you must sell to cover your costs.
- Data Size ConverterConvert between bytes, KB, MB, GB and TB — decimal and binary, side by side.
- Discount CalculatorWork out a sale price and how much you save.
- Percentage CalculatorThe three percentage questions people actually ask — each with its own formula.
- Profit Margin & Markup CalculatorCalculate profit margin from cost and sale price.
- Random Number GeneratorGenerate random numbers in a range, without modulo bias.
- ROI CalculatorCalculate return on investment as a percentage.
- Time Duration CalculatorWork out the time between two dates or times.
- Unit ConverterConvert length, weight, temperature and volume between common units.
- VAT / Sales Tax CalculatorAdd or remove VAT (sales tax) from a price.
Ordinary sums, done the way people actually mean them
The failure mode for everyday calculators is not arithmetic, it is ambiguity, and each of these picks the interpretation people intend and then says which one it picked. Percentage change is measured against the starting value, which is why a 50% rise and a 50% fall do not cancel out and a price that doubles and halves ends where it began. Margin and markup are different quantities computed from different denominators, and confusing them is how a business prices itself into a loss. Date differences count the way people count them — whole days between two dates, with the weekday total available separately, rather than a raw division of milliseconds.
VAT, discounts and the reverse calculations
The direction of a tax or discount calculation matters more than the rate. Adding 20% VAT to a net price is a multiplication by 1.2; removing 20% VAT from a gross price is a division by 1.2, not a subtraction of 20%, and doing it the wrong way understates the net figure by four percent of the total. The same trap catches stacked discounts: 20% off then a further 10% off is 28% off, never 30%, because the second reduction applies to an already reduced price. Both tools show the intermediate figures so the number you pass to an accountant is one you can explain.
Why a “1 TB” drive shows up as 931 GB
Storage is sold in decimal units, where a terabyte is a trillion bytes, and reported by most operating systems in binary units, where a tebibyte is 2^40 bytes — about 10% larger. Nothing has been lost and nobody has cheated; the two systems simply use the same three letters for different quantities. The data size converter shows both side by side for exactly this reason, which also settles the related confusion between bits and bytes: a 100 Mbps connection is megabits per second, so its ceiling is around 12.5 megabytes per second, and expecting eight times that is the most common reason a broadband line is reported as underperforming when it is not.
Random numbers without the bias nobody sees
Generating a number in a range by taking a random value modulo the range size is the standard approach and it is subtly wrong: unless the range divides the generator’s output space evenly, the lowest values come up slightly more often than the highest. The skew is invisible in a handful of draws and very much measurable over thousands, which matters for a prize draw, a sampling decision or anything auditable. The generator here uses rejection sampling against the browser’s cryptographic random source instead, discarding the values that would cause the bias rather than folding them back in.
Frequently asked questions
Why is removing 20% VAT not the same as subtracting 20%?
Because the tax was added to the net price, not to the gross one. If the net is 100 and VAT is 20%, the gross is 120 — and getting back to 100 means dividing by 1.2, not subtracting 20% of 120, which would give 96. That four-percent gap is one of the most common errors in small-business bookkeeping, which is why the calculator shows the intermediate figures rather than just the answer.
Is profit margin the same as markup?
No, and mixing them up is expensive. Margin is profit as a percentage of the selling price; markup is profit as a percentage of the cost. An item costing 100 and selling for 150 carries a 50% markup but a 33.3% margin. Pricing to a target margin using the markup formula leaves you short on every unit sold, and the error grows with the percentage.
Why does my 1 TB drive show as 931 GB?
Storage is sold in decimal units where a terabyte is a trillion bytes, and reported by most operating systems in binary units where the equivalent figure is 2^40 bytes — about ten percent larger. Nothing is missing and no space has been consumed; the same abbreviation is being used for two different quantities. The data size converter shows both conventions side by side so the discrepancy stops looking like a fault.
Are the random numbers here suitable for a prize draw?
They are drawn from the browser’s cryptographic random source and use rejection sampling, so they carry no modulo bias — the subtle skew that makes low values slightly more likely when a range does not divide evenly into the generator’s output space. That makes them appropriate for a draw or a sample. What no browser tool can provide is an auditable record that the draw happened as described, so for anything with a legal requirement attached, keep your own evidence.