Converters & calculators
Unit, money and grade calculators that show the method, not just the answer.
Written for: Everyone else, quickly · People moving text between systems · Students and language learners
What this family is for
A calculator is only useful if you can see where the number came from. Unit conversion goes wrong when the factor is hidden — a mile is 1.609344 kilometres exactly, a pound is 0.45359237 kilograms, and a megabyte is a thousand kilobytes while a mebibyte is 1,024. Debt payoff and GPA go wrong when the method is hidden: interest charged monthly, spare cash rolled onto the next debt, quality points divided by credits. These tools run the arithmetic in your browser and state the rule behind every result.
Which calculator answers which question
The finance calculators look interchangeable and are not. Percent change, markup, margin, ROI, break-even and compound interest answer different questions, and using the wrong one produces a number that is arithmetically correct and commercially wrong. The difference is always the denominator: margin divides profit by the selling price, markup divides the same profit by the cost, and ROI divides the net gain by what was invested. On a £100 cost sold at £150 the profit is £50, which is a 50% markup and a 33.3% margin — both true, and one of them will get a pricing decision wrong.
Break-even is the other one people reach for by mistake: it asks how many units cover fixed costs, so it needs a per-unit margin (price minus variable cost) and a fixed cost figure that does not change with volume. It cannot tell you whether a price is profitable — that is margin — only when the fixed costs are paid off. And compound interest is not the same as simple growth: the year-by-year split on that page shows how much of the final value is contributions and how much is interest on interest, which is the part people underestimate.
The non-financial side is simpler but shares one trap: units. A unit converter that silently treats a megabyte as 1,024 kilobytes disagrees with a storage vendor and agrees with an operating system; both conventions exist here and are labelled. Grade and debt calculators show their method because the method is the answer — snowball and avalanche order debts differently and therefore cost different amounts, and a weighted GPA is not an unweighted one.
| If you have… | Use | Because |
|---|---|---|
| Convert between units of length, mass, temperature or data | Unit converter | States the exact factor, including the offset terms temperatures need |
| A number in another base | Number base converter | Exact for long values; any base from 2 to 36 |
| Profit as a share of the selling price | Profit margin calculator | Margin divides by price; markup divides by cost — the page shows both |
| The selling price for a target markup | Markup calculator | Solves for price from cost and markup, with a conversion table |
| Return on an investment, annualised | ROI calculator | Net gain over amount invested, plus CAGR to compare different holding periods |
| How many units before fixed costs are covered | Break-even calculator | Needs fixed cost and per-unit margin, not a total revenue figure |
| Commission on a sale, or the rate behind a payout | Commission calculator | Both directions, plus what the seller keeps |
| What a loan costs per month | Loan calculator | Amortisation schedule, so the interest total is visible |
| How a lump sum and monthly savings grow | Compound interest calculator | Year-by-year split of contributions and interest earned |
| How long until a savings target | Savings goal calculator | Solves for the number of months, not the final balance |
| A percent of something, or a percent change | Percentage calculator | Four percentage questions on one page, each with its own formula |
| Which debt to clear first | Debt payoff calculator | Snowball and avalanche compared side by side |
| A GPA, weighted or on a target | GPA calculator | Quality points over credits, and the grades still needed to hit a target |
| Scaling an image or video to a ratio | Aspect ratio calculator | Simplifies a ratio and finds matching standard resolutions |
14 tools in Converters & calculators
Common mistakes with these tools
- Quoting markup as margin — The same £50 profit is a 50% markup and a 33% margin. Reporting the markup as if it were margin overstates profitability by half at these numbers.
- Using ROI to compare different holding periods — A 20% return over one year and over five are not the same investment. The annualised (CAGR) figure is the one that can be compared; the raw ROI cannot.
- Feeding total revenue into break-even — Break-even needs price and variable cost per unit, plus fixed costs. A lump revenue figure has no per-unit margin to divide by.
- Assuming a megabyte is always 1,024 kilobytes — SI says 1,000 (kB); the binary prefix says 1,024 (KiB). Storage is sold in the former and often displayed in the latter, which is why a “500 GB” drive shows less in the OS.
- Treating a monthly interest rate as an annual one divided by twelve — Twelve monthly compounding periods at r/12 produce more than r per year. The nominal and effective rates differ, and the calculator states which one it applies.
- Comparing a weighted GPA with an unweighted one — They are computed over different credit totals. A 3.7 on a weighted scale and a 3.7 unweighted are not the same standing, and a target calculation has to use the same scale as the input.
Frequently asked questions
Why is temperature different from other conversions?
Most units scale from zero, so a single factor multiplies them. Temperature scales do not share a zero: 0 °C is 32 °F and 273.15 K, so the conversion needs an offset as well as a factor.
Is a kilobyte 1,000 or 1,024 bytes?
SI says 1,000 (kB) and the binary prefix says 1,024 (KiB). Both are offered here and labelled, because storage vendors and operating systems disagree in practice.
How precise are the results?
The factors are the exact defined values (for example 1 in = 0.0254 m), and results are shown rounded for reading rather than truncated.
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