The formula, always shown.

I built these because I kept needing them for work. I work in ICT: cloud, telecommunications and data networks. It is a field full of small, unglamorous calculations you have to get right. Capacity, ratios, conversions, percentages, dates and elapsed time. Most of them take thirty seconds. Almost none of them are worth building a spreadsheet for.

So I would search for a calculator, and the experience was reliably worse than the problem. A page demanding an email address before it would divide two numbers. A cookie banner, then a newsletter pop-up, then an interstitial. An answer with no indication of how it was reached, on a site with every incentive to keep me clicking rather than to be correct.

SwiftSums is what I wanted instead: the calculator at the top of the page, the answer immediately, and the formula printed next to it so I can check the working rather than take it on faith.

No account, ever. Nothing here is gated. There is no sign-up, no email wall, no free tier and no upgrade.

Your numbers stay in your browser. Every calculation runs on your own device in JavaScript. The figures you type are never transmitted to a server, because there is no server-side calculation to transmit them to. If you save a calculator or switch between imperial and metric, that preference is stored in your browser's local storage, on your device, where I cannot see it.

No pop-ups and no interstitials. Nothing slides over the page asking for anything.

And the honest part: the site is funded by advertising, which is Google's, and advertising uses cookies. I would rather say that plainly than claim a purity the arrangement does not have. What I can promise is where the ads are not: never in the middle of a calculation, never covering the answer, and never as a pop-up. The privacy policy sets out exactly what is collected and how to opt out of personalised advertising, and it now includes a live panel showing everything this site has stored on your device, with a button to clear it.

An answer you cannot check is just an assertion, so every calculator prints the formula it used, alongside a worked example with the arithmetic shown step by step.

Behind that, each page carries a worked example computed from its own default inputs and baked into the page itself. An automated check reloads every calculator, lets its code run, and compares the live answer against that baked-in one. Two independent statements of the same result, and any disagreement between them is a bug worth finding. That check runs across all 82 calculators and currently finds no mismatches.

Automation only proves the code matches the formula printed beside it. It cannot tell me the formula is the right one to have chosen, or that the answer matches what a professional would expect. Both of those need people, and that review is ongoing. Where a figure depends on rules that change, such as tax rates and student loan thresholds, the figures and the worked examples are updated together, because an example left contradicting the calculator beside it is worse than no example at all.

These are quick tools for quick jobs, and it matters to be clear about the edges. The finance calculators are estimates, deliberately simplified, and are not financial advice. The health calculators use standard population formulas that describe averages rather than individuals, and are not medical advice. Where a figure is genuinely an estimate, such as a due date or a body fat percentage, the page says so rather than presenting it as more certain than it is.

For anything where the answer carries real consequences, treat these as a sanity check on a professional's figure, not a replacement for it.

SwiftSums is built and maintained by Anthony Ide, working in ICT in Australia. It is a one-person project, not a company: there is no team behind the “we” because there is no we. If something on this site is wrong, it is my mistake, and I am the person who fixes it.

Then I would genuinely like to know. A calculator that is quietly wrong is worse than no calculator, and an outside pair of eyes catches what the person who built it cannot. Email hello@swiftsums.com or use the contact page. Corrections go to the front of the queue, ahead of anything else.