About this site

What this site is for

Most calculator websites give you a number and nothing else. You cannot tell what formula was used, what it assumed, or whether it is current. If the answer matters — a mortgage payment, a tax bill, a dosage of anything — that is not good enough.

Every page here shows the formula it used, works through an example with real numbers, states what it does not account for, and links to the source its figures came from. You should be able to check us. That is the point.

How the calculators are built

Each one goes through the same process before it is published:

• The formula is taken from an authoritative source — the published law, a standards body, or a definition rather than another website.
• The maths is tested against known-good values that were verified independently, not produced by running our own code. Where a public reference exists, such as a bank calculator or an official table, results are cross-checked against it.
• Money never passes through ordinary decimal arithmetic. Computers get 0.1 + 0.2 slightly wrong, and over 360 mortgage payments that becomes a figure that disagrees with your bank. All currency here uses exact decimal maths.
• Awkward inputs are handled deliberately — zero, negative numbers, absurdly large values, text in a number field. You should get a helpful message, never a crash and never a nonsense answer.
• Every figure quoted in the written content is calculated before it is written.An example that disagrees with the calculator above it would undermine both.

There are currently 15 calculators live, with 404 planned. They are published in small batches rather than all at once, because each one is checked before it goes up.

How this site is paid for

Advertising, and in future affiliate links on some finance pages. Two commitments that follow from that:

• Advertising never changes an answer. No calculation, ranking or recommendation is influenced by who pays for space.
• Affiliate links, when they appear, will be marked. If a link earns us a commission, the page will say so.

The calculators are free, need no account, and your inputs are never sent anywhere — every calculation runs inside your own browser.

What this site is not

It is not professional advice. The finance pages are not a financial adviser, the health pages are not a doctor, and the tax pages are not an accountant. They are tools for understanding your own situation well enough to ask better questions of the people who are.

Corrections

If a number here looks wrong, please tell us. Errors are fixed quickly, and the review date on each page shows when its formula and sources were last checked. Get in touch through thecontact page.