About Calculator Castle

Why this exists

Most online calculators give you a number and nothing else. You cannot see what formula produced it, which assumptions it made, or whether it applies to your situation. That is fine until the number matters, and then it is useless.

Calculator Castle takes the opposite approach. Each calculator shows its working, names the formula and the source it comes from, and is followed by an article explaining what the result means and where it stops being reliable. If a calculator uses a coefficient from a 1995 paper, the article says so.

How the calculators are built

Every calculator is written by hand for its subject rather than generated from a template. The process for each one is the same:

• Find the authoritative formula and read the original source, whether that is a federal regulation, a clinical guideline, a published standard or a peer-reviewed paper.
• Reproduce the arithmetic exactly, including the rounding conventions, then check the output against worked examples published by the body that defined it.
• Write the calculator so the intermediate steps are visible, because a result you can follow is a result you can trust.
• Write the article that goes with it, covering the assumptions, the edge cases where the formula misleads, and the questions people actually ask.

Where authorities disagree, the calculator shows both answers rather than picking one quietly. The kidney function calculator runs four equations side by side for exactly that reason, and they differ by around 30 units on the same blood sample.

Where the numbers come from

Sources are named on the page that uses them. Across the site they include the Consumer Financial Protection Bureau, the Federal Housing Administration, the Department of Veterans Affairs, the Internal Revenue Service, the Federal Reserve, the American College of Obstetricians and Gynecologists, the World Health Organization, the National Weather Service, the International Union of Pure and Applied Chemistry and the relevant published standards for construction and electrical work.

Live data, such as exchange rates and average mortgage rates, comes from third-party feeds and falls back to stored values when a feed is unavailable.

How the site pays for itself

Advertising. There is no paywall, no subscription and no charge for any calculator. Ads are kept away from the inputs and the results so they do not get in the way of the thing you came for.

Accounts are optional

Every calculator works fully without signing in. An account does one thing: it saves a calculation so you can come back to it later, which helps when you are comparing scenarios across days rather than minutes. Nothing you type is sent anywhere unless you press Save.

Use our calculators on your own site

The embeddable widgetsare free for any site, commercial included. Copy one snippet, choose the colours, and it runs in your visitors' browsers with nothing sent back to us.

Corrections

If something here is wrong, we want to know. Send the inputs you used and the figure you expected to info.calculatorcastle@gmail.com, and see the disclaimer for what these calculators can and cannot tell you.