Why pair a reference guide with a calculator?

Because most of these questions arrive while you are already doing something else. Getting the rule and the answer in one place is faster than reading the rule and then finding a calculator.

Are the conversion factors exact?

For defined units, yes - an inch is exactly 25.4mm and a pound is exactly 0.45359237kg. Where a unit is ambiguous, such as a cup or a gallon, the guides say which definition is used.

Converters and everyday reference: guides and benchmarks

Guides

Barcode formats: Code 128, EAN-13 and UPC compared
What separates the common one-dimensional barcode symbologies, why a retail barcode needs a GS1 prefix, and how to print one that actually scans.
3 min read
Metric and imperial: which conversions are exact
Which unit conversions are defined exactly, where US and imperial measures diverge, and the volume and temperature traps that produce wrong answers.
3 min read
Morse code: how it works and how to learn it
The structure of International Morse Code, the timing ratios that make it readable, and why learning it by lookup table caps you at a few words a minute.
3 min read
Percentage calculations: the four formulas
Percentage of, percentage change, reverse percentages and percentage points - the four calculations people mix up, and the arithmetic behind each.
3 min read
Roman numerals: the rules and the 3999 limit
How subtractive notation works, which pairs are legal, why 3999 is the ceiling, and what to do about numbers the system cannot express.
3 min read

About converters and everyday reference

This hub covers the questions that interrupt a task rather than start one. How to work out a percentage change without inverting the formula. Whether a recipe's cup is 237ml or 250ml. What the check digit on a barcode is for, and why a number you generated yourself will not scan at a till. Each has an exact answer, and each is the sort of thing people get subtly wrong in a spreadsheet and never notice.

The guides are written to be correct at the edges, because that is where these conversions actually fail. Adding 20% and then removing 20% does not return you to where you started. A US gallon and an imperial gallon differ by a fifth. Roman numerals have no zero and stop at 3999. These are the details that turn a rough answer into a wrong one.

Every guide is paired with a browser tool that does the conversion on the page with nothing sent to a server, so you can check the answer immediately rather than trusting the worked example.

Common questions

Why pair a reference guide with a calculator?: Because most of these questions arrive while you are already doing something else. Getting the rule and the answer in one place is faster than reading the rule and then finding a calculator.
Are the conversion factors exact?: For defined units, yes - an inch is exactly 25.4mm and a pound is exactly 0.45359237kg. Where a unit is ambiguous, such as a cup or a gallon, the guides say which definition is used.