🧭 Guides

Guides & explainers

A converter gives you a number; the interesting part is knowing whether to trust it. These guides explain where the conversion factors come from, which conventions are law and which are habit, and how the same quantity can mean two different things in two different countries.

Written and reviewed by SM Arijul Islam · Last reviewed · Editorial standards · Report an error

All guides

Nine pieces, roughly ten minutes each. Every one is tied to a tool on this site, so you can read the explanation and then run the numbers yourself.

Metric vs imperial Where the two systems came from, what the factors are exactly, and the handful worth memorising 9 min read · Length, weight, volume Cooking measurements Why a cup of flour is not 250 g, US versus UK cups, and when a scale beats a measuring cup 8 min read · Cooking, weight, volume Loans and compound interest The amortisation formula, APR versus nominal rate, and what an extra payment really saves 10 min read · Loan, payment, interest Screen resolution and PPI Why "retina" is a viewing distance rather than a pixel count, and how to size an image for print 8 min read · PPI, CSS units, print Baker's percentages and hydration How professional bread recipes are written, and how to scale one up or down without ruining it 9 min read · Baking, hydration, plato Ten unit mistakes that cost money From a lost Mars orbiter to a plane gliding into a drag strip — and the checks that prevent a repeat 11 min read · All converters Wire gauge systems AWG, SWG and mm² — how to compare them, read ampacity and substitute safely 9 min read · Wire, area, current Coffee brew ratios Dose, water, brewed yield and how to fix a cup by taste instead of guessing 8 min read · Coffee, weight, volume Solar panel output Nameplate watts, peak sun hours, derating and realistic annual kWh 9 min read · Solar, power, energy

How these guides are written

Each guide is written in-house rather than assembled from other people's summaries. The reference tables come from the standards that define the units — the BIPM's SI brochure, NIST Special Publication 811 for US customary units, the US Code of Federal Regulations for food labelling, and the relevant trade or engineering standard for industry units such as mesh, rebar, yarn count, water hardness, wire gauge and PV performance.

Where a definition is a legal one, the guide says so and gives the exact factor. Where a definition is a convention that varies — cup sizes, "a cup of flour", bigha, yarn count systems, mesh openings — the guide states the range and the reason it varies instead of pretending there is one answer. Calculations in the guides are rounded for readability, while the tools themselves carry more digits and tell you the precision they are working to.

Every guide ends with the sources used and a date. Unit definitions change rarely, but conventions and standards do, so a guide that has not been reviewed in a long time is a guide worth double-checking.

Corrections and feedback

If a number here disagrees with a standard you trust, we want to know. The error report form sends the details straight to our inbox, and every message reaches a person. Verified corrections are made to the page itself and noted in the review date, not buried in a comment thread.

The tools that these guides support are all free, work without an account, and run entirely in your browser. Start with the converter index or the calculator index if you would rather just get the number.