The “right” unit is the one your reader can use without a second conversion and without a safety mix-up. Precision, industry habit, and who will read the number all matter more than which system you personally prefer.
Who will read it?
International engineering notes almost always want millimetres or metres, kilograms, and Celsius. A US-only DIY post can use feet and inches if that is what the hardware store labels. Mixed readers get both: 10 mi (16 km) on first mention, then stick to one system in the calculations so you do not add metres to feet.
Example: a US recipe says 2 cups of water. For a scale in grams, 2 × 237 ≈ 474 g of water (density ≈ 1 g/mL at room temperature is a cooking approximation, not a lab value). Put 474 g on the scale; do not convert cups to “grams of flour” with the same factor — flour is not water.
Match the size of the number
Prefer values that sit roughly between 1 and 1000. “2.5 m” is easier than “0.0025 km” or “2 500 mm” in a paragraph about a room. Microscopic work wants micrometres or nanometres; road distances want kilometres or miles. The length converter is for changing label, not for choosing which label belongs in the sentence.
Follow the field you are in
| Context | Usual choice |
|---|---|
| Lab science | SI (m, kg, s, K or °C as specified) |
| US residential plans | feet and inches |
| Aviation (many ops manuals) | feet altitude, nautical miles, knots — do not freelance |
| US kitchen | cups and spoons, or grams if you have a scale |
| Medicine | whatever the protocol says (often SI mass); never improvise a unit in a dose |
Traps with the same English word
- US gallon ≈ 3.785 L; imperial gallon ≈ 4.546 L.
- Ounce (mass) versus fluid ounce (volume).
- Ton: short, long, or tonne — three different masses.
- Unlabeled “degrees” for temperature.
Spell the unit once: “500 grams (g)”.
When the unit is a safety control
Dose calculations, structural loads, and altitude are not the place to mix systems in one formula. Convert in a clearly labeled step, or use a dedicated converter and copy the output with its unit. If two people can check the work, they should.
A five-question pass
- Who is the reader?
- What size is the thing?
- What does this industry already print on drawings and labels?
- Could this unit be confused with another of the same name?
- If I get it wrong, is anyone harmed?
Need the other system’s number?
Convert after you have chosen the unit for the document — do not choose the unit by what the converter defaulted to.
Converter list →Tables and dual units
If a table must serve two regions, put SI in the primary column and customary in parentheses, or give two columns with headers that name the unit. Do not put mixed units in a single unlabeled column. Spreadsheets that later sum those cells will silently add feet to metres.
When you convert for a table, convert the source values once, then round for display. Rounding first and then converting stacks error.
Takeaway
Pick units for the reader and the risk level, keep one system inside each calculation, and name gallons, ounces, and tons explicitly. Conversion tools come after that decision.