Is BMI accurate for children?

No — children use age-and-sex-specific BMI percentile charts, because body composition changes rapidly during growth. An adult BMI band applied to a child is meaningless.

Does BMI work during pregnancy?

No. Weight gain during pregnancy is expected and is tracked against pre-pregnancy BMI instead, so a mid-pregnancy BMI reading has no clinical interpretation.

Why do some countries use 23 as the overweight threshold?

Populations differ in the BMI at which type 2 diabetes and cardiovascular risk begin to climb. Several Asian health bodies lowered the cut-off to 23 after studies showed risk appearing well below the WHO figure of 25.

What is a healthy body fat percentage instead?

Broadly 8–19% for men and 21–32% for women, varying with age and measurement method. Body fat percentage requires a scan, calliper or bioimpedance device — BMI is the free approximation people reach for when none is available.

August 4, 2026 · 3 min read

A Practical Guide to Metric and Imperial Conversion

The handful of conversion factors worth memorising, and where rounding gets you in trouble.

Unit conversion is arithmetic until it is not. Temperature has an offset, fluid ounces differ between countries, and a US ton is not a tonne — the mistakes cluster in a handful of predictable places.

The conversions worth memorising

From	To	Multiply by	Quick mental version
Inches	Centimetres	2.54	×2.5 then add 2%
Feet	Metres	0.3048	÷3 then subtract 1.5%
Miles	Kilometres	1.60934	×1.6
Pounds	Kilograms	0.453592	÷2, then subtract 10%
US gallons	Litres	3.78541	×3.8
Ounces	Grams	28.3495	×28
Acres	Hectares	0.404686	÷2.5

Temperature has an offset, not just a factor

°F = (°C × 9/5) + 32
°C = (°F − 32) × 5/9
K  = °C + 273.15

Because of the offset, temperature differences convert differently from temperatures. A rise of 10 °C is a rise of 18 °F, not 50 °F. Getting this wrong is the classic error in recipes, HVAC specifications and lab notes.

Where the same word means two things

Unit	US	UK / Imperial	Difference
Fluid ounce	29.57 ml	28.41 ml	~4%
Pint	473 ml	568 ml	~20%
Gallon	3.785 l	4.546 l	~20%
Ton	907 kg (short)	1,016 kg (long)	Tonne = 1,000 kg

A recipe calling for a pint of stock means noticeably different amounts on either side of the Atlantic, and shipping quotes priced per ton need the type spelled out.

Precision and significant figures

Converting 6 feet to 1.8288 metres implies millimetre precision that the original measurement never had. Carry the same number of significant figures you started with, and round only at the end of a chain of conversions to avoid accumulated drift.

Cooking measures are volume versus weight

A cup is a volume, so a cup of flour and a cup of sugar have very different masses. Baking converted by volume is the usual reason a translated recipe fails; weigh instead where the original gives grams.

Flour: 1 US cup ≈ 120 g
Granulated sugar: 1 US cup ≈ 200 g
Butter: 1 US cup ≈ 227 g
Water: 1 US cup ≈ 237 g

The unit converter covers length, mass, volume, area and speed, and the temperature converter handles the offset scales.

Frequently asked questions

Why is the US the main holdout on metric?

The US formally authorised metric in 1866 and again in 1975, but adoption was voluntary and industrial retooling costs kept customary units in everyday use. Science, medicine and the military there are already metric.

Is a metric tonne the same as a ton?

No. A tonne is 1,000 kg, a US short ton is 907 kg and a UK long ton is 1,016 kg — differences large enough to matter in freight pricing.

How do I convert speed quickly?

Multiply mph by 1.6 for km/h, or divide km/h by 3.6 for metres per second. For a rough mph from km/h, halve it and add 10%.

What about fuel economy?

mpg and l/100km are inversely related, so you cannot convert by multiplying. Use 235.2 ÷ mpg to get l/100km.