One: squared and cubed factors
Length scales by 10, so area scales by 100 and volume by 1000: 1 metre is 100 centimetres, but 1 square metre is 10,000 square centimetres and 1 cubic metre is 1,000,000 cubic centimetres. Likewise 1 hectare is 10,000 square metres (not 1,000), and 1 mu is about 666.67 square metres. The check is plain: read the magnitude aloud — "this wall is 6 square metres, so 600 square centimetres?" A moment’s pause tells you it is wrong.
Two: jin, kilograms and liang
The mainland jin is 500 grams, so 1 kilogram is 2 jin or 1,000 grams, and 1 liang is 50 grams. But jin means something different in some regions (a Taiwanese jin is 600 grams), so confirm the convention when shopping across borders or following foreign recipes. Wet markets also distinguish gross and net weight: packaging counts towards gross while the food itself is net, and across 100 jin that difference is real money. Whenever "jin" appears, ask whether it means the local jin or a kilogram.
Three: time is not decimal
1.5 hours is 1 hour 30 minutes, not 1 hour 50 minutes — the classic "decimal treated as minutes" slip. Convert properly: the integer part is hours, and the fraction times 60 gives minutes (1.75 hours is 1 hour 45 minutes). Similarly, seconds and milliseconds are base-1000 while minutes and seconds are base-60, so a value like "1 min 30.5 s" needs each part handled separately. For timers, shifts and video durations, keep everything in seconds or milliseconds and only format as h:m:s at the very end.
Four: orders of magnitude and bit versus byte
Chinese and English magnitudes do not map one to one, and the mega in megawatts is not the mega in MB; meanwhile 100 million is 10^8 and a trillion is 10^12, easily confused with older senses of the characters. When money is involved, always spell out the unit. In storage, the most common error is bit versus byte: a 100 Mbps connection delivers about 12.5 MB/s (divide by 8), which is why "100-meg broadband" downloads at roughly 12 megabytes per second. Bases differ too: drive makers use 1000 while operating systems often use 1024, so a 1 TB disk showing about 931 GB is normal.
Five: temperature is not proportional
Celsius and Fahrenheit are related by an affine transform: °F = °C × 9 ÷ 5 + 32, and that +32 offset means 20 °C is not 40 °F (it is 68 °F), nor can you scale one from the other by simple ratios. Three anchors are worth memorising: 0 °C = 32 °F, 100 °C = 212 °F, and −40 °C = −40 °F where the scales meet. Physical calculations need an absolute scale: 0 °C is 273.15 K, and a 100 °C change is 100 K — use kelvin whenever ratios are involved.
Six: litres, cubic metres and gallons
1 cubic metre is 1,000 litres, 1 litre is 1,000 millilitres, and cc is simply millilitres (1 cc = 1 ml) — so a 2.0-litre engine is 2,000 cc. For water there is a handy equivalence: 1 cubic metre weighs about 1 tonne. Gallons need care because there are two standards: a US gallon is about 3.785 litres and an imperial gallon about 4.546, a gap of nearly 20% that bites when reading imported manuals or renting a car abroad. Whenever you see gal or oz, establish which system applies before calculating.
Seven: when you round
Rounding before or after converting gives different answers. Do the arithmetic at full precision and round only for display; never round mid-way through a multi-step calculation (unit price × area × floors), because the error accumulates. For money, convert to the smallest unit (cents) before operating to avoid floating-point drift — 0.1 + 0.2 is not 0.3. Quotes and contracts should state the conversion basis and the number of decimals kept, for example "at 1 m² = 10.7639 ft², rounded to two decimals", so the other side reaches the same figure when checking.
想马上动手：常用换算在 单位换算 里选好类别直接算，教学与作业场景可用 单位换算练习；金额相关建议配合 人民币换算 与 金额大写；时间进制换算用 时间戳转换 更不容易错。Try it now: pick a category in the unit converter, practise with the unit conversion drill, handle money via RMB units and amount in words, and avoid time-base slips with the timestamp converter.
FAQ
Why is 1 square metre 10,000 square centimetres?
Because the factor of 100 is squared: 1 m = 100 cm, so squaring both sides gives 1 m² = 100² = 10,000 cm². Volume works the same way, cubed: 100³ = 1,000,000.
Can I put a converted figure straight into a contract?
State the conversion basis and the decimals kept, and cite the reference rate or standard. Contracts and settlements should follow what both parties confirm — these tools are for estimating and cross-checking.
Do I still need imperial units?
Depends on your life: cross-border shopping, imported equipment and travel abroad bring inches, pounds, gallons and Fahrenheit. A few anchors (1 inch = 2.54 cm, 1 lb ≈ 454 g, 1 mile ≈ 1.609 km) cover most daily needs.