Length and area
| 单位Unit | 等于Equals | 备注Note |
|---|---|---|
| 1 m | 100 cm = 1000 mm | 公制基准Metric base |
| 1 km | 1000 m | 常用距离单位Everyday distance |
| 1 in | 2.54 cm | 英寸，屏幕与管径Inch, screens and pipes |
| 1 ft | 30.48 cm = 12 in | 英尺Foot |
| 1 mi | 1609.344 m | 英里Mile |
| 1 nmi | 1852 m | 海里，航空航海Nautical mile |
Area is where most mistakes happen: length steps by 10, so area steps by 100 and volume by 1000. That makes 1 m² equal to 10,000 cm² (not 100) and 1 m³ equal to 1,000,000 cm³; meanwhile 1 hectare is 10,000 m², 1 mu is about 666.67 m², and 1 km² equals 100 hectares or about 1,500 mu.
Volume, capacity and mass
| 单位Unit | 等于Equals | 备注Note |
|---|---|---|
| 1 m³ | 1000 L | 1 立方米水约 1 吨1 m³ of water ≈ 1 tonne |
| 1 L | 1000 mL = 1000 cc | cc 就是毫升cc equals mL |
| 1 US gal | 3.785 L | 美制加仑US gallon |
| 1 UK gal | 4.546 L | 英制加仑，差约 20%Imperial gallon, ~20% larger |
| 1 t | 1000 kg | 1 斤 = 500 g，1 两 = 50 g1 jin = 500 g, 1 liang = 50 g |
| 1 lb | 453.59237 g | 磅Pound |
| 1 oz | 28.3495 g | 盎司（常衡）Ounce (avoirdupois) |
Time, speed and data
| 单位Unit | 等于Equals | 备注Note |
|---|---|---|
| 1 h | 60 min = 3600 s | 1.5 小时 = 1 小时 30 分1.5 h = 1 h 30 min |
| 1 s | 1000 ms | 秒与毫秒是 1000 进制Seconds to ms is base-1000 |
| 1 m/s | 3.6 km/h | 乘 3.6 换 km/hMultiply by 3.6 |
| 1 kn | 1.852 km/h | 节，航速Knot |
| 1 Byte | 8 bit | 100 Mbps ≈ 12.5 MB/s100 Mbps ≈ 12.5 MB/s |
| 1 MB | 1000 KB（厂商标称） | 系统常用 1024：1 MiB = 1024 KiBSystems often use 1024: 1 MiB = 1024 KiB |
| 1 GiB | 1024 MiB ≈ 1.074 GB | 1 TB 硬盘显示约 931 GBA 1 TB drive shows ≈ 931 GB |
Pressure, energy and power
| 单位Unit | 等于Equals | 备注Note |
|---|---|---|
| 1 atm | 101.325 kPa | 标准大气压Standard atmosphere |
| 1 bar | 100 kPa | 轮胎压力常用Common for tyre pressure |
| 1 kcal | 4.184 kJ | 食物热量Food energy |
| 1 kWh | 3.6 MJ | 电表计量单位Electricity billing |
| 1 公制马力 | 735.5 W | 汽车功率常用Metric horsepower |
| 1 英制马力 | 745.7 W | 与公制差约 1.4%~1.4% above metric |
Four places people go wrong
First, area and volume factors are the square and cube of the length factor — do not copy 100 and 1000 across blindly. Second, time is not decimal: 1.75 hours is 1 hour 45 minutes, while seconds to milliseconds and bytes to kilobytes really are base-1000. Third, data has two conventions, 1000 and 1024, and network speeds are usually quoted in bits (divide by 8 for bytes per second). Fourth, US and imperial gallons, ounces (avoirdupois versus troy) and horsepower (metric versus imperial) are separate standards — when you see gal, oz or hp, establish which one applies. For everyday work use the unit converter, and practise with the conversion drill.
FAQ
Why is 1 hectare 10,000 square metres?
A hectare is defined as the area of a 100 m square, so 100 × 100 = 10,000 m². One square kilometre is 100 hectares — do not mistake a hectare for 1,000 m².
Why is my hard drive smaller than advertised?
Manufacturers use base-1000 (1 TB = 10¹² bytes) while operating systems display base-1024 (1 TiB = 2⁴⁰ bytes), so a 1 TB drive shows about 931 GB. That is a convention, not a shortfall.
How do I convert temperatures?
Celsius and Fahrenheit are related affinely: °F = °C × 9 ÷ 5 + 32, so no simple ratio works. Anchors: 0 °C = 32 °F, 100 °C = 212 °F, and −40 °C = −40 °F.