SI prefixes

Kilo means 10³, milli 10⁻³ and micro 10⁻⁶. Thus 1 km is 10³ m and 1 μm is 10⁻⁶ m. Changing the unit does not change the physical quantity. Larger units require smaller numerical values for the same quantity: 4 700 m = 4.7 km. Prefix factors are exact, although 4.7 may originate from an imprecise measurement.[2]

The 24 rows are complete. Quetta Q and ronna R together with ronto r and quecto q extended the older list in 2022; the extremes are now 10³⁰ and 10⁻³⁰. There is no separate 10⁰ prefix because an unchanged unit symbol needs none. Additional power tables are calculation aids, not new prefix names.[2]

Lowercase m means milli and uppercase M means mega: 1 mm = 10⁻³ m and 1 Mm = 10⁶ m differ by 10⁹. Position distinguishes m as metre: the first m in mm is the prefix, the second the unit. Kilo uses lowercase k; uppercase K denotes kelvin. μ is the micro symbol. Prefix and unit symbols join directly, while a space separates the numerical value from the complete unit symbol.[3][2]

Only one prefix attaches to a unit. Use nm for 10⁻⁹ m, not a combined milli and micro prefix. First identify each complete unit in a compound expression: mA·s is milliampere times second, whereas A/ms is ampere per millisecond. The same factor in a denominator affects the numerical value inversely.[1]

Write the source prefix as 10ᵃ and the target prefix as 10ᵇ before the same unit u. From x·10ᵃu = y·10ᵇu, solve y = x·10ᵃ⁻ᵇ. This avoids an easily confused left-or-right rule. For 4.8 μm in nm, a = −6 and b = −9, so a − b = 3: 4.8 μm = 4 800 nm.

Check direction: nanometres are smaller than micrometres, so the number of nanometres must increase. Then reverse the calculation: 4 800·10⁻³ μm = 4.8 μm. Scientific notation is safer than long strings of zeros for large exponents. For example, 3 Rm = 3 × 10²⁷ m stays readable even though that length is not typical of a school experiment.

An exponent on a unit symbol applies to the entire unit including its prefix. mm² means (mm)² = (10⁻³ m)² = 10⁻⁶ m², not milli times square metre. For mm³ the factor is multiplied three times, giving 10⁻⁹ m³. The additional table compares these relationships for common length prefixes.[1]

For 35 mm², 35 is already the area value and must not be squared again. The conversion is 35·10⁻⁶ m² = 3.5·10⁻⁵ m². If a square instead has side length 35 mm, square the number too: (35 mm)² = 1 225 mm². The distinction is between a stated area and a stated length.

Kilogram is the only SI base unit whose name already contains a prefix. Further mass prefixes attach to gram, not kilogram. Thus 1 μg = 10⁻⁶ g = 10⁻⁹ kg and 1 mg = 10⁻⁶ kg. μkg may look understandable as an informal expression, but is not an allowed SI construction. This is a rule, not merely a stylistic preference.[1]

A denominator divides by the prefix factor. 1 A/ms = 1 A/(10⁻³ s) = 10³ A/s, whereas 1 mA/s = 10⁻³ A/s. A prefix does not simply make an entire compound unit smaller: position matters. Units such as mg/mm³ combine the gram rule with a third power in the denominator; insert both factors explicitly.

Prefixes change presentation, not relative measurement uncertainty. (5.00 ± 0.02) mm becomes (0.00500 ± 0.00002) m, retaining 0.4% relative uncertainty. A suitable prefix makes comparison easier but must not imply extra accuracy. Keep intermediate values unrounded where possible and round the result consistently with inputs or the stated uncertainty.

SI prefixes represent powers of ten only. For data quantities, kB therefore means 1 000 bytes, whereas KiB uses an IEC binary prefix and means 1 024 bytes. These do not belong in one list of SI prefixes. If a source ambiguously writes KB, clarify its convention before converting; the numerical value alone does not reveal it.[1]

Use readable complete unit symbols. No space separates μ from m, but a space separates 5 from μm. A centred dot clarifies products such as mA·s. For multiple denominator factors, parentheses or negative exponents are safer than repeated slashes.[3]