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Physics · Glossary

What is SI prefixes?

Also known as: prefix (SI)

Definition 1.3 High School Physics · Chapter 1 — Orders of Magnitude: Measuring the Universe

A prefix glued to a unit multiplies it by a fixed power of ten:

femtof10−1510^{-15}centic10−210^{-2}
picop10−1210^{-12}kilok10310^{3}
nanon10−910^{-9}megaM10610^{6}
microμ\mu10−610^{-6}gigaG10910^{9}
millim10−310^{-3}teraT101210^{12}

Thus 1 nm=10−9 m1\,\mathrm{nm} = 10^{-9}\,\mathrm{m} and 1 km=103 m1\,\mathrm{km} = 10^{3}\,\mathrm{m}.

Examples

Example 1.4 (Reading prefixes)

Green light has wavelength about 500 nm=5.00×10−7 m500\,\mathrm{nm} = 5.00 \times 10^{-7}\,\mathrm{m}; a human hair is about 70 µm=7.0×10−5 m70\,\text{µ}\mathrm{m} = 7.0 \times 10^{-5}\,\mathrm{m} thick; the Moon orbits at 384 Mm=3.84×108 m384\,\mathrm{Mm} = 3.84 \times 10^{8}\,\mathrm{m}. One raindrop has mass about 30 mg=3.0×10−5 kg30\,\mathrm{mg} = 3.0 \times 10^{-5}\,\mathrm{kg} — note that the SI base unit of mass, alone among the seven, carries a prefix already.

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