SI Units in the Laboratory: Volume, Amount, Concentration and Length

Every number in a lab notebook carries a unit. The International System of Units (SI) gives those units precise definitions and consistent prefixes, so a value written in one lab means the same in another.

Base units you use every day

Seven SI base units define everything else. Four come up constantly at the bench:

• kilogram (kg): mass. The gram is the unit used with prefixes.
• mole (mol): amount of substance, exactly 6.022 140 76 × 10²³ entities.
• metre (m): length.
• kelvin (K): thermodynamic temperature (see temperature scales).

Volume: the litre

The SI unit of volume is the cubic metre, but the litre (L, = 1 dm³) is accepted for use with the SI and is universal in the lab. Useful equivalences: 1 mL = 1 cm³, and 1 µL = 1 mm³. Both l and L are permitted symbols, but L avoids confusion with the digit 1.

Concentration: derived units

• Amount concentration: mol/L, often written M. The coherent SI unit is mol/m³, and 1 mol/m³ = 1 mM.
• Mass concentration: g/L, which is the same as mg/mL.

Length at the bench

Wavelengths are quoted in nanometres (280 nm), filter pore sizes in micrometres (0.22 µm), and molecular dimensions in nanometres or ångströms. The ångström (Å = 0.1 nm = 10⁻¹⁰ m) is not an SI unit, but it remains common in structural biology.

Writing values the SI way

• Leave a space between number and unit: 25 µL, 37 °C. Percent is also written with a space: 5 %, though 5% is widespread.
• Unit symbols are not plurals or abbreviations: 10 mL, not 10 mls. Do not add a full stop after a symbol unless it ends a sentence.
• Use one prefix per unit: nm, never mµm. Prefixes attach directly to the symbol: µmol.
• Choose a prefix that keeps the number between about 0.1 and 1000.
• Write a solidus once at most: mol/(L·s), not mol/L/s.

Converting reliably

The converter takes the same route every time: convert the value to the base or coherent unit (L, mol, mol/L, g/L or m), then convert to the target unit. Going through the base unit avoids chaining several conversions in your head, which is where factor-of-1000 errors creep in.