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Units and Unit Conversions

SI units, prefixes, mass, volume and temperature conversions, and scientific notation.

Chemistry Maths 5 min read Updated 14 questions Jump to quiz Feedback

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In short: Chemistry uses SI units such as the kilogram, metre, second, kelvin and mole, plus everyday units like grams, millilitres and degrees Celsius. To convert, multiply when going from a big unit to a small one and divide the other way, with 1 L = 1000 mL and K = °C + 273.

SI units

Scientists around the world use the same base units, called SI units.

Quantity SI unit Symbol Common in chemistry
Mass kilogram kg gram (g)
Length metre m centimetre (cm), nanometre (nm)
Time second s minute (min)
Temperature kelvin K degree Celsius (°C)
Amount of substance mole mol mol
Volume cubic metre m³ litre (L), millilitre (mL)

Prefixes

A prefix in front of a unit makes it bigger or smaller by a power of ten.

Prefix Symbol Means Example
kilo k 1 kg = 1000 g
hecto h 1 hL = 100 L
deka da 1 dam = 10 m
deci d 10 dL = 1 L
centi c 100 cm = 1 m
milli m 1000 mL = 1 L
micro μ 1 000 000 μg = 1 g
nano n Atoms are about 0.1 nm across

The conversion rule

kg g mg × 1000 down × 1000 down ÷ 1000 going up
Step down the stairs: multiply. Step up: divide.

Volume

Volume units trip up many students. Learn these links.

Temperature

Chemists use two scales. The kelvin scale starts at absolute zero, the coldest possible temperature, where particles have the least possible energy.

A change of 1 °C is the same size as a change of 1 K. There are no negative kelvin temperatures, and we write "300 K", not "300 °K".

Converting with unit factors

For conversions with more than one step, multiply by unit factors: fractions that are equal to 1, such as . Put the unit you want on top and the unit you have on the bottom, so the old unit cancels.

This method is called dimensional analysis. It works for any conversion, even ones you have never seen, as long as you know the links between the units.

Scientific notation

Chemists deal with very big and very small numbers. Scientific notation writes them as a number between 1 and 10 times a power of ten.

  • Moving the decimal point left makes a positive power (big number).
  • Moving it right makes a negative power (small number).
Units
SI units
  • kg, m, s, K, mol
Prefixes
  • kilo: × 1000
  • milli: ÷ 1000
  • micro, nano: very small
Rule
  • Big to small: multiply
  • Small to big: divide
Volume
  • 1 L = 1000 mL
  • 1 mL = 1 cm³
Temperature
  • K = °C + 273
Scientific notation
  • a × 10ⁿ

Quick summary

  • SI units: kg, m, s, K, mol. Chemists often use g, mL, L and °C too.
  • kilo = × 1000, centi = ÷ 100, milli = ÷ 1000.
  • Big unit to small unit: multiply. Small to big: divide.
  • 1 L = 1000 mL and 1 mL = 1 cm³.
  • K = °C + 273.
  • Scientific notation: a number from 1 to 10 times a power of ten.

Quiz

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Try 3 questions now

  1. How many grams are in 3.2 kg?

    1. A 0.0032 g
    2. B 32 g
    3. C 3200 g
    4. D 320 g
    Show answer

    C: 3200 g

    kg to g is big to small, so multiply by 1000.

  2. Convert 750 mg to grams.

    1. A 0.75 g
    2. B 7.5 g
    3. C 75 g
    4. D 750 000 g
    Show answer

    A: 0.75 g

    mg to g is small to big, so divide by 1000.

  3. 1.5 L is the same as:

    1. A 15 mL
    2. B 150 mL
    3. C 1500 mL
    4. D 0.0015 mL
    Show answer

    C: 1500 mL

    1 L = 1000 mL, so 1.5 × 1000 = 1500 mL.

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