Good news: the litre is a cube in disguise. One litre is exactly a 10 cm × 10 cm × 10 cm cube — the milk-carton cube — so $1\ \mathrm{L} = 1000\ \mathrm{cm}^3$. Work a volume in cm³, slide the decimal three places, and you're speaking litres. Going up a size, one cubic METRE holds a thousand of those litre cubes: $1\ \mathrm{m}^3 = 1000\ \mathrm{L}$ — a full hot tub, a tonne of water.

Now the trap this lesson exists for: lengths convert by 100, but VOLUMES convert by 100 cubed. A metre holds 100 centimetres, so a metre cube holds $100 \times 100 \times 100 = 1{,}000{,}000$ centimetre cubes — a million, not a hundred. Convert the lengths first, or cube the bridge — never let a flat ×100 anywhere near a volume.