How much potting soil does a planter need? Fill it and see

You are standing in the garden-centre aisle holding a planter, and the bags of potting soil say 20, 40 or 50 litres. Nobody tells you how many litres your planter takes. Buy too little and you are back next weekend; buy too much and a half-open bag lives under the sofa until spring.

The sum is short once you know it, and you can do it with a tape measure. Put your planter's measurements in below and watch the soil fill up.

Measure the inside, not the outside

Pot walls are often 1–2 cm thick, and a chunky trough can have walls much thicker than that. Measure across the inside at the top, then across the inside at the bottom (many pots narrow towards the base), then the inside height from the base to the rim. For a trough, measure the length and the width too.

A tapered pot holds a good deal less than the top width suggests, so the bottom measurement matters. If your pot is straight-sided, put the same number in both boxes.

The sums behind it

One litre is 1,000 cubic centimetres, so every volume in centimetres divides by 1,000 to give litres. A trough is a box: length × width × soil depth ÷ 1,000. A straight round pot is π × radius × radius × soil depth ÷ 1,000.

A tapered pot is a truncated cone. Its volume is the soil depth × π ÷ 3 × (r_b² + r_b × r_s + r_s²), where r_b is the radius at the bottom and r_s is the radius at the soil surface. That is what the planner is doing when you drag the line. The soil depth is the inside height minus the gap at the top.

Why not fill to the brim?

The Royal Horticultural Society advises leaving a slight gap between the compost and the container's top edge so compost doesn't spill out when you water, and lightly firming it as you go. A few centimetres is typical; where the soil sits depends on the plant, because the root ball takes up room too. That is why there is a box for the space the plants take: a nursery pot of about a litre or two comes out of the soil you need.

Fresh potting mix is fluffy and drops as it gets wet, so the planner adds 10% as a cushion. If you would rather not have any left, buy slightly less and top up from a neighbour's bag.

Wet soil is heavy

Bag labels quote the dry weight. Peat- and coir-based mixes run roughly 0.25–0.5 kg per litre dry, and about double that once soaked, so a soaked litre is somewhere around 0.6–1 kg. Water itself is 1 kg a litre, so a 40-litre trough is not a light thing to carry or to put on a balcony.

The planner's weight is for the soil only and ignores the pot, the plants and rainwater. It is not a structural check. Ask your landlord or building manager what your balcony is rated for, and put the heaviest planters where you are allowed to.

Ways to need less soil

• Choose the pot to fit the plant. A bigger pot means more soil, more weight and a root ball that stays wet for longer.
• Reuse what you have. Last year's potting mix can often be topped up with some fresh mix rather than thrown away, and second-hand pots work as well as new ones.
• Fill deep pots for shallow roots. For plants with shallow roots in very deep containers, some gardeners put a lightweight filler such as upturned empty plastic pots in the bottom. Keep the drainage holes clear and leave enough depth for the roots.
• Check the drainage holes. Holes wider than about 2.5 cm are best covered with stones or crocks so the soil stays in; small holes need nothing.

Measure before you buy

Write down the inside top width, inside bottom width and inside height of each planter before you leave the house, and take the numbers with you. Check the bag size on the shelf, and if the pot is awkward (an odd shape, a thick rim), do the sum once with water: fill the pot with a measuring jug and count the litres. That figure beats any formula.