Calculate pot volume: how many litres of soil do you need?

You calculate the volume of a pot by multiplying the base area by the height you fill with soil, then dividing the result in cubic centimetres by 1,000. That gives you the number of litres of potting soil you need.

How do you calculate the volume of a pot in litres?

One litre is exactly 1,000 cubic centimetres. So measure everything in centimetres, work out the volume in cubic centimetres and divide by 1,000. Don't measure the height up to the rim, but up to where the soil will end: leave about three to five centimetres free for watering, otherwise the soil washes over the edge. If the pot has a drainage layer, subtract its thickness from the fill height as well.

All you need is a tape measure, the inside dimensions of the pot and a calculator. Always measure the inside, because the wall of a clay or earthenware pot can be quite thick. With a handmade pot this matters even more, because no two pieces are exactly alike.

How do you calculate a round pot with straight sides?

For a cylinder you multiply the base area by the height. The base area is 3.14 times the radius squared, and the radius is half the diameter. Two examples:

• Pot of 30 cm diameter, 25 cm fill height: radius 15 cm, 3.14 × 15 × 15 × 25 = about 17,700 cm³. That is roughly 18 litres of potting soil.
• Pot of 40 cm diameter, 35 cm fill height: radius 20 cm, 3.14 × 20 × 20 × 35 = about 44,000 cm³. That is roughly 44 litres.

Notice how quickly the volume grows. A 40 cm pot is only a third wider than a 30 cm pot, yet holds more than twice as much soil. That is why a pot that is too big for a small plant so often goes wrong: the soil stays wet for too long. You can read more about matching plant and pot in our article on which size plant pot you need.

How do you calculate a square or rectangular planter?

This one is simpler: length times width times fill height, in centimetres, divided by 1,000. A square planter of 40 × 40 cm with a 35 cm fill height holds 56,000 cm³, so 56 litres. A long planter of 100 × 30 cm with a 25 cm fill height holds 75,000 cm³, so 75 litres.

For a long planter along a wall or terrace edge, a useful rule of thumb is about 75 litres per metre for a planter 30 cm wide and 30 cm deep, depending on how far you fill it. For how to build up the layers, including drainage at the bottom, read our article on filling a planter to prevent root rot.

How do you calculate a tapered pot?

Most beautiful pots and vases are wider at the top than at the bottom. In that case you use the formula for a truncated cone: 3.14 × height ÷ 3 × (R² + R × r + r²), where R is the radius at the top and r the radius at the bottom, measured at the height of the fill line.

An example: a pot of 40 cm diameter at the top and 28 cm at the bottom with a 35 cm fill height. R is 20, r is 14. Then R² is 400, R × r is 280 and r² is 196, together 876. Multiply by 3.14 × 35 ÷ 3 and you get about 32,100 cm³: some 32 litres. That is considerably less than the 44 litres of a straight pot with the same top, a difference of almost a third.

What if the pot is rounded, like an amphora?

With a bulging or amphora shaped pot a formula doesn't work neatly. The practical solution is to measure with water. Fill a bucket of known size, for example 10 litres, and pour it into the pot up to the level where the soil should end. Count the buckets and you have the volume. Do this outside or in a tub, and let the pot drain and dry thoroughly before you fill it.

For larger pieces, up to about 120 cm tall, another option is to mentally cut the pot into a few parts: a cylinder for the narrow part and a cone for the wide part. Add up the results. It is never exact, but it gets you within a few litres.

How many bags of potting soil do you need?

Many bags of potting soil hold 20, 40 or 50 litres. First subtract the volume of the drainage layer. A 5 cm layer of clay pebbles in a pot of 40 cm diameter already takes up more than 6 litres. A 44 litre pot with such a layer therefore needs about 38 litres of soil, and one 40 litre bag is enough.

Better to buy slightly too much than too little: potting soil always settles a few centimetres in the first weeks after planting and watering. Milieu Centraal advises choosing peat free potting soil, because peat extraction damages natural areas. Keep a little spare to top up.

How heavy will the filled pot be?

Wet potting soil weighs roughly between half a kilo and a full kilo per litre. A pot of 44 litres can therefore hold 25 to 40 kilos of soil, before counting the pot itself and the plant. A large handmade pot is heavy to begin with. Think about where it will stand before you fill it, because moving it afterwards won't work without help. For tips, see our article on moving a heavy plant pot safely.

What does this mean for an inner pot in a decorative pot?

If you place a nursery pot in a decorative pot, the nursery pot decides the amount of soil, not the outer pot. A C3 pot holds far less than the decorative pot it fits into. For the litre sizes that go with C2, C3 and C5, read our article on nursery pot sizes C2, C3 and C5. How to place an inner pot safely in a pot without a hole is explained in our guide to using an inner pot in a decorative pot.