Potting Mix and Planter Volume Calculator

Work out how much potting mix you need before you buy it. The calculator covers rectangular troughs and planter boxes, round pots and cylinders, and round or square tapered planters. Enter your dimensions, the shape, the wall thickness, how many planters you are filling, the gap you want below the rim and the bag size you are buying. It gives you litres per planter, total litres, cubic metres and the number of bags to pick up.

Treat the results as estimates. An existing root ball, a gravel drainage layer, an internal false base, the thickness of the container walls and the gap you leave below the rim will all reduce the amount of mix you actually need. The calculator works out volume only. It does not give you the planted weight, which is the pot, soil, drainage gravel and mature plant together, or tell you what a deck or balcony can carry.

Volume formulas

Each formula uses internal dimensions in centimetres (cm) and converts the result straight to litres (L). Pi is calculated as approximately 3.1416.

• Rectangular trough or planter box: litres = length x width x planting depth / 1,000
• Round pot or cylinder: litres = pi x (diameter / 2)^2 x planting depth / 1,000
• Round tapered planter: litres = pi x height x (top radius^2 + top radius x base radius + base radius^2) / 3,000
• Square tapered planter: litres = height x (top side^2 + top side x base side + base side^2) / 3,000

On a tapered pot, take the top and base radii at the internal planting depth rather than treating the walls as straight.

How to measure your planter

Measure inside the pot where you can, because that is the space the mix has to fill. For a rectangular trough, take the internal length, the internal width and the depth you intend to plant to. For a round pot or cylinder, take the internal top diameter and the internal depth. For a tapered planter, take the internal top opening, the base dimensions and the internal vertical height.

If all you have is the outside measurements, enter those and add the wall thickness. The calculator deducts the walls for you and works back to the soil capacity inside. It accepts centimetres and millimetres, and it will total the quantities across a run of matching planters.

Below-rim allowances, bag counts and weight

Use the below-rim allowance to take out the space loose mix will not fill: the root ball on the plant you are potting, drainage aggregate, an internal false riser, or the usual 30 mm to 50 mm left below the rim so water does not run over the edge when you water.

Bag counts are worked out against standard retail bag volumes, such as 25 L, 30 L or 50 L bags, and should be rounded up to whole bags. Remember that 1,000 litres of potting mix equals 1 cubic metre (1 m³).

Volume is not weight. Planted weight covers the empty weight of the pot, the dry mix, the water the mix holds after a good soaking, any drainage gravel and the plant itself, and a big planter of wet soil is heavier than most people expect. On a balcony or rooftop, check the floor rating before you fill it; for a suspended slab, have your builder confirm the load. Lighter composites make that easier, and they are simpler to carry through a standard garden gate and shift around a patio later. Ring us and we will give you the empty weight and filled volume for whatever you are considering, so you have real figures to hand over.

Have a look through our trough planters and rectangular planter boxes, oversize planters, or the full garden pots and planters range.

If you would rather size it up in person, visit the Malaga showroom, where more than 200 styles are on display and you can stand a plant beside the pot before deciding.

Common questions