Gardeners Toolkit
Free pot & planter volume calculator

Pot & Container Compost Calculator

Measure the inside of your pots and planters, choose the shape, and calculate the litres of compost they actually hold — including tapered round pots.

Tapered potsInternal dimensionsMultiple containers
Diagrams of cylindrical, tapered and rectangular plant containers with internal measurement arrowsinside toproundtaperedplanter

Measure the space the compost will occupy

Use internal measurements, not the outside of the pot. Wall thickness, a narrower base and the fill level below the rim all reduce usable compost volume.

Why taper matters

A pot that is wider at the top than at the bottom is a truncated cone, not a cylinder. This calculator uses the frustum formula so it does not automatically overestimate the compost.

Calculate compost for pots and planters

Choose the container shape and enter internal dimensions to the compost fill line.

Measuring help
Step 1

Choose the container

Step 2

Match your compost bags

L

Measure only to the compost fill level. If an existing rootball or old compost already occupies part of the pot, the empty-container result will be higher than the fresh compost you actually need.

Your container estimate

Per container—

Total for — container(s): — litres.

Total volume—
Bag size—
Whole bags—
Empty-space estimate

The tool calculates the space inside the container down to the fill depth you entered. A rootball, existing compost or a deliberate drainage layer will displace some of that volume.

Use the number for the job you are actually doing

Not sure how to measure the pot?See internal dimensions, taper, watering space and repotting examples.Large raised planter?Use the raised-bed tool where the footprint and existing fill matter.Spreading compost on a bed?Switch to the area × depth calculator for borders.Check the geometrySee how volumes, litres and rounding are handled.

How to measure a pot for compost

Measure the usable inside, not the outside rim. For a straight-sided round pot, measure the internal diameter and the depth you will actually fill. For a tapered pot, measure both the top and bottom internal diameters. For a rectangular planter, use internal length, width and fill depth.

inside diameterfill depthStraight round pot
top insidebottom insideTapered round pot
lengthdepthRectangular planter

The tapered-pot formula

For a round tapered pot, the calculator uses the volume of a truncated cone: V = πh(R² + Rr + r²) ÷ 3, where R and r are the top and bottom radii and h is fill depth. You only need to provide the measurements; the tool handles the formula.

Leave room for watering and plants

RHS container guidance commonly positions plant rootballs so the finished compost surface sits below the rim; its general container guide uses about 5cm below the rim when positioning rootballs. For some specific edible-container examples the stated gap is smaller. The practical point is not to calculate to the physical brim unless that is genuinely your intended fill line.

Repotting is not the same as filling an empty pot

If a rootball moves into a larger pot, it already occupies volume. Calculate the empty container capacity only as an upper bound, then account for the plant and any compost you are retaining. RHS repotting guidance describes back-filling around an existing rootball rather than replacing the whole container volume.

Worked examples

A straight round pot with a 40cm internal diameter filled to 35cm has a volume of about 44 litres. A rectangular 80cm × 30cm planter filled to 30cm holds 72 litres. Three identical planters therefore need 216 litres before allowing for rootballs, old compost or a lower fill line.

Common mistakes

  1. Measuring the outside of a thick-walled pot.
  2. Treating a tapered pot as a full cylinder.
  3. Using the full physical height when the compost will stop below the rim.
  4. Buying for the full empty volume when a rootball or existing material occupies part of it.
  5. Forgetting to multiply by the number of matching containers.