GardenCalcs.uk
Pot Volume Calculator — Calculate Plant Pot Volume
Calculate the volume of round, square, or rectangular plant pots.
Round pots taper, so top and bottom diameters differ
Internal diameter across the rim of the pot
Internal diameter across the base — usually narrower than the rim
Internal depth from the base up to the rim
Unfilled space at top for watering
Crocks or polystyrene layer depth
How We Calculate This
Round tapered pot (frustum): V = (π × h / 3) × (R² + r² + Rr) where R = top radius, r = bottom radius, h = height.
Square pot: V = Side × Side × Depth
Rectangular: V = Length × Width × Depth
Volume in cm³ is divided by 1000 to get litres.
Frequently Asked Questions
Measure the internal diameter at the top and bottom, and the internal depth. For round pots, use the frustum formula for an accurate volume.
Cordon tomatoes need at least 20-30 litres of compost per plant. A typical 30cm pot only holds about 12-15 litres, so it is too small for most tomatoes. Use a 40cm+ pot (around 30 litres), a dedicated tomato planter, or a grow bag.
A pot size in cm refers to the rim diameter, not the volume — litres depend on depth and taper. A typical 30cm round pot (about 24cm deep) holds roughly 12 litres; a deeper, less-tapered 30cm pot can reach 15 litres or more. Enter your actual top diameter, bottom diameter and depth above for an exact figure.
Pot size (e.g., 2L, 3L) refers to the volume of compost it holds. Named sizes (e.g., 9cm pot) refer to the diameter.
Leave 2-3cm of space at the top for watering. For very deep pots (40cm+), you can fill the bottom third with broken polystyrene to save on compost.
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Last updated: February 2026
All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.