How do I measure a tapered pot?

Measure the internal diameter at the top and bottom, and the internal depth. For round pots, use the frustum formula for an accurate volume.

What size pot for tomatoes?

Tomatoes need at least 20-30 litres. A 30cm diameter pot is about 15L — too small for most tomatoes. Use 40cm+ pots or grow bags.

Pot sizes in cm refer to diameter. A 30cm round pot is roughly 15-20 litres depending on depth and taper.

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.

Calculate the volume of round, square, or rectangular plant pots.

Pot Shape

Top Diameter (internal)

Bottom Diameter (internal)

Depth (internal)

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.

Work out how much compost you need to fill your containers.

Calculate compost volume and plant quantities for window boxes.

Work out compost, liner, and plant quantities for hanging baskets.

Calculate how many grow bags you need and plants per bag.

Work out crocks or gravel needed for container drainage layers.

Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.