Plant Pot Size Converter Tool

How to use the plant pot size converter

Pot sizes are sold in at least four different systems, and almost none of them agree with each other. A grower in Oregon lists a shrub in a #3 container. A garden centre in Manchester sells the same shrub in a 10 litre pot. A hardware store labels an identical pot 10 inch. The converter above translates between all of them.

1. Pick what you already know. Diameter if you have a tape measure, volume if the label shows litres or gallons, trade size if you are buying from a nursery, or measurements if you want an exact answer for an unusual planter.
2. Choose the pot shape. This matters more than most people expect — a squat azalea pot holds roughly half the soil of a deep pot of the same width, and a square pot holds about 27% more than a round one.
3. Read the results and the soil line. Set how many pots you are filling and which bag size your supplier stocks, and the tool tells you how many bags to buy.

Plant pot size chart: inches, cm, litres and gallons

This chart covers standard round pots where the height is roughly nine-tenths of the width — the shape of most plastic nursery pots and the majority of decorative planters. Soil volume is the amount of mix actually needed, filled to just below the rim.

Converting pot sizes in your head

Two shortcuts get you close without a calculator. To go from inches to centimetres, multiply by 2.5 — a 12 inch pot is a 30 cm pot. To estimate litres for a standard round pot, cube the diameter in inches and divide by 110: a 12 inch pot gives 1,728 ÷ 110 ≈ 16 litres, which is almost exactly right.

The important consequence is that volume grows far faster than width. Moving from a 6 inch pot to a 12 inch pot doubles the diameter but multiplies the soil needed by eight. This is why potting up one size at a time is cheaper and healthier for the plant than jumping straight to a large container.

Nursery trade sizes explained (#1, #2, #3, #5, #7)

North American nurseries sell shrubs and trees in trade gallon containers, written as #1, #2, #3 and so on. The number is a class name, not a measurement. A #1 “one gallon” pot holds roughly three quarts — about 80% of a real US gallon. The ranges are set out in the ANSI Z60.1 American Standard for Nursery Stock, and individual growers vary within them, so treat the figures below as typical rather than exact.

European and UK pot sizes

Outside North America, pots are named by their width in centimetres, and often carry a “P” code — a P9 is a 9 cm square pot holding around half a litre, the standard size for mail-order perennials. Common steps are 9, 10.5, 12, 13, 15, 17, 19, 21, 23 and 25 cm, after which sizing usually switches to litres. Because most European pots at these sizes are square, they hold noticeably more than the round equivalent: a 9 cm square pot holds about 0.5 L, while a 9 cm round pot holds closer to 0.4 L.

How much potting soil do you need?

For a standard round pot, the working formula is simple:

Litres of mix ≈ 0.0092 × (diameter in cm)³ ÷ 1000, or in imperial, cubic inches ≈ 0.51 × (diameter in inches)³.

Three practical adjustments matter more than the arithmetic:

• Leave headspace. Fill to about 2 cm (¾ in) below the rim so water does not run off the surface. That is roughly 10% of the pot's volume.
• Buy about 10% extra. Bagged mix is compressed and peat- or coir-based blends settle after the first few waterings. The converter already builds this into its bag count.
• Check whether bags are dry quarts or cubic feet. A US dry quart is 1.1 litres, not the 0.95 litres of a liquid quart, and a 2 cubic foot bag is 56.6 litres. Mixing these up is the most common reason people come home with half the mix they needed.

Drainage material, a root ball, or a filler layer at the base all reduce the amount of mix required. If you are transplanting a plant that already fills a #3 container into a #7, subtract the smaller volume: you need about 11 litres of new mix, not 21.

What size pot does your plant need?

Root volume, not plant height, decides the pot. As a working guide:

• Under 1 L (up to 4 in / 10 cm) — seedlings, succulents, small cacti, propagation, single herbs.
• 1–3 L (5–7 in / 13–18 cm) — most houseplants, bedding annuals, salad greens, a clump of chives.
• 3–8 L (8–10 in / 20–25 cm) — peppers, bush beans, compact perennials, medium houseplants such as a young monstera.
• 8–20 L (10–14 in / 25–36 cm) — tomatoes, aubergines, roses, hydrangeas, large indoor plants.
• 20–45 L (14–18 in / 36–46 cm) — potatoes, courgettes, dwarf fruit trees, mature shrubs.
• 45–100 L (18–24 in / 46–61 cm) — citrus, olives, Japanese maples, small ornamental trees.
• Over 100 L (24 in / 61 cm and up) — mature patio trees, mixed permanent plantings, large architectural specimens.

Potting up: how much bigger?

Go up roughly 5 cm (2 in) in diameter for pots under 25 cm, and 10 cm (4 in) for anything larger. Oversizing is a real risk rather than a cautious choice — a small root system in a large volume of mix cannot dry that mix out between waterings, and the constantly wet zone around the roots invites rot. Most houseplants need repotting every 12–24 months; container shrubs and trees every 2–3 years, or when roots circle the base.

Round, square and fabric pots

A square pot holds about 27% more mix than a round pot of the same width and height, and squares pack together with no wasted bench space — which is why nurseries use them. Fabric grow bags are sized in nominal gallons like nursery pots, but they lose water quickly through the walls, so a 5 gallon fabric bag needs watering more often than a 5 gallon plastic pot in the same position. Terracotta is more tapered than plastic, holds slightly less mix than its diameter suggests, and dries out faster because the clay is porous.

Frequently asked questions