Pot Size Calculator

Plant pot size and soil volume matching is critical for healthy plant growth. Too small a pot restricts roots and causes stress; too large a pot holds excess moisture and can cause root rot.

Pot volume formula (cylinder approximation): Volume = π × r² × h

Where:

• r = inner radius (half the inner diameter)
• h = inner height of the pot

Most pots are tapered, wider at the top. That needs the frustum formula: Volume = (π × h ÷ 3) × (R² + R×r + r²)

Where R = top inner radius, r = bottom inner radius, h = pot height.

Real pots taper, narrowing to about three quarters of their rim diameter at the base. That takes roughly 23% off what a straight cylinder would hold, and it is why a “12 inch pot” holds nowhere near what π × 6² × height suggests. With r = 0.75R the frustum formula collapses to a single factor: 0.771 × the cylinder volume. This page uses that, and so does the plant pot soil calculator, so the two agree on the same pot.

Standard pot sizes and soil volumes:

Tapered, height about 0.85 × the diameter, filled to an inch below the rim:

Notice the jumps. Going from a 10-inch pot to a 12-inch one is not a 20% increase in soil, it is nearly a doubling, because volume grows with the square of the diameter and the extra depth on top of that. This is exactly why the repotting rule below exists.

Repotting rule: Move up one pot size at a time, typically 2 inches larger in diameter. Doubling the pot size surrounds a small root ball with a large volume of wet mix that no roots are drinking from, and that is how healthy plants get root rot in the week after a well-meant repotting.

Soil type and drainage:

• Standard potting mix: 60% peat or coco coir, 30% perlite, 10% compost
• Succulents and cacti: half gritty sand or perlite, half regular mix
• Orchids: bark-based and very open, not soil at all
• One cubic foot bag of potting soil holds about 28.3 liters

What the soil costs: US potting mix ran roughly $8 to $12 per cubic foot in 2025, which works out near $0.35 a litre. That puts a 10-inch pot around $2.50 of mix and a 24-inch planter around $39. Prices move with peat supply and by region, so enter your own bag price in the field above and the panel will work out the real figure for your pot.

Worked example: A monstera in a 10-inch pot (7.4 litres) has gone root-bound. Moving it to a 12-inch pot (13.1 litres):

• The root ball comes across with the plant and fills roughly 60% of the old pot: 7.4 × 0.6 = 4.4 litres
• Fresh mix needed = 13.1 − 4.4 = 8.7 litres
• That is about 0.31 cubic feet, so a third of a standard 1 cu ft bag
• Refresh the bottom few inches with fresh mix first, then centre the root ball and fill around the sides