gizmobench

Potting Soil Calculator

A 12 inch pot filled with 10 inches of mix takes 0.65 cubic feet, which is about 19 litres, so one 1.5 cubic foot bag fills two of them with a little left over. This page always opens in pot mode, with that pot in the fields the first time you land on it, and it asks for the three things a pot actually has: the diameter across the top, the height of mix you want in it, and how many pots you are filling. It uses the cylinder formula, pi times the radius squared times the height, so the shape is the pot's rather than a rectangle standing in for one, and the count multiplies it, which is the part that matters when you are potting up a dozen at once. The bag size is a field, in cubic feet, because what a brand puts in a bag is the brand's decision and not something this page should assume.

10 in of mix12 in across1 pot
Soil needed12 in across at 10 in deep
Cubic feet0.650.02 cu yd · 19 litres1 bag at 1.5 cu ft, rounded up
Mode
Diameter in
Height in
Pots
Bag cu ft
Units
Common bag sizes

The bag size is a field because it is the one number a brand decides: bagged soil and compost are sold at anything from 0.75 to 3 cubic feet, and 40 litre and 50 litre sacks work out at 1.41 and 1.77 cubic feet. Type what is printed on yours and the count follows. Switching between feet and metres rewrites what you already typed into the other system, to four decimals, rather than clearing it, and the volume is then worked from the figures on the screen.

  • A 4 by 8 raised bed10 in of mix, /soil-calculator/raised-bed
    26.67 cu ft · 18 bags
  • A hole to fill3 ft by 3 ft, 2 ft deep, /soil-calculator/fill
    18.00 cu ft · 12 bags
  • A 12 in round pot10 in of mix, /soil-calculator/pot
    0.65 cu ft · 1 bag
Exact on the shape you describe, and it says so about the rest. Exact arithmetic on what you enter: length by width by depth, or the cylinder formula for a pot, converted without rounding until the last step. It cannot know how far soil settles after its first watering, how much room root balls take, or what a brand actually puts in a bag. There is no price here and no supplier, and no allowance is folded in behind the figure: if you want a margin for settling, raise the depth a little and watch the number move.

Common questions

How much potting soil does a 12 inch pot need?
With 10 inches of mix in it, 0.65 cubic feet, or about 19 litres. Fill the same pot an inch higher and it is 0.72 cubic feet. Three of them come to 1.96 cubic feet, which is 2 bags of 1.5 cubic feet; six come to 3.93 cubic feet, which is 3 bags of 1.5 or 4 bags of 1; ten come to 6.54 cubic feet and 5 bags of 1.5. Type your own diameter, height and count and the figures follow, in cubic feet, cubic yards, litres and bags.
How much soil do the common pot sizes take?
Filled to about an inch below the rim, a 6 inch pot with 5 inches of mix is 0.08 cubic feet, an 8 inch pot with 7 inches of mix is 0.20, a 10 inch pot with 9 inches is 0.41, a 12 inch pot with 10 inches is 0.65, a 14 inch pot with 12 inches is 1.07, a 16 inch pot with 14 inches is 1.63, a 20 inch pot with 16 inches is 2.91, and a 24 inch planter with 20 inches of mix is 5.24 cubic feet. The jump between sizes is steeper than it looks, because the diameter is squared: a 20 inch pot with 16 inches of mix holds about four and a half times what a 12 inch pot with 10 inches does.
How many pots will one bag of potting mix fill?
Divide the bag by the pot. A 1.5 cubic foot bag fills 2 of the 12 inch pots at 0.65 each with 0.19 cubic feet left over, or 7 of the 8 inch pots at 0.20 each with 0.07 left over. It does not fill a 16 inch pot: that one takes 1.63 cubic feet with 14 inches of mix in it, so one bag falls 0.13 short and the page asks for 2. A 1 cubic foot bag is one 12 inch pot and half of another. The page does the division the other way round, from the pots you are filling to the bags you need, and it always rounds the bag count up, because half a bag is not something a shop sells.
Does the calculator allow for the pot tapering and for the plant?
No, and it says so. It measures a cylinder of the diameter and height you give it, so a pot that narrows toward its base holds a little less than the figure, and the figure is the top of the range rather than the middle. A root ball takes up room the calculator cannot see either: potting on a plant from a 6 inch pot into a 12 inch one displaces roughly 0.08 cubic feet of that 0.65. Drainage material in the bottom comes off the total as well. For one pot the difference is small, and for twenty it is worth measuring the depth of mix you will really add rather than the full height of the pot.
Is potting mix measured in litres or cubic feet?
Both, depending on where you are buying. This page prints litres for every answer, so a 12 inch pot with 10 inches of mix reads 19 litres as well as 0.65 cubic feet, and a 30 centimetre pot with 25 centimetres of mix reads 18 litres. The bag field itself is in cubic feet, so convert a litre sack once by dividing by 28.32: a 40 litre sack is 1.41 cubic feet and a 50 litre sack is 1.77. Type that in and every bag count on the page is worked from your sack rather than from a size we guessed.

Exact arithmetic on what you enter: length by width by depth, or the cylinder formula for a pot, converted without rounding until the last step. It cannot know how far soil settles after its first watering, how much room root balls take, or what a brand actually puts in a bag.