What to measure
How to use the potting soil calculator
Choose the kind of container. For a round pot, measure across the top inside the rim and the depth you will fill. For a rectangular planter or window box, measure the inside length, width and soil depth. For grow bags and nursery pots sold by size, enter the gallons on the label. Then enter how many containers of that size you are filling.
For containers of different sizes, work out each size separately and add them up.
The formula
- Round pot = 3.1416 × (diameter ÷ 2)² × depth
- Planter = length × width × depth
- Grow bag = gallons × 231 cubic inches
- Dry quarts = cubic inches ÷ 67.2, or cubic feet × 25.71
- Bags = quarts (or cubic feet) ÷ bag size, rounded up
Worked example: four 12 in pots, 10 in deep
- 3.1416 × 6² × 10 = 1,131 cubic inches = 16.8 quarts per pot
- 4 pots: 67.3 quarts, or 2.62 cubic feet
- 67.3 ÷ 8 → 9 bags of 8 qt, or 2.62 ÷ 1.5 → 2 bags of 1.5 cu ft
Potting mix for round pots
Straight-sided pots, filled to the depth shown. Tapered pots take a little less.
| Pot (across × soil depth) | Dry quarts | Cubic feet | 8 qt bags |
|---|---|---|---|
| 6 × 5 in | 2.1 | 0.08 | 1 |
| 8 × 7 in | 5.2 | 0.2 | 1 |
| 10 × 8 in | 9.3 | 0.36 | 2 |
| 12 × 10 in | 16.8 | 0.65 | 3 |
| 14 × 12 in | 27.5 | 1.07 | 4 |
| 16 × 13 in | 38.9 | 1.51 | 5 |
| 20 × 16 in | 74.8 | 2.91 | 10 |
Potting mix for grow bags
| Grow bag | Dry quarts | Cubic feet | 1 cu ft bags |
|---|---|---|---|
| 1 gallon | 3.4 | 0.13 | 1 |
| 3 gallon | 10.3 | 0.4 | 1 |
| 5 gallon | 17.2 | 0.67 | 1 |
| 7 gallon | 24.1 | 0.94 | 1 |
| 10 gallon | 34.4 | 1.34 | 2 |
| 15 gallon | 51.6 | 2.01 | 3 |
| 20 gallon | 68.7 | 2.67 | 3 |
Common mistakes
- Reading liquid quarts as dry quarts. A 5 gallon pot holds 20 liquid quarts but only 17.2 dry quarts of mix. Compare bags by the dry quarts or cubic feet printed on them.
- Measuring the outside of the pot. Thick plastic, terracotta and wood walls take up space. Measure inside.
- Counting the full height. Leave room below the rim for watering, and subtract any drainage layer at the bottom.
- Buying exactly the volume. Fresh mix settles after the first few waterings. Keep a little extra for topping up.
Frequently asked questions
How many quarts of potting soil are in a cubic foot?
About 25.7 dry quarts. Potting mix is sold in US dry quarts of 67.2 cubic inches each, and a cubic foot is 1,728 cubic inches, so 1,728 ÷ 67.2 = 25.71.
How much potting soil do I need for a 12 inch pot?
About 16.8 quarts (0.65 cu ft) for a straight-sided pot 12 inches across filled 10 inches deep. That is a little over two 8 quart bags. Most pots taper toward the base, so they take somewhat less.
How much soil fills a 5 gallon grow bag?
About 17.2 dry quarts, or 0.67 cubic feet. Five US gallons is 1,155 cubic inches, and a dry quart is 67.2 cubic inches.
Why is a 5 gallon pot not 20 quarts of potting mix?
Gallons on pots are liquid gallons, and bagged mix is measured in dry quarts. A dry quart is about 16% bigger than a liquid quart (67.2 vs 57.75 cubic inches), so 5 gallons holds 17.2 dry quarts, not 20.
How many 8 quart bags make a cubic foot?
A little over three. One cubic foot is 25.7 dry quarts, and 25.7 ÷ 8 = 3.2, so a 1 cu ft bag holds about as much as three 8 qt bags plus a bit.
How much potting mix is in a bag labeled in liters?
Divide the liters by 28.3 to get cubic feet, since one cubic foot is 28.3 liters. The calculator also gives the total in liters, so you can compare it with the bag directly.
Should I fill the pot to the rim?
No. Leave about an inch below the rim so water can pool and soak in instead of running over the edge. Enter the depth you will actually fill.
Sources
Formulas and defaults last reviewed October 30, 2026. How we check our math.