Potting Soil Calculator
Find out exactly how much potting mix your containers hold. Choose round, tapered or rectangular pots (or a nominal gallon size), enter the size and how many you have, and get quarts, liters, cubic feet and the number of bags to buy.
Last updated:
Inside, across the rim.
Grow bags hold close to their size. Nursery "#5" trade pots hold only about 70-85% of real gallons.
Space below the rim for watering, in your units. About 1 in / 2-3 cm.
Optional gravel or filler depth, in your units.
How to use the Potting Soil Calculator
- Choose the pot shape. Most plastic and terracotta pots are tapered round.
- Measure inside the pot: diameter at the rim and the bottom, or length and width, and the depth.
- Enter the number of pots, headspace and any drainage layer.
- Pick the bag size sold at your garden center.
- Read the volume and bag count; compare bag sizes in the table to find the best deal.
Pot volume formulas
- Straight round pot: V = π × r² × h.
- Tapered round pot (frustum): V = π × h ÷ 3 × (R² + R·r + r²), with R the top radius and r the bottom radius.
- Rectangular planter: V = length × width × h.
- Conversions: 1 cu ft = 1,728 cu in = 25.71 dry qt = 28.32 L = 7.48 US gal; 1 L = 61.02 cu in.
Example: four tapered pots, 12 in top, 9 in bottom, 11 in deep, filled to 10 in. At the soil surface the pot is about 11.7 in wide, so V = π × 10 ÷ 3 × (5.86² + 5.86 × 4.5 + 4.5²) ≈ 848 cu in per pot = 12.6 qt; four pots need about 50.5 qt (56 L), just under two 1 cu ft bags.
Approximate potting mix per pot
| Pot (top diameter) | Typical depth | Potting mix (tapered pot) | Liters |
|---|---|---|---|
| 4 in | 3.5 in | ≈ 0.3 qt | ≈ 0.4 L |
| 6 in | 5.5 in | ≈ 1.4 qt | ≈ 1.5 L |
| 8 in | 7 in | ≈ 3.3 qt | ≈ 3.6 L |
| 10 in | 9 in | ≈ 7 qt | ≈ 7.7 L |
| 12 in | 11 in | ≈ 12.6 qt | ≈ 14 L |
| 14 in | 12 in | ≈ 19 qt | ≈ 21 L |
| 16 in | 14 in | ≈ 29 qt | ≈ 32 L |
| 20 in | 16 in | ≈ 53 qt | ≈ 58 L |
| 24 in | 20 in | ≈ 97 qt | ≈ 107 L |
Values assume a bottom diameter about three quarters of the top and 1 in of headspace. Pot shapes vary a lot, so use the calculator with your own measurements.
Frequently asked questions
How much potting soil do I need for a 12 inch pot?
A typical tapered 12 in pot (12 in across the top, 9 in at the bottom, 11 in deep) filled to 1 in below the rim holds about 3.7 gallons, or roughly 12.6 dry quarts (14 L) of potting mix. A straight-sided 12 in cylinder filled to the same level holds about 17 quarts.
How do I calculate the volume of a pot?
For a straight round pot: π × radius² × depth. For a tapered pot use the frustum formula: π × depth ÷ 3 × (R² + R × r + r²), where R and r are the top and bottom radii. For a rectangular planter: length × width × depth. Work in inches and divide cubic inches by 1,728 for cubic feet or multiply by 0.0164 for liters.
How many quarts are in a cubic foot of potting soil?
About 25.7 dry quarts. Potting mix is sold by dry volume; a 1 cu ft bag holds 25.7 dry quarts (28.3 L), and an 8 qt bag holds about 0.31 cu ft.
Why are bag sizes in dry quarts?
Soil and mulch are sold by dry volume. A US dry quart is 67.2 cubic inches (1.10 L), slightly larger than the 57.75 cubic-inch liquid quart used for milk or water, so this calculator uses dry quarts for bag counts.
Is a 5-gallon pot really 5 gallons?
Grow bags are usually close to their stated size. Nursery containers sold as #5 or 5-gallon are trade sizes and often hold only about 3.5-4.5 real gallons, so measure the pot if the exact amount matters.
Should I put gravel in the bottom of pots?
Research shows a gravel layer doesn't improve drainage; it raises the saturated zone in the pot. Use a well-draining mix and a pot with drainage holes instead. If you do use a filler layer, enter its depth to subtract it.
How much extra potting mix should I buy?
Buy about 10% more than calculated. Fresh mix is fluffy and settles after the first few waterings, and you'll want some to top up pots later.
Are all the tools on FreeToolCabin tested for quality and accuracy?
Yes! Every tool on our site undergoes automated Q/A testing in real browser environments to ensure high precision, correct output, and zero errors. If you happen to spot a bug or have a suggestion, please contact us at [email protected] and our team will review it right away.