Potting Soil Calculator
Calculate container volume, bag counts, and optional potting mix component volumes.
Last reviewed: June 2026Soil Mix Ratios (optional)
Adjust percentages to calculate component volumes. Totals should add up to 100.
Bag Calculator
What this potting soil calculator estimates
This potting soil calculator estimates how much potting mix is needed to fill containers with common shapes: round cylinder pots, rectangular planters, tapered frustum pots, and hemisphere bowls. It converts the calculated volume into liters, gallons, cubic feet, quarts, and common bag counts.
The calculator does not choose a plant or location. It is a container-volume tool. Plant selection, pot size, root depth, drainage holes, watering frequency, fertilizer, pH, and soil mix ingredients still need plant-specific judgment.
How the geometry works
- Round pot: cylinder volume = pi x radius^2 x height.
- Rectangular planter: box volume = length x width x height.
- Tapered pot: frustum volume = pi x height / 3 x (top radius^2 + top radius x bottom radius + bottom radius^2).
- Hemisphere pot: half-sphere volume = 2 / 3 x pi x radius^3.
Imperial dimensions are entered in inches. Metric dimensions are entered in centimeters and converted internally before the same volume calculation. Bag counts round up because partial bags usually are not useful when shopping.
Worked examples
With the default round 12-inch diameter by 10-inch high pot, the modeled volume is 18.5 L, or 4.90 gal and 0.65 cu ft. That rounds up to 3 eight-quart bags, 1 one-cubic-foot bag, 1 one-and-a-half-cubic-foot bag, or 1 two-cubic-foot bag.
For three tapered pots with 14-inch top diameter, 10-inch bottom diameter, and 12-inch height, the calculator estimates 67.3 L total and 22.4 L per pot. With a 60% potting soil, 25% perlite, and 15% peat moss mix, component volumes are 40.4 L, 16.8 L, and 10.1 L.
In metric mode, two rectangular planters that are 60 cm by 30 cm by 25 cm need about 90.0 L total, or 45.0 L per planter.
Buying and filling tips
- Buy a buffer: Add roughly 5% to 15% for settling, uneven pot interiors, root balls, and spills.
- Do not block drainage: Container gardening guidance consistently emphasizes drainage holes and a suitable potting mix.
- Do not use garden soil by default: Garden soil can compact in containers and drain poorly compared with container mix.
- Match depth to roots: The calculator estimates fill volume; it does not decide whether the container is deep enough for the plant.
- Normalize mix percentages: If the mix ratios do not total 100, the calculator normalizes them against their sum.
Sources and further reading
For container-gardening guidance, see Oregon State University Extension on container gardening basics, University of Maryland Extension on growing media for containers, and NC State Extension's gardener handbook chapter on plants grown in containers.
Frequently Asked Questions
Quick reference
| Output | Meaning |
|---|---|
| Total soil needed | Total modeled volume across all pots. |
| Per pot volume | Modeled volume for one selected pot shape. |
| Bag counts | Rounded-up shopping estimates for 8 qt, 1 cu ft, 1.5 cu ft, and 2 cu ft bags. |
| Component volumes | Total volume split by normalized mix percentages. |