How Do You Calculate How Much Potting Soil You Need?
Potting soil volume equals the container's inside volume up to the fill line. A tapered round pot is a frustum: V = π × h ÷ 3 × (R² + R×r + r²), with R and r the top and bottom radii. Divide cubic inches by 67.2 for dry quarts.
Measure the inside of the pot and subtract about 1 inch from the height for watering space. In the 12-inch example, the fill height is 9 inches, the diameter at the fill line is 11.7 inches, and the volume is π × 9 ÷ 3 × (5.85² + 5.85 × 4.5 + 4.5²) = 761 cubic inches. That equals 11.3 dry quarts, 12.5 liters or 3.3 liquid gallons. Four of these pots need 45.3 dry quarts, or three 16 quart bags.
Straight-sided containers are simpler. A cylinder holds π × radius² × fill height, and a rectangular planter or window box holds length × width × fill height. A 24 × 8 inch window box filled 9 inches deep holds exactly 1,728 cubic inches — 1 cubic foot, or 25.7 dry quarts. The calculator is one of PlantoScan's free garden planning calculators.
Plant Pot Size Guide: How Much Soil Does Each Pot Hold?
A 6-inch pot holds about 1.5 dry quarts (1.7 L) of potting mix, an 8-inch pot about 3.4 quarts (3.8 L), a 10-inch pot about 7 quarts (7.7 L), a 12-inch pot about 12 quarts (13.6 L) and a 16-inch pot about 30 quarts (33 L).
Pot sizes in the US refer to the inside diameter across the top. Volume grows with roughly the cube of the diameter, so a 12-inch pot holds about 3.6 times as much soil as an 8-inch pot, not 1.5 times. Shallow bulb pans and tall, narrow pots of the same diameter hold noticeably less or more, so the chart is a starting point rather than a spec.
| Pot size (top) | Typical top × base × height | Potting mix | Liters | Liquid gallons |
|---|---|---|---|---|
| 4 in (10 cm) | 4 × 3 × 3.6 in | 0.4 qt | 0.5 L | 0.12 gal |
| 6 in (15 cm) | 6 × 4.5 × 5.4 in | 1.5 qt | 1.7 L | 0.45 gal |
| 8 in (20 cm) | 8 × 6 × 7.2 in | 3.4 qt | 3.8 L | 1 gal |
| 10 in (25 cm) | 10 × 7.5 × 9 in | 7 qt | 7.7 L | 2.03 gal |
| 12 in (30 cm) | 12 × 9 × 10.8 in | 12.4 qt | 13.6 L | 3.6 gal |
| 14 in (36 cm) | 14 × 10.5 × 12.6 in | 20 qt | 22 L | 5.82 gal |
| 16 in (41 cm) | 16 × 12 × 14.4 in | 30.3 qt | 33.3 L | 8.81 gal |
Approximate volumes for a standard round pot (height ≈ 90% and base ≈ 75% of the top diameter), filled ½ in below the rim for 4–6 in pots and 1 in below for 8 in and larger. Quarts are US dry quarts.
How Much Soil Fits in a 1-, 3- or 5-Gallon Nursery Pot?
Nursery “gallon” pots hold less than their name suggests. A #1 container typically holds about ¾ of a liquid gallon (2.8 L), a #3 about 2¾ gallons (10.4 L) and a #5 about 4 gallons (15 L). Volumes vary by manufacturer.
The nursery trade names containers by class number (#1, #2, #3, #5), and the “gallon” label is a trade size rather than a measured liquid volume. A root ball from a #1 container takes up about 2.6 dry quarts, so a plant moved from a #1 container into a 10-inch pot needs roughly 7 − 2.6 ≈ 4.4 quarts of fresh mix. Floor-standing large houseplants such as fiddle-leaf figs and monsteras often arrive in #3 or #5 grow pots and move into 14- to 16-inch decorative pots.
| Nursery container | Typical capacity | Potting mix | Common use |
|---|---|---|---|
| #1 (“1 gallon”) | ≈ 0.75 gal | 2.6 qt / 2.8 L | Perennials, small shrubs, houseplants |
| #2 (“2 gallon”) | ≈ 1.75 gal | 6 qt / 6.6 L | Shrubs, ornamental grasses |
| #3 (“3 gallon”) | ≈ 2.75 gal | 9.5 qt / 10.4 L | Larger shrubs, young fruit trees |
| #5 (“5 gallon”) | ≈ 4 gal | 13.7 qt / 15.1 L | Trees, large shrubs, patio vegetables |
Approximate; trade container volumes vary by manufacturer. Quarts are US dry quarts; gallons are US liquid gallons (231 in³).
How Many Bags of Potting Soil Do You Need?
Divide the total potting mix volume by the bag size and round up. Common bag sizes are 8 quarts (8.8 L), 16 quarts (17.6 L), 1 cubic foot (25.7 quarts, 28.3 L), 2 cubic feet (51.4 quarts, 56.6 L) and 50 liters in metric markets.
Potting soil bags in the US are labeled in dry quarts or cubic feet. A US dry quart is 67.2 cubic inches (1.101 L), slightly larger than a liquid quart (57.75 cubic inches, 0.946 L), so one liquid gallon of pot volume equals about 3.4 dry quarts. The calculator uses dry quarts for bag math and liquid gallons for pot capacity. Compressed bales list both a compressed and an expanded volume; use the expanded (loose) figure.
Many gardeners blend their own mix for large outdoor containers, typically adding compost at up to about one-quarter of the volume for extra nutrients. The compost calculator for beds and lawns covers the compost side of that blend and any in-ground beds nearby.
How Full Should You Fill a Pot With Potting Soil?
Fill a pot with potting soil to about 1 inch (2.5 cm) below the rim, or ½ inch for pots under 6 inches. The gap holds water long enough to soak in instead of running over the edge.
Moisten dry potting mix before filling, firm it gently around the root ball, and keep the plant at the same depth it grew before. Fresh mix settles after the first thorough watering, often by an inch or more in deep pots, so top it up once. Every pot needs a drainage hole. A layer of gravel or pot shards in the bottom does not improve drainage; it raises the saturated zone closer to the roots and takes up space that soil could fill.
Cacti and succulent plants that need fast-draining soil grow best in a gritty mix with extra perlite, pumice or coarse sand, and in shallow pots that dry out quickly between waterings.
What Size Pot Does a Plant Need?
A plant needs a pot 1 to 2 inches (2.5–5 cm) wider than its current root ball or nursery pot. Trees and large floor plants can move up 2 to 4 inches. Pots that are much too large hold wet soil that leads to root rot.
Repot when roots circle the bottom, poke out of the drainage holes or the plant dries out within a day or two of watering. Moving up one size at a time keeps the soil volume in proportion to the roots, which use up the water before the mix stays soggy. Small houseplants for desks and shelves usually stay happy in 4- to 6-inch pots for years, and a single 8 quart bag fills about five 6-inch pots.
Frequently Asked Questions
- How many pots will a 16 quart bag of potting soil fill?
- A 16 quart bag fills about ten 6-inch pots, four 8-inch pots, two 10-inch pots or one 12-inch pot, filled to about an inch below the rim. Pot shapes vary, so measure your containers for an exact figure.
- How many quarts are in a cubic foot of potting soil?
- One cubic foot holds about 25.7 US dry quarts, the unit printed on potting soil bags, or about 28.3 liters. A 2 cubic foot bag therefore holds about 51 dry quarts, a little more than three 16 quart bags.
- Can I use garden soil instead of potting soil in pots?
- Garden soil is a poor choice for containers because it compacts in a pot, drains slowly and leaves roots short of air. Potting mix is a lighter blend of peat or coir, bark, perlite or pumice and often compost, built to drain freely in a container.
- Can you reuse old potting soil?
- Old potting soil can be reused for healthy plants once old roots are removed and it is refreshed with about one part fresh mix or compost to three parts old mix. Discard mix from plants that had root rot, pests or disease.