How much potting mix do you need?
Buying potting soil by guesswork often means a second trip to the garden center, or a half-used bag drying out in the garage. The volume of a pot depends on its diameter, its height and how much it tapers. This calculator works it out for round pots, rectangular planters and raised beds, then converts it into bags.
The formulas
- Tapered round pot (frustum of a cone): V = π × h ÷ 12 × (D² + D × d + d²), with D the top diameter, d the bottom diameter and h the height
- Straight-sided pot: set the bottom diameter equal to the top
- Rectangular planter or bed: V = length × width × depth
- Soil needed = volume × filling level × number of containers
- Bags = total ÷ bag size, rounded up
Worked example
You are filling four 12-inch pots, 10 inches deep, that narrow to 9 inches at the bottom.
- Volume of one pot: π × 10 ÷ 12 × (144 + 108 + 81) in³ = 872 in³, or 14.3 liters.
- Filled to 90%: 11.7 dry quarts (12.9 L) per pot.
- Four pots: about 1.8 cubic feet.
- With 1 cubic foot bags, buy 2 bags.
Soil for common pot sizes
Tapered pots, filled to 90%:
| Top diameter | Typical height | Soil per pot |
|---|---|---|
| 6 in | 5 in | 1.5 qt |
| 8 in | 7 in | 3.6 qt |
| 10 in | 9 in | 7.3 qt |
| 12 in | 10 in | 11.7 qt |
| 14 in | 12 in | 19 qt |
| 16 in | 14 in | 29 qt |
US potting soil bags are labeled in dry quarts or cubic feet: 1 cubic foot is about 25.7 dry quarts, or 28.3 liters.
Raised beds
A 4 × 8 ft bed filled 10 inches deep holds about 24 cubic feet at 90%, nearly a cubic yard. At that scale, bulk soil delivered by the cubic yard is usually cheaper than bags. A common mix for beds is roughly 60% topsoil, 30% compost and 10% potting mix or other amendments.
Tips
- Leave a gap: about an inch below the rim lets water soak in without spilling over.
- Do not put gravel at the bottom. It raises the water table in the pot rather than improving drainage. Use a pot with holes instead.
- Mixes settle by 10 to 20% after the first few waterings, so keep a little extra to top up.
- Large containers can be partly filled with a light filler such as upturned plastic pots, if the plants have shallow roots.
- Store leftover mix in a closed bag so it stays moist and usable for next season.