Potting Soil Calculator
Potting mix is sold in quarts, liters or cubic feet, while pots are described by their diameter in inches or centimeters – so it is hard to tell how many bags will fill your containers. This potting soil calculator finds the volume of round pots (including tapered ones), square planters and window boxes, multiplies by the number of containers, and converts the result into quarts, gallons, liters, cubic feet and bags.
How to use this calculator
- Choose imperial (inches) or metric (centimeters).
- Select round or square/rectangular. Measure the inside of the container, not the rim.
- For tapered pots, also enter the bottom diameter – this gives a much more accurate volume than treating the pot as a cylinder.
- Enter the depth and how many identical containers you are filling.
- Keep the headroom option ticked to leave space for watering, then read the quarts or liters and the number of bags.
Container volume formulas
A straight-sided round pot is a cylinder: volume = π × (diameter ÷ 2)² × depth. A tapered pot is a frustum of a cone: volume = π × depth ÷ 12 × (D² + D × d + d²), where D is the top diameter and d is the bottom diameter. A square or rectangular planter is simply length × width × depth. In imperial units the result in cubic inches is divided by 1,728 to get cubic feet; one cubic foot is about 7.48 US gallons, or 29.9 quarts. In metric, cubic centimeters ÷ 1,000 = liters.
Worked examples
A 12 inch diameter pot that is 12 inches deep holds π × 6² × 12 = 1,357 cubic inches, or 0.79 cu ft – about 5.9 gallons or 23.5 quarts. If the same pot tapers to 8 inches at the base, its volume drops to about 0.66 cu ft (roughly 19.7 quarts), which is why the bottom diameter matters. A 40 × 40 cm planter 50 cm deep holds 80 liters; two of them need 160 liters, or four 40 L bags.
What do nursery pot sizes mean?
Nursery containers labeled “#1”, “#5” or “1 gallon” are trade sizes, not exact volumes. A #1 pot actually holds about 2.5–3.5 quarts, less than a true gallon. If you are repotting into a nursery-size container, measure it and use the calculator rather than trusting the label.
Choosing a potting mix
Use a soilless potting mix for containers rather than garden soil or topsoil. Garden soil compacts in pots, drains poorly and can carry weed seeds and diseases. Good potting mixes are based on peat moss, coconut coir, composted bark or a blend, with perlite or pumice for drainage. Vegetables and annual flowers appreciate a mix with added compost or slow-release fertilizer; cacti and succulents need a gritty, fast-draining mix; and long-lived shrubs in large containers benefit from a heavier, bark-rich mix that holds its structure.
Saving soil in large planters
Tall planters can swallow a surprising amount of mix. Most annuals, herbs and vegetables root in the top 12 to 18 inches, so many gardeners fill the bottom of very deep containers with an upturned plastic pot, crushed empty bottles or a false bottom, then add potting mix above. Avoid a layer of gravel at the bottom – it actually raises the water table in the pot rather than improving drainage.
Frequently asked questions
How much potting soil for a 12 inch pot?
A straight-sided 12 × 12 in pot holds about 23.5 quarts (0.79 cu ft). A typical tapered 12 inch pot holds closer to 15–20 quarts, depending on its depth and base.
How many quarts are in a cubic foot of potting soil?
One cubic foot equals about 29.9 US dry-volume quarts by the gallon conversion used on most bags (7.48 gallons × 4). Many brands round this to 25–30 quarts per cubic foot.
How many liters of soil for a window box?
Multiply length × width × depth in centimeters and divide by 1,000. A 60 × 18 × 18 cm window box holds about 19 liters.
Can I reuse old potting soil?
Yes, if the previous plants were healthy. Remove old roots, loosen the mix and refresh it with about a quarter to a third fresh compost or potting mix plus some slow-release fertilizer.
Should I fill the pot to the top?
No. Leave about an inch (2–3 cm) of space below the rim so water can pool and soak in rather than running off. The headroom option in the calculator accounts for this.