Potting Soil Calculator
This potting soil calculator turns container size, depth and count into quarts, cubic feet and bags of mix for round pots, planters, window boxes and baskets.
By CalcBuilt Editorial TeamUpdated September 9, 2026Formulas checked against manufacturer specs, see how we calculate.
Results
- Potting mix needed
- 114 quarts (4.42 cu ft)
- Round pot x 6, filled to 1 in below the rim. Quarts on a mix bag are dry quarts, so 1 cu ft is 25.7 of them
- Per container
- 18.9 quarts
- 0.74 cu ft of mix each. Fresh mix settles about 10 to 15 percent after the first few waterings, so top it back up before you plant
- Quart bags to buy
- 8 bags of 16 qt
- Or 15 bags of 8 qt. Espoma and most brands sell 4, 8 and 16 qt bags; a 16 qt bag is 0.62 cu ft, so two of them are not 2 cu ft
- Cubic foot bags to buy
- 3 bags of 2 cu ft
- Or 5 bags of 1 cu ft. A 2 cu ft bag holds 51.4 dry quarts and is usually the cheapest way to buy mix by the cubic foot
- Nursery trade size it matches
- #7
- Each container holds 1,424 cu in to the rim. ANSI Z60.2-2025 Table 1 puts a #1 pot at 152 to 251 cu in and a #5 at 785 to 1,242 cu in, so the class number is a trade name, not a gallon count
Potting Soil formula
Fill depth (in) = Inside depth - Headspace below the rim
Round pot (cu in) = pi / 12 x Fill depth x (d² + d x dt + dt²), d = 0.75 x top diameter
Window box (cu in) = Width x Length x Fill depth; Grow bag = pi / 4 x Diameter² x Fill depth
Dry quarts = Cubic inches / 67.2; Cubic feet = Cubic inches / 1,728Worked example
Take two 36 in window boxes, 8 in wide and 7 in deep, filled to 1 in below the rim. Each holds 36 x 8 x 6 = 1,728 cubic inches, exactly one cubic foot. Two boxes take 2 cubic feet, or 51.4 dry quarts. That is one 2 cu ft bag, and nothing left over for the top-up after it settles.
Now six 5 gallon fabric grow bags, 12 in across and 12 in tall, filled to 1 in below the rim. Each holds 3.1416 / 4 x 12 x 12 x 11 = 1,244 cubic inches. Six come to 7,464 cubic inches, which is 4.32 cubic feet or 111 dry quarts. Three 2 cu ft bags cover it with about 1.7 cubic feet spare.
Quick reference
| Trade size | Volume range | Dry quarts | Cubic feet | Actual US gallons |
|---|---|---|---|---|
| #1 | 152 to 251 cu in | 2.3 to 3.7 qt | 0.09 to 0.15 cu ft | 0.66 to 1.09 gal |
| #2 | 320 to 474 cu in | 4.8 to 7.1 qt | 0.19 to 0.27 cu ft | 1.39 to 2.05 gal |
| #3 | 628 to 742 cu in | 9.3 to 11 qt | 0.36 to 0.43 cu ft | 2.72 to 3.21 gal |
| #5 | 785 to 1,242 cu in | 11.7 to 18.5 qt | 0.45 to 0.72 cu ft | 3.4 to 5.38 gal |
| #7 | 1,337 to 1,790 cu in | 19.9 to 26.6 qt | 0.77 to 1.04 cu ft | 5.79 to 7.75 gal |
| #15 | 2,768 to 3,696 cu in | 41.2 to 55 qt | 1.6 to 2.14 cu ft | 11.98 to 16 gal |
How to size potting mix for pots and planters
Work in cubic inches, because that is the only unit every container shape shares. A window box is width times length times fill depth. A straight-sided grow bag is pi / 4 times the diameter squared times the fill depth. A tapered patio pot of the same top diameter holds roughly 75 percent of that. Then divide by 67.2 for dry quarts or 1,728 for cubic feet.
The quart is where most orders go wrong. Potting mix is a dry commodity, so its quart is the dry quart of 67.200625 cubic inches from NIST Handbook 44, not the 57.75 cubic inch liquid quart. That makes a cubic foot 25.7 quarts. A 16 qt bag is therefore 0.62 cubic feet and a 2 cu ft bag is 51.4 quarts. In 2026 a 16 qt bag is typically $7 to $14, which works out to $11 to $23 a cubic foot. The same mix in a 2 cu ft bag is typically $16 to $32, or $8 to $16 a cubic foot. Buy the big bag once you need more than about 3 cubic feet.
Pots are sold by trade number, and the number is not a capacity. ANSI Z60.2-2025 Table 1 lets a #1 container be anywhere from 152 to 251 cubic inches, so it may hold 0.66 gallons or 1.09. A #5 runs 785 to 1,242 cubic inches, almost 7 dry quarts apart. If a spec really needs the gallon figure, the standard asks for a true-to-size pot, written #5T, at gallons times 231 cubic inches.
Why garden soil and topsoil fail in a container
A container has no subsoil under it to pull water down. Water leaves only by gravity and by evaporation, so a saturated layer sits on the floor of the pot no matter what is in it. NC State Extension is blunt about it. Field soils hold too much water, have too little pore space for oxygen, are too heavy, and can carry disease, insects and weed seed. Illinois Extension finds that drainage and aeration are both badly impeded once garden soil goes into a pot.
That is why bagged potting mix is mostly not soil. Commercial mixes are sphagnum peat moss, perlite, vermiculite, composted bark, coir and compost, limed to about pH 6.2. University of Maryland Extension keeps topsoil to 5 to 10 percent of the volume, even in very large containers. Bagged garden soil is built for beds and belongs in beds. Weight helps on a windy balcony. For that, Illinois Extension suggests one part garden soil, one part peat moss and one part perlite or coarse sand.
Skip the gravel, shards and packing peanuts at the bottom. The coarse layer raises the saturated zone into the root ball instead of draining it, which is exactly backwards. Use the full depth of the pot for mix and check that the drain holes are open. Stop filling about 1 in below the rim so water has somewhere to pond while it soaks in. UGA Extension moves that to 2 in on large containers.
What this calculator leaves out
The result is mix volume only. It does not price the pot, the saucer, the liner for a coir basket, the slow-release fertilizer, or the wall brackets a loaded window box needs. It assumes a standard taper on round and square containers, so a straight-sided cachepot or a heavily tapered nursery can will differ by a quart or two. It does not account for the root ball already in the pot, which can displace a third of the mix on a #3 shrub. It also leaves out the annual refresh: peat breaks down, so most gardeners replace or cut in half the mix in a food container each spring.
Code limits that apply
- ANSI Z60.2 Table 1
A #1 pot is 152 to 251 cu in, a #3 is 628 to 742, a #5 is 785 to 1,242
If you miss it: Sizing mix by the gallon on the label overshoots a #5 pot by up to 5.5 quarts and falls at most 1.3 quarts short
- ANSI Z60.2 1.1.3.4
A class number is a range, not a capacity; gallons x 231 cu in needs a true-to-size pot
If you miss it: Two suppliers both ship compliant #5 pots and the planting plan no longer matches the mix order
- NIST HB 44 App. C
One dry quart is 67.200625 cu in, so a cubic foot of mix is 25.71 dry quarts
If you miss it: Using the 57.75 cu in liquid quart calls a cubic foot 29.9 quarts and short-buys every bag count
- NIST HB 130 2.5.2.2
Peat and peat moss sold by cubic measure must be labeled in liters or cubic feet
If you miss it: A quart-only bag has no legal cubic declaration, so two brands' quarts need not agree
Rows marked checked produce a warning in the results when an entry crosses the limit. The others are on the plans or on site and this calculator does not test them. Local amendments can be stricter; the adopted edition and the inspector govern.
Mistakes that cost money
- A gravel layer in the bottom for drainage collects water in the mix just above it; NC State Extension says never add one.
- Two 16 qt bags are 1.24 cubic feet, not 2, so they fall nearly two fifths short of a 2 cu ft planter.
- Ordering by trade number alone: the ANSI #5 range runs 785 to 1,242 cubic inches, nearly 7 quarts apart.
- Filling flush with the rim leaves nowhere for water to pond, so it sheets off and the center of the root ball stays dry.
- Garden soil or bagged topsoil in a pot packs down and drowns roots, worst of all in a container under 10 in deep.
- A 14 in hanging basket 7 in deep holds about 8.4 quarts, under half a 14 in pot 12 in deep, so it dries first.
Key facts
- ANSI Z60.2-2025 Table 1 sets a #1 nursery container at 152 to 251 cubic inches, or 0.66 to 1.09 US gallons. A pot sold as one gallon can hold two thirds of one.
- A dry quart is 67.200625 cubic inches under NIST Handbook 44 Appendix C, so one cubic foot of potting mix is 25.7 quarts. A 2 cu ft bag holds 51.4 quarts.
- A #5 nursery container holds 785 to 1,242 cubic inches under ANSI Z60.2-2025. That is a spread of 6.8 dry quarts between the smallest and largest pot sold under one number.
- University of Maryland Extension limits topsoil to 5 to 10 percent of the volume in a container mix, and only in very large containers.
- University of Georgia Extension advises filling a container to within 2 inches of the top so water soaks in rather than running over the rim.
Frequently asked questions
How many quarts of potting soil are in a cubic foot?
25.7. Potting mix is measured in dry quarts of 67.200625 cubic inches each under NIST Handbook 44, and a cubic foot is 1,728 cubic inches. Dividing gives 25.71. The 29.9 you get from the liquid quart is the wrong conversion for bagged mix.
How much potting soil do I need for a 12 inch pot?
About 12.6 dry quarts, or 0.49 cubic feet, for a tapered 12 in pot 11 in deep filled to 1 in below the rim. One 16 qt bag fills it with a little left for the top-up after it settles.
Does a 1 gallon nursery pot hold a gallon?
Usually not. ANSI Z60.2-2025 Table 1 puts a #1 container at 152 to 251 cubic inches and a US gallon is 231, so most #1 pots hold 0.66 to 1.09 gallons. The number is a trade class covering a range, not a capacity.
Can I use garden soil or topsoil in pots?
Not on its own. It compacts in a container, cuts off oxygen to the roots and holds water at the base of the pot. University of Maryland Extension keeps topsoil to 5 to 10 percent of the volume, and only in very large containers.
Sources and references
- American Standard for Nursery Stock ANSI Z60.2-2025, Table 1 container class volume ranges (AmericanHort)
- NIST Handbook 44, Appendix C general tables of units (1 dry quart = 67.200625 cubic inches) (NIST)
- Growing Media (Potting Soil) for Containers (University of Maryland Extension)
- Extension Gardener Handbook, Chapter 18 Plants Grown in Containers (NC State Extension)
Next steps for this project
The usual order for a landscaping & outdoor project. See all landscaping & outdoor calculators.
Results are estimates based on standard formulas and typical product specifications. Confirm quantities with your supplier and local code before ordering.