Potting Soil Calculator
A potting soil calculator turns container size and fill depth into 1.5 cu ft bags. A 4 × 2 ft planter filled 10 in deep holds 6.67 cu ft, so 5 bags ($45).
What your order looks like
A few bags or one pickup trip will cover it.
0.25 yd³
2 wheelbarrow trips (4 cu ft each)
1 pickup load (2 cubic yards each)
Rules of thumb: A wheelbarrow trip of light material is 4 cubic feet. A pickup load of light material is 2 cubic yards.
A potting soil calculator converts the size of your container and how deep you will fill it into bags. The default is a 4 ft by 2 ft planter filled 10 inches deep. That is 4 × 2 × (10 ÷ 12) = 6.67 cu ft, or 4.44 bags at 1.5 cu ft each, which rounds up to 5 bags. At $9 per bag the cost is $45.
The formula in plain words
Multiply length, width, and fill depth to get the volume of soil. Depth is entered in inches, so the calculator divides it by 12 to match the feet. Then it divides the volume by the size of one bag.
You cannot buy 0.44 of a bag, so the count rounds up to a whole bag. Under the hood the calculator works in cubic yards first, then multiplies by 27 to get back to cubic feet. The result is the same.
There is no waste box for this calculator, because potting mix is poured into a container and little is lost. If you want a cushion for topping off after watering, add a bag by hand.
Worked examples
Example 1, the default: 4 ft by 2 ft planter, 10 inches deep, $9 per bag.
- Volume: 4 × 2 × 0.8333 = 6.67 cu ft
- Bags: 6.67 ÷ 1.5 = 4.44, rounded up to 5
- Cost: 5 × $9 = $45
Example 2, a window box: 3 ft by 1 ft, filled 8 inches deep.
- Volume: 3 × 1 × 0.6667 = 2.0 cu ft
- Bags: 2.0 ÷ 1.5 = 1.33, rounded up to 2, or $18
Example 3, a round pot, worked by hand since the calculator uses rectangles: 14 inch diameter, 12 inches deep.
- Area: 3.1416 × 7² = 153.94 sq in
- Volume: 153.94 × 12 = 1,847 cu in, and 1,847 ÷ 1,728 = 1.07 cu ft
- Bags: 1.07 ÷ 1.5 = 0.71 of a bag
Bags needed by container size
All at a 10 inch fill and 1.5 cu ft bags.
| Container footprint | Volume | Bags (rounded up) |
|---|---|---|
| 2 × 1 ft | 1.67 cu ft | 2 |
| 4 × 2 ft | 6.67 cu ft | 5 |
| 6 × 2 ft | 10.0 cu ft | 7 |
| 8 × 4 ft | 26.67 cu ft | 18 |
How to measure
Measure the inside length and width of the container at the soil line. Then measure the depth you will fill, which is less than the container’s height, since you leave an inch or two of headspace below the rim.
Raised containers with sloped sides hold less than a straight box. Measure the middle of the slope for the width. For tapered pots, average the top and bottom diameters.
Common mistakes
- Filling the container to the rim. Water pours over the top, and soil settles after the first watering.
- Assuming the bag size. Check the cubic feet printed on the bag before you buy.
- Forgetting drainage material. If you put gravel or large stones at the bottom, subtract that depth from the fill.
- Using the container’s outside dimensions. The walls take up space.
- Not counting the volume taken by a large plant’s root ball when you transplant.
Mixing your own
If you blend ingredients yourself, the same volume applies. For the 6.67 cu ft planter, a mix of three ingredients in equal parts is 6.67 ÷ 3 = 2.22 cu ft of each. Read the recipe for your plants, since seed-starting and succulent mixes differ from a general container mix. Fresh mix settles after the first watering, so keep a spare bag for topping off.
Large containers
Big planters hold a lot. A raised container of 6 ft by 2 ft filled 12 inches deep holds 12 cu ft and takes exactly 8 bags of 1.5 cu ft. Some gardeners fill the lower third with lightweight material to save soil and weight, in which case enter only the depth of potting mix. Heavy planters on balconies and roofs need a check on the structure before you load them with wet soil.
Related calculators
For in-ground or raised beds, use the garden soil calculator or the raised bed soil calculator. The compost calculator and the peat moss calculator help you blend your own mix.
Frequently asked questions
Why can't I just use garden soil in pots and containers?
Garden soil packs down in a container, drains slowly, and can bring weeds and pests with it. Potting mix is a lighter blend, often built on peat or coir with perlite or bark, that stays open so roots get air and water drains. Use potting mix for pots and garden soil for in-ground beds.
What's actually in a bag of potting soil?
Brands differ, but most are a blend of peat moss or coir for water holding, perlite or vermiculite for air space, and sometimes compost or bark. Some include slow-release fertilizer. The bag label lists the ingredients, so check it against what your plants prefer.
How big of a bag does this calculator assume?
1.5 cu ft per bag, priced at $9 by default. Bags range from smaller 1 cu ft sizes to 2 cu ft and larger, so check the size printed on yours. If you buy 2 cu ft bags, divide the cubic feet needed by 2 instead. The 6.67 cu ft example needs 4 of those.
Do I need to replace potting soil in containers every year?
Not always. Many gardeners refresh the top few inches and mix in compost or fertilizer each season, and replace all of it every few years or when it has shrunk, compacted, or hosted a diseased plant. Annual vegetables in pots usually do better with fresh mix.
How much soil does a typical window box need?
A box 3 ft long and 1 ft wide filled 8 inches deep holds 2.0 cu ft, which is 1.33 bags, so you buy 2 bags. A single round pot 14 inches across and 12 inches deep holds about 1.07 cu ft, or 0.71 of a bag.
Estimates only. Verify quantities with your supplier before purchasing.