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Landscaping & Outdoor

Potting Soil Calculator

Soil volume for pots, planters, raised beds and window boxes, in the bag sizes garden centres sell.

Your measurements

Container
Fill depth, not container depth. Leave an inch or two at the rim for watering.
Soil
Potting mix settles after the first few waterings. Most people under-order without this.

Your result

Sample values

7

Bags of 1.5 ft³

11.0

Cubic feet

0.41

Cubic yards

82

US gallons

Six 24 × 12 in planters filled to 10 in, 10% settling allowance.

What to order

Bag size Quantity
1.5 ft³ bags 7 bags
2 ft³ bags 6 bags
1 ft³ bags 11 bags
Bulk, by the yard 0.41 yd³

How to use this calculator

Measure the inside of the container and the depth you actually intend to fill, leaving an inch or two below the rim so water does not run straight over the edge. For round pots, measure the internal diameter at the soil line rather than the rim, which is usually wider.

Add a settling allowance. Bagged potting mix is compressed and full of air; after a fortnight of watering it will sit noticeably lower than the day you filled it, and topping up later disturbs roots.

Formula

Rectangular: Volume = Length × Width × Depth

Round: Volume = π × Radius² × Depth

Bags = Total volume ÷ Bag size

Depth
fill depth, not container depth
1 ft³
about 7.5 US gallons

Worked example

Six rectangular planters, 24 in by 12 in, filled to 10 in, 1.5 ft³ bags.

  1. 24 × 12 × 10 = 2,880 in³ per planter
  2. 2,880 ÷ 1,728 = 1.67 ft³ per planter
  3. 1.67 × 6 = 10.0 ft³
  4. 10.0 × 1.10 = 11.0 ft³ with settling
  5. 11.0 ÷ 1.5 = 7.3, so 7 bags plus a little spare

11 cubic feet, about 7 bags of 1.5 ft³

Understanding your results

Potting mix is not topsoil and the two are not interchangeable. Container plants need a light, free draining mix that holds air as well as water; garden soil in a pot compacts, waterlogs and suffocates roots. For raised beds deeper than about a foot, a blend of topsoil and compost is usually both better and far cheaper than bagged potting mix.

Bulk delivery becomes worth pricing somewhere around a cubic yard, which is roughly eighteen of the 1.5 ft³ bags. Below that the bags usually win once you account for delivery.

Questions

About this calculator

Ask something else

No. It is a persistent myth. A gravel layer raises the saturated zone in the pot rather than lowering it, so roots sit wetter, not drier. Use a free draining mix and make sure the drainage holes are clear.

For most ornamentals, yes, if you refresh it. Tip it out, break up the root mass, mix in a third fresh compost and some slow release feed. Do not reuse mix from plants that had root disease, and start fresh for seedlings.

Work it out the same way, using the internal dimensions and your fill depth. Beds are large enough that bagged mix gets expensive fast — a 4 by 8 ft bed filled to 12 in needs about 32 cubic feet, which is a bulk delivery rather than a trolley of bags.

Bagged mix is compressed for transport and contains a lot of air. Watering collapses those voids and organic matter breaks down over the season. Ten percent is a reasonable allowance for the first settling; expect to top up annually after that.

Keep going

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Estimates, not specifications. Results are based on the values you enter and on typical material properties. Densities, coverage rates and product sizes vary between suppliers, so confirm quantities against your supplier's data before ordering. Structural work must be signed off by a qualified professional and meet your local building code.