Potting Soil Calculator
Calculate how many bags of potting soil you need to fill containers or window boxes at your depth, plus total cost.
How this calculator works
Potting soil (potting mix) is sold in bags for filling containers and planters. This calculator converts your container’s dimensions and fill depth into cubic yards, then divides by the standard 1.5 cubic foot bag size to give you a bag count.
Formula: Cubic yards = (Length ft × Width ft × Depth ft) ÷ 27. Bags = (Cubic yards × 27) ÷ 1.5.
Worked example
A 4 ft × 2 ft container area at a 10-inch fill depth:
- Depth in feet: 10 ÷ 12 ≈ 0.8333 ft
- Cubic yards: (4 × 2 × 0.8333) ÷ 27 ≈ 0.2469 yd³
- Bags: (0.2469 × 27) ÷ 1.5 ≈ 4.44 bags
- At $9/bag, that’s $40.00.
Coverage reference
| Container size | Bags needed (10 in) |
|---|---|
| 2 × 1 ft | 1.11 bags |
| 4 × 2 ft | 4.44 bags |
| 8 × 4 ft | 17.78 bags |
How to measure your containers
Measure each container’s footprint (length and width) at the soil line and the depth you’ll actually fill, not the container’s full height, since most pots leave an inch or two of headspace below the rim. Add multiple containers together by total footprint if you’re buying potting soil in bulk for several planters at once.
Frequently asked questions
Why can't I just use garden soil in pots and containers?
Garden soil compacts badly in a container, where it doesn't have surrounding soil structure or earthworm activity to keep it loose, which suffocates roots and drains poorly. Container potting mix is formulated lighter and airier specifically to avoid that compaction problem.
What's actually in a bag of potting soil?
Most potting mixes are largely peat moss or coco coir blended with perlite or vermiculite for drainage and aeration, often with a starter dose of fertilizer. Many products marketed as 'potting soil' contain no actual soil at all, which is why some brands call it 'potting mix' instead.
How big of a bag does this calculator assume?
This calculator uses a standard 1.5 cubic foot bag, the most common size sold for containers and raised planters. Check your specific bag's printed cubic footage if it's a different size, since some brands sell smaller 1 or 2 cubic foot bags.
Do I need to replace potting soil in containers every year?
Annual containers usually benefit from fresh mix each season since the old mix compacts and loses nutrients, but perennial containers and larger planters can often go 2-3 years between full refreshes if you top-dress with compost annually. Root-bound or waterlogged containers are the clearest sign it's time for a full change.
How much soil does a typical window box need?
This calculator's default 4 ft × 2 ft, 10-inch-deep frame is sized for a generous window box or small raised planter — a standard single window box is usually a fraction of that footprint, so adjust the dimensions to your actual box before ordering.
Estimates only. Verify quantities with your supplier before purchasing.