Calculate Exactly How Much Soil Your Bed or Pot Needs

This tool was built by Serge (MSc) using established science and government sources. My academic background covers plant biology, environmental biology, and biogeochemistry. My tools are for educational purposes and give practical guidance, not professional advice.

Filling a raised bed or a large pot takes more soil than most people expect, and buying too little means a second trip to the store.

This calculator works out how much soil you need from your bed or pot size.

It gives you the volume in litres and cubic feet, plus a rough number of bags to buy.

The volume itself is exact geometry, but a small buffer is added to the buy amount, because fresh soil settles once you water it.

How this works

The maths here is plain volume.

For a rectangular bed, the calculator multiplies length by width by depth.

For a round pot, it uses the area of the circle, which is pi times the radius squared, then multiplies that by the depth.

Everything is converted to one unit first so the numbers line up.

The result is then shown in litres, cubic feet, and cubic yards, since soil is sold in all three depending on where you shop.

Understanding your result

The volume figure is what your bed or pot holds when filled to the depth you entered.

The bag counts are a guide, since bag sizes differ between brands, so check the size printed on the bag before you buy.

The recommended amount to buy sits a little above the exact volume.

Fresh soil and compost settle by around 10 to 20 percent after the first few waterings, so buying a bit extra saves you topping up later.

Important limitations

This gives you the volume of the space, not a promise of how many bags you will use, because soil density and settling differ between products.

It does not account for large rocks, soil already in the bed, or a drainage layer at the bottom.

If your bed sits on open ground and roots will grow into the soil below, you may not need to fill the full depth.

Treat the buy figure as a sensible starting point, not an exact order.

Scientific basis

The volume formula and the soil depth guidance behind this tool come from university extension services, the public gardening advice arm of the land-grant university system:

• University of Maryland Extension: Soil to Fill Raised Beds
• Oregon State University Extension: Raised Bed Gardening
• Mississippi State University Extension: Constructing Raised Beds

Keep Planning With These Calculators

I recommend these calculators alongside this one because each answers the next number question that usually follows once the soil volume is known.

The Repotting Soil Calculator sizes the potting mix for a houseplant moving into a bigger container, before a bag is opened.

Once the bed is filled, the Mulch Calculator works out how much is needed to cover the top at the right depth.

To check the balance of the mix itself, the Compost Ratio Calculator compares browns to greens.

When a soil test shows the pH is off, the Soil pH Amendment Calculator estimates how much lime or sulphur a bed needs.

Related reading

If you are filling a new bed, my guide on how to improve garden soil helps you choose what to put in it.

My article on what compost is and how it works explains the compost side of a good mix.

My piece on no-till gardening covers filling beds without digging.

My guide to soil test kits for gardeners explains what to look for before adding amendments to a new bed.

This calculator is for educational purposes and general guidance only. It is not professional horticultural or landscaping advice. Always check product labels and your local supplier before buying.