How to Calculate Soil for Raised Garden Beds
Raised garden beds require enough soil to fill the entire planting volume. The calculation is straightforward: multiply the length by the width by the depth of each bed to get the volume. This calculator adds a 10% overfill factor to account for soil settling after watering, which is a common issue gardeners often forget to budget for.
Once you have the total cubic volume, the calculator converts it to bag equivalents — the most practical unit for buying soil at a garden centre. Common bag sizes are 1 cubic foot, 2 cubic foot (the most popular size in the US), and 50-litre bags (common in the UK, Australia, and Canada).
Ideal Raised Bed Soil Depth
- 6 inches (15 cm): minimum for shallow-rooted crops like lettuce, herbs, and radishes.
- 12 inches (30 cm): recommended for most vegetables including tomatoes, peppers, and beans.
- 18 inches (45 cm): best for deep-rooted crops like carrots, parsnips, and potatoes.
- 24 inches (60 cm): ideal if you are building over compacted soil or paving.
What Soil Mix to Use in Raised Beds
The classic Mel's Mix for raised beds is one-third each of peat moss (or coconut coir), coarse vermiculite, and blended compost. A simpler and more affordable option is 60% topsoil, 30% compost, and 10% perlite or coarse horticultural grit for drainage. Avoid using garden soil on its own — it compacts badly and drains poorly in a contained raised bed.
- Topsoil provides bulk and nutrients — look for screened topsoil without large clumps or rocks.
- Compost improves fertility and moisture retention — aged compost is better than fresh.
- Perlite or vermiculite improves drainage and aeration, especially important in deep beds.
- Coco coir is a sustainable alternative to peat moss for moisture retention.
Frequently Asked Questions
Why add a 10% overfill factor?
Soil settles after it is watered, particularly in the first season. Organic matter decomposes and shrinks over time. Adding 10% extra ensures your beds stay full and you have enough to top up the following spring without another trip to the garden centre.
How many bags of soil do I need for a 4×8 raised bed?
A standard 4×8 ft raised bed at 12 inches deep holds about 32 cubic feet (with the 10% overfill that becomes roughly 35 cubic feet). You would need approximately 35 one-cubic-foot bags, 18 two-cubic-foot bags, or 20 fifty-litre bags. The exact number depends on how firmly the soil is packed in each bag.
Can I use the same soil calculator for in-ground beds?
Yes — simply enter the length, width, and the depth of soil you plan to add on top of or to mix into the existing ground. For top-dressing, a depth of 3–4 inches is typical.