Raised Bed Soil Calculator

Enter your bed dimensions to get the volume in cubic metres, cubic feet and bags. No more guessing at the garden centre.

How the calculation works

Volume = length × width × depth, all in the same unit. That is the whole formula — the calculator just does the conversions and rounds the bag count up.

Soil mix ratios

For herbs and Mediterranean plants, push the sand/perlite share to 20%. For hungry crops like tomatoes and squash, go 50% topsoil and 40% compost.

What to put in the bottom

• Cardboard — one layer, to smother weeds. It breaks down within a season.
• Not plastic or landscape fabric — it creates a saturated layer and blocks worms.
• Not gravel — it does not improve drainage in a raised bed; it just raises the water table inside the bed.
• Logs and branches — a hugelkultur base works, but expect the soil level to drop as they rot. Fill to full depth anyway.

Cheaper ways to fill a deep bed

A 40 cm bed needs a lot of bought soil. Two options that work:

1. Fill the bottom third with organic matter — logs, leaves, grass clippings, kitchen scraps. Top the last 25 cm with proper mix.
2. Build shallower — a 25 cm bed costs about 40% less to fill and grows most vegetables fine.

Common questions

How much soil does a 4x8 raised bed need?

A 4x8 ft bed at 12 inches deep needs about 32 cubic feet, or roughly 0.9 cubic metres — around 23 bags of 40 litres.

How many bags of soil for a raised bed?

Divide the volume in litres by the bag size and round up. A 1.2 x 2.4 m bed at 30 cm needs about 864 litres, so 22 bags of 40 litres.

Should I fill a raised bed with only compost?

No. Pure compost compacts, holds too much water and is expensive. Mix it with topsoil and a drainage component.

How deep should the soil be?

30 cm suits most vegetables. 15 cm works for lettuce and herbs; 40 cm or more for carrots, tomatoes and potatoes.