This garden bed calculator works out how much soil or compost a raised bed needs to fill it, and how many plants fit at a given spacing. It saves the frustration of ordering too little soil to finish a bed — or far too much — and helps you plan a productive planting layout.
Soil volume
Filling volume is simply length × width × depth. The calculator multiplies the bed's footprint by the fill depth to give the volume in litres or cubic metres (or cubic feet/yards), which is what you order soil and compost by. Bagged compost is sold by volume, so converting straight to litres tells you how many bags to buy.
Two things catch people out. Soil settles after watering, so filling a little proud and topping up later is normal. And you rarely want to fill to the very brim — leaving a margin stops soil washing over the edge when you water — so set the fill depth slightly below the bed height.
Plant spacing and yield
How many plants a bed holds depends on the spacing each crop needs. Dividing the bed area by the area each plant occupies (spacing squared, or using a grid as in square-foot gardening) gives a realistic count. Crowding reduces yield and invites disease, so the spacing figure matters as much as the bed size when planning what to grow.
Filling a 2 m × 1 m raised bed to a depth of 0.3 m.
- Volume = 2 × 1 × 0.3 = 0.6 m³.
- Convert to litres: 0.6 m³ × 1000 = 600 litres.
- At 50 litres per bag, that's 12 bags of compost.
The bed needs about 600 litres of fill — roughly 12 fifty-litre bags.
