Raised Bed Soil Calculator — Volume & Soil Mix

A widely-used soil mix for raised garden beds is 70% topsoil and 30% compost (or well-rotted organic matter) — a common landscaping convention rather than a single published standard. This provides good structure from the topsoil with excellent nutrient content and drainage from the compost. The RHS suggests bulking screened topsoil with around a third organic matter, adding sharp sand on heavier soils. For vegetable growing, some gardeners prefer 60% topsoil, 30% compost and 10% perlite for extra drainage. Need topsoil for a larger area? Use our Topsoil Calculator.

A minimum depth of 200mm (8 inches) is needed for most plants. 300mm (12 inches) is ideal for vegetables, herbs and most flowers. For root vegetables like carrots and parsnips, aim for 400-450mm deep. If the raised bed sits on existing soil, roots can grow deeper into the ground below. This raised bed volume calculator supports any depth you enter.

A standard 4ft x 8ft (1.2m x 2.4m) raised bed at 300mm deep needs approximately 0.86 m³ of soil — roughly 1-2 bulk bags depending on bag size. Using the 70/30 mix, that is about 0.6 m³ topsoil and 0.26 m³ compost.

Popular options include timber sleepers, treated softwood planks, concrete blocks, or gabion baskets. For taller retaining walls (over 600mm), consider our Retaining Wall Calculator to estimate materials. For a lawn around the raised bed, see our Turf Calculator.

For beds over 300mm deep, you can line the bottom with cardboard or newspaper to suppress weeds, then add a layer of small branches, leaves or garden waste (hugelkultur method) before adding soil. This saves on soil and improves drainage. Do not use landscape fabric at the bottom as it restricts root growth.