About SoilMath
SoilMath helps home gardeners turn a bed, pot or planting area into a practical amount of soil, compost or potting mix to buy. The tools are for growing spaces: raised beds, containers, lawns and garden top-dressing, with imperial inputs and metric equivalents.
From garden measurements to a shopping quantity
The raised-bed soil calculator multiplies length, width, fill depth and the number of identical beds. It then converts that volume into cubic feet, cubic yards and litres, divides by the selected bag size, and rounds bags up so a partial bag does not leave the bed short. Container and topsoil tools use the geometry that matches their growing area; the compost tool applies the chosen amendment depth over the measured area.
Mix recipes and public sources
Ingredient splits are made by volume. The Mel's Mix calculator divides the finished bed volume into equal thirds of blended compost, peat moss or coco coir, and coarse vermiculite, following the Square Foot Gardening Foundation recipe. Raised-bed volume and compost guidance also cite Clemson Cooperative Extension. The exact source and assumption for each result appear on that calculator's page.
Every length is converted at the input edge, and volume is calculated in a consistent internal unit before being formatted as cubic feet, cubic yards, quarts, buckets or litres. That keeps a 30 cm depth and its inch equivalent tied to the same underlying quantity.
How the math is checked
Automated reference cases check the basic geometry, metric and imperial conversions, whole-bag rounding and recipe splits. For example, the tests verify that a 4 × 4 ft bed filled 6 in deep holds 8 cu ft and that Mel's Mix assigns one third to each ingredient. These checks detect arithmetic and conversion changes; they do not test the soil sold at a particular garden center or predict how it behaves after watering.
Limits in the garden
Loose products settle, compact and vary in moisture, so a measured volume is not a guarantee of delivered or final depth. Irregular beds, slopes, roots and stones can change the amount required. A recipe quantity also cannot decide whether a mix suits a particular plant, climate or drainage condition. Allow a sensible margin and use local extension guidance for soil and plant decisions.
Report a calculation problem
Email [email protected] with the calculator URL, the dimensions and units entered, the result shown and what you expected. Include a public extension or standards link if it is relevant to the correction.