Formula and method
Volume = bed length × width × depth. Apply the extra-material percentage before dividing by bag volume and rounding up.
Worked example
A 5 m × 4 m bed at 5 cm depth contains 1 m³. A 10% allowance raises the order estimate to 1.1 m³.
Bed shapes and bag sizes
Circular beds use π/4 × diameter² × depth and triangular beds ½ × base × height × depth. Bag presets of 0.75, 1, 1.5, 2 and 3 ft³ divide the order volume in cubic feet and round up; the custom field accepts any labeled volume in cubic meters.
Topsoil mode
Choose topsoil, compost or garden soil to add weight. Weight = volume × bulk density; the 1,200 kg/m³ default (about one short ton per cubic yard) is an editable assumption for loose, moist screened topsoil. Weight is shown in metric tonnes and US short tons.
Worked example: topsoil
A 9 ft by 3 ft raised bed filled 12 in deep holds 27 ft³, exactly 1 yd³. With the allowance set to 0%, that is 36 bags of 0.75 ft³, and at 1,200 kg/m³ about 917 kg, or 1.01 short tons. The default 10% allowance raises the order to 29.7 ft³ and 40 bags.