Measure the space the soil will occupy
Use inside length × inside width × planned fill depth. A product’s outside or nominal length includes walls, corners, or hardware and can overstate the soil space. Wall height is also not automatically fill depth.
Measure from the inside faces at the intended fill line. Record headspace separately: if the inside wall is 17 inches high and you plan a 15-inch fill, the headspace is 2 inches. For an open-bottom bed, this calculation covers only the material placed inside the bed; it does not count native soil below it.
- If only outside or nominal dimensions are available, measure the assembled interior before ordering fill.
- If the maker publishes a soil capacity but no internal dimensions, keep that capacity as a separate manufacturer value; do not reverse-engineer an inside size from it.
- For a container, measure inside diameters at the fill line and bottom. Do not use nursery or nominal gallons as measured liquid gallons.
Calculate volume once, then convert it
For a rectangle, cubic feet = inside length (ft) × inside width (ft) × fill depth (ft). Convert inches to feet before multiplying. Divide cubic feet by 27 for cubic yards, or multiply by 28.316846592 for liters.
Worked example: a bed with a 6 ft × 3 ft interior and a 10 in fill depth needs 6 × 3 × (10 ÷ 12) = 15 cubic feet. That is about 0.556 cubic yard or 424.8 liters. Keep the unrounded calculation for bag counts and round only the displayed result.
- Round a bag count up: 15 cubic feet requires 10 bags labeled 1.5 cubic feet.
- A round bed uses
π × inside radius² × fill depth. - A tapered round pot uses both inside top and bottom diameters; the soil calculator applies the frustum formula.
Separate the soil that fits from extra soil you order
Start with the amount that fits inside the bed. If you want extra for settling or spills, add it to the order separately. For a custom mix, use the proportions you have chosen for your bed.
Placement changes the soil question without changing the geometry. University of Maryland Extension separates existing garden soil, native soil, and hard-surface scenarios. Use the volume result first, then choose materials for the actual surface and local soil conditions; do not apply one scenario’s mix to every bed.
- Record the bed surface: existing garden soil, other native soil, uncertain or potentially contaminated soil, or a hard surface.
- For a container, preserve drainage openings and planned headspace. Maryland Extension says gravel or broken pot pieces at the bottom do not improve drainage.
- Do not approve a deck, balcony, roof, or retaining structure from a soil-volume result; filled weight needs a site-specific structural answer.
Turn the result into an order
Use the exact volume printed on each bag. For bulk material, use the supplier’s quoted unit, minimum order, and delivery amount rather than assuming every supplier sells the calculated fraction of a yard.
Open the soil calculator with your inside dimensions. Use its Soil order tab to convert the fill into bags or a bulk quantity. If an inside measurement is missing, the useful next step is to measure it or choose a product with a published capacity; an outside dimension alone cannot produce a precise fill order.
References
- Soil to Fill Raised Beds, University of Maryland Extension
- Growing Vegetables in Containers and Salad Tables, University of Maryland Extension