How much soil does a raised bed actually need?

Start with the gap, not the lumber

A bed framed with 12-inch boards rarely needs a full 12 inches of new mix. The frame may sit below grade, the bottom may already contain usable soil, or an old bed may only need topping up. Measure from the current soil surface to the finished level you want in several places and use the average gap as the depth input.

Leave a little headspace below the rim. It helps keep water and mulch inside the bed and makes it easier to add compost later without spilling material over the path.

Use one unit system all the way through

For imperial measurements, multiply length in feet by width in feet by depth in feet. Convert an inch depth to feet by dividing by 12. The result is cubic feet; divide by 27 for cubic yards. In metric, length in meters times width in meters times depth in meters gives cubic meters, and one cubic meter is 1,000 liters.

Bag labels may use cubic feet, liters, or quarts. Convert the planned volume to the bag’s stated volume before dividing, and always round the final bag count up because partial bags are not sold.

Reserve is for settling, not for hiding bad measurements

Loose mixes settle after watering and weathering, but no single reserve fits every product. The calculator's 10% default is a visible planning convention, not horticultural guidance: change it from supplier information, product texture, moisture, compaction, and delivery method.

• Re-measure irregular or sloped beds in smaller rectangles and add them together.
• Ask bulk suppliers whether their yardage is measured loose or settled.
• Do not fill deep beds with construction debris or unknown treated wood waste.

The calculator stops where soil judgment begins

Volume math cannot choose a soil mix for your site. Drainage, native soil, crop, climate, and available compost all matter. Local extension guidance and a soil test are more useful than a universal recipe when pH or nutrients are uncertain.