Raised Bed Soil Calculator

Volume math you can trust

Soil volume is length × width × depth in feet. A 4×8 bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet, or 32 ÷ 27 ≈ 1.19 cubic yards. Bag counts round up: 32 ÷ 1.5 ≈ 21.33 → 22 bags of 1.5 cu ft, or fewer larger 2 cu ft bags. Buy bulk by the yard when the total climbs past a dozen bags — delivery often wins on price and back strain.

Mel's mix breakdown

Square Foot Gardening’s Mel’s mix is equal parts by volume: compost, peat moss or coco coir, and coarse vermiculite (33.33333333333333% / 33.33333333333333% / 33.33333333333333%). For 32 cu ft that is about 10.7 cu ft of each. Use coarse vermiculite, not fine, so the mix stays open. Coco coir is a peat alternative with different wetting behavior — pre-hydrate bricks fully before blending.

Depth choices

Twelve inches suits most vegetables on a fabric or gravel base. Shallower beds dry faster and limit carrots; deeper beds cost more fill but buffer drought. If you hugelkultur the bottom with logs, subtract that volume from what you buy — this calculator assumes a full soil fill to the depth you enter. Measure the clear inside of the box after the liner is in place; boards, corner posts, and landscape fabric can steal an inch of usable width that you will notice when the bag count comes up short on fill day.

Settling and top-ups

Fresh mixes settle. Many gardeners order about 10% extra the first season or plan a compost top-dress (see the soil amendment calculator) after the first winter. Screen compost so woody chunks do not leave voids under seedlings. Moisten as you fill; bone-dry peat sheds water and makes dry pockets. Plan a spring top-up of an inch of finished compost rather than dumping more peat each year — compost feeds biology while peat alone mostly holds structure.

Bag vs bulk

Bags are convenient for one small bed and for matching Mel’s components precisely. Bulk compost and screened topsoil blends suit long runs of beds if you trust the supplier’s mix. Ask what is in “garden mix” — some loads are mostly mineral soil that compacts hard in a box. For true Mel’s mix, keep the thirds intentional rather than hoping a single bulk product matches. Weigh a sample bag at home once so you know whether a “1.5 cu ft” bag was packed fluffy or compressed.

Common mistakes: measuring inside dimensions then buying for outside lumber sizes, forgetting paths when estimating multiple beds, and filling with heavy clay native soil that defeats drainage. Another: using fine vermiculite that collapses. Print the bag count before you leave for the store so you do not under-buy by a frustrating half yard. If you build several beds, multiply volume by bed count before rounding bags — one shared bulk order beats three separate under-filled trips.

Frequently asked questions