Raised Bed Soil Calculator
Bed dimensions in, soil volume out — plus bag counts and the recipe split for Mel's Mix or a simple 50/50 blend.
The short answer
Formula. Cubic feet = bed length × width × (depth in inches ÷ 12) × number of beds; bags = cubic feet ÷ bag size, rounded up.
Example. One 4 ft × 8 ft bed filled 12 in deep needs 32 cubic feet of soil — twenty-two 1.5-cu-ft bags; a Mel's Mix splits that into three equal parts of about 10.7 cu ft (compost, peat moss, vermiculite).
How this calculator works
The volume of a rectangular raised bed is length × width × soil depth, all in feet — so a 4 × 8 ft bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet of soil. Divide by 27 and you get 1.19 cubic yards, which is the unit bulk soil and compost are sold by; multiply the bed's volume by the number of beds if you are filling several. Bags are sold by the cubic foot (1, 1.5, 2 and 3 cu ft are the common sizes), and the calculator rounds bag counts up because nobody sells half a bag.
The recipe split is by volume, not weight. Mel's Mix — the Square Foot Gardening recipe — calls for equal volumes of compost, peat moss (or coco coir), and coarse vermiculite, so each part is one third of your total: about 10.7 cubic feet each for the 4 × 8 bed. The simpler and cheaper 50/50 fills half the bed with topsoil and half with compost, which most vegetables handle fine at a fraction of the vermiculite cost. Whichever you pick, compost is the part that feeds the plants; the rest is mainly structure and water-holding.
Two cost tricks the volume math makes obvious. First, deep beds are mostly empty promise: roots of lettuce, spinach and herbs live in the top 6 inches, so a 24-inch bed growing greens wastes the bottom foot — fill it with logs, branches and leaves (the hugelkultur approach) and buy soil only for the top. Second, buying "raised bed mix" by the bag for a full yard is the most expensive possible path; for anything above about 1.5 cubic yards, bulk delivery of a soil-compost blend usually costs less than half the bagged equivalent, with the same one-time delivery minimum caveats as mulch.
Sources & review date
- Mel's Mix (equal thirds compost, peat moss, coarse vermiculite) comes from Mel Bartholomew's All New Square Foot Gardening.
- Bag sizes are standard US retail soil bags (1.5 and 3 cubic feet).
Last reviewed: October 2026. Numbers that drift (fees, prices, densities) are editable inputs on this page.
Raised bed soil FAQs
How many bags of soil do I need for a 4x8 bed?
At 12 inches deep, 32 cubic feet: 22 bags of 1.5 cu ft, 16 bags of 2 cu ft, or 11 bags of 3 cu ft. At 6 inches deep, half those counts.
How many cubic yards is a 4x8 raised bed?
1.19 cubic yards at 12 inches of soil (32 cu ft ÷ 27). Bulk suppliers will round you to 1.5 or 2 yards delivered — extra is never wasted if you have pots or a compost heap.
Is Mel's Mix worth it?
It is light, drains beautifully, and is close to weed-free — great for small beds and beginners. For larger beds the vermiculite and peat cost adds up fast, and a topsoil-compost blend at half the price grows equally good vegetables when you mulch and feed through the season.
How do I account for the soil settling?
Filled beds drop an inch or two after the first watering as pockets close. If your target is a true 12 inches of settled soil, mound slightly at filling or top up after a week — the calculator's numbers are for settled volume, which is what you actually need to buy.