How Much Soil You Need for a Raised Bed (With Formula)

How many bags of soil should you actually buy for a new raised bed? Ordering soil is one of the few garden tasks with a genuinely exact answer. Get the formula right once and you never have to guess, round up nervously, or make a second trip to the garden center for the bags you forgot.

What's the Core Formula?

Volume equals length times width times depth, all in the same unit. For a 4 by 8 foot raised bed filled to 12 inches (1 foot) deep, that's 4 times 8 times 1, or 32 cubic feet. Depth is the number gardeners most often get wrong — measure the actual inside height of the bed frame you intend to fill, not an assumed standard, since bed heights commonly range from 6 to 18 inches.

Converting to Cubic Yards

Bulk soil is sold by the cubic yard, and a cubic yard is 3 feet by 3 feet by 3 feet, or 27 cubic feet. Take your cubic-foot total and divide by 27 to get cubic yards. Our 32-cubic-foot example converts to 32 divided by 27, or about 1.19 cubic yards — meaning you'd order 1.25 yards from most suppliers, since they typically sell in quarter-yard increments.

Converting to Bags

If you're buying bagged soil instead of bulk, check the bag's cubic-foot rating, usually printed on the front (commonly 1.5 or 2 cubic feet per bag). Divide your total cubic feet by the bag size: 32 cubic feet divided by 1.5 equals about 21.3 bags, so you'd buy 22. At 2 cubic feet per bag, that's 32 divided by 2, or exactly 16 bags. Always round up, since a partial bag still costs you a full bag.

Multiple Beds at Once

For several beds, calculate each one's volume separately if depths differ, then add the cubic-foot totals before converting to yards. Three beds at 4 by 8 by 1 foot (32 cubic feet each) total 96 cubic feet, or 96 divided by 27, about 3.6 cubic yards — order 3.75 yards to round to the nearest quarter-yard most bulk suppliers sell in.

Accounting for Settling

Fresh garden soil mix, especially blends heavy in compost or peat, compresses as organic matter breaks down and air pockets collapse under watering. Expect 10 to 20 percent settling in the first season. A bed filled exactly to 32 cubic feet often reads closer to 26 to 28 cubic feet of visible volume by midsummer. Either fill slightly proud of the bed's rim initially, or plan on a top-up delivery of 3 to 5 cubic feet per bed later in the season.

Bulk vs. Bagged Cost Comparison

Bagged soil commonly runs the equivalent of 150 to 200 dollars per cubic yard once you add up individual bag prices, while bulk delivery from a landscape supplier often runs 40 to 70 dollars per cubic yard for a comparable garden mix. For any project needing more than roughly 1 cubic yard, bulk delivery typically saves more than half the cost, even after accounting for a delivery fee.

Filling in Layers for Deep Beds

For beds deeper than 18 inches, many gardeners fill the bottom third with a cheaper bulk fill (like untreated logs, straw, or coarse compost) and reserve the quality garden soil mix for the top 12 to 15 inches where roots concentrate. Recalculate your soil-mix volume using only that top depth — for an 18-inch-deep bed with 6 inches of filler, your soil-mix order is based on a 1-foot depth, not 1.5 feet, cutting your soil cost by a third.

Soil ordering doesn't have to involve guesswork or a return trip. Measure length, width, and depth, multiply, divide by 27 for yards or by your bag size for bags, and round up. The only judgment call left is how much to budget for settling later in the season.