How much soil do I need for a 4x8 raised bed?

A standard 4 × 8 foot raised bed that is 12 inches deep holds 32 cubic feet, which is about 1.2 cubic yards. Using a 60/30/10 Mel's Mix ratio, that is roughly 19 cubic feet of topsoil, 10 cubic feet of compost, and 3 cubic feet of aeration material like perlite or coarse bark. A bed that is 6 inches deep holds exactly half that volume, and one that is 18 inches deep holds 48 cubic feet.

Mel's Mix is the soil recipe popularized by Square Foot Gardening author Mel Bartholomew: one-third compost, one-third peat moss or coir, and one-third coarse vermiculite, by volume. It drains well, holds moisture, and supplies nutrients without native topsoil. The original Mel's Mix avoids topsoil entirely; modern variations often replace the vermiculite with perlite because it is cheaper and more available, and some gardeners substitute a quality topsoil for the peat to save money.

Do I need to fill the whole raised bed with soil?

You do not have to, especially for deep beds. Beds deeper than 12 inches can be filled at the bottom with layers of logs, branches, wood chips, leaves, and straw — the hugelkultur method — which saves money on soil and releases nutrients as it breaks down. Only the top 8 to 12 inches needs to be premium soil blend because that is where most plant roots actually grow.

How much does it cost to fill a raised bed?

A 4 × 8 × 12 inch raised bed filled with bagged Mel's Mix costs roughly 150 to 250 dollars depending on component prices. Using bulk topsoil and compost from a landscape supplier drops the cost to 50 to 100 dollars for the same bed. The savings grow with bed count — filling four beds bulk is about 250 dollars total, versus 800 to 1,000 dollars in bags.

Can I reuse raised bed soil next year?

Yes — raised bed soil should last many years. Each spring, top up with 1 to 2 inches of fresh compost to replace settled volume and replenish nutrients. Every 3 to 5 years, add a fresh dose of aeration material (perlite or coarse bark) because it compacts over time. Only replace all the soil if the bed gets root knot nematodes or a soil-borne disease.

Raised Bed Soil Calculator - Mel's Mix & Volume

How much soil do you need for a raised bed? Enter dimensions and your preferred soil mix ratio and see cubic yards, cubic feet, bags, and the breakdown by component.

How to Calculate Raised Bed Soil

Volume is length × width × height in feet. If your height is in inches, divide by 12 before multiplying. A 4 × 8 × 1 foot bed holds 32 cubic feet. The calculator multiplies that by the number of beds, converts to cubic yards at 27 cubic feet per yard, and then splits the total three ways using the Mel's Mix ratio you choose. Default is 60 percent topsoil, 30 percent compost, and 10 percent aeration (perlite or coarse bark). Your own mix can use anything as long as the percentages add up to 100.

Mel's Mix and Other Raised Bed Soil Recipes

The original Mel's Mix from Square Foot Gardening is one-third compost, one-third peat moss or coconut coir, and one-third coarse vermiculite. It is lightweight, drains well, holds moisture, and needs no native topsoil. The drawback is cost — vermiculite and peat are both expensive. Most home gardeners today build a hybrid: 60 percent good topsoil, 30 percent mixed compost (use two or three different compost types for nutrient diversity), and 10 percent perlite or coarse bark for aeration. This mix is cheaper, drains almost as well as Mel's Mix, and holds up better over multiple seasons.

Hugelkultur: Fill the Bottom with Wood

For beds deeper than 12 inches, you can save significant money and soil by filling the bottom 50 percent with logs, branches, wood chips, leaves, straw, or cardboard. This is called hugelkultur, and it mimics the rotting logs on a forest floor. The wood slowly decomposes over several years, releasing nutrients, holding moisture like a sponge, and providing habitat for beneficial fungi and bacteria. The top 8 to 12 inches of the bed should still be premium soil mix because that is where most vegetable roots actually live.

Topping Up Existing Beds

Raised bed soil settles 1 to 2 inches per year as organic matter decomposes. Every spring, top off with a fresh layer of compost and a light dusting of aeration material. Do not till the whole bed — that destroys the soil structure and fungal networks you have built. Every 3 to 5 years, dig down 8 inches and refresh the mix with fresh compost if the soil has become dense or hydrophobic. Only fully replace the soil if you have had a major pest or disease outbreak.

Frequently Asked Questions

How much soil for a 4x8x12 raised bed?

32 cubic feet (about 1.2 cubic yards) for a single bed.

What's the best soil mix for raised beds?

60% quality topsoil, 30% compost, 10% perlite or coarse bark is a good all-purpose blend.

Can I put cardboard at the bottom?

Yes — it blocks weeds, decomposes in a season, and is free. Use plain, uncoated cardboard only.

Do raised beds need drainage holes?

If the bed sits on bare soil, no. If it sits on concrete, yes — drill or space slats on the bottom.

How often should I replace raised bed soil?

Not for 5+ years. Just top up with compost each spring and refresh aeration every 3 to 5 years.

Raised Bed Soil Calculator

Soil You Need