How Much Soil Do I Need for a Raised Bed? Calculator and Examples

To find how much soil a raised bed needs, multiply length x width x depth in feet to get cubic feet, then divide by 27 for cubic yards. A standard 4 x 8 foot bed that is 12 inches deep holds 32 cubic feet, about 1.2 cubic yards, or sixteen 2-cubic-foot bags. Add 10 to 20 percent extra for settling, and you are ready to order. Below are the formula, worked examples, a chart of common bed sizes with bag counts, and ways to fill deep beds without buying a mountain of soil.

Part of our Vegetable Gardening & Raised Beds guide →

In this guide

1. The Raised Bed Soil Formula
2. Raised Bed Soil Chart for Common Sizes
3. Round, Oval and Odd-Shaped Beds
4. How Deep Should the Soil Be?
5. Filling Deep Beds for Less
6. What to Fill a Raised Bed With
7. Bags vs. Bulk
8. Common Measuring Mistakes
9. Plan for Settling and Top-Ups
10. Frequently Asked Questions

The Raised Bed Soil Formula

Every rectangular raised bed calculation comes down to three steps.

1. Measure inside dimensions in feet: length, width and the depth you plan to fill. Use the inside measurement, because thick boards or block walls take up space. If you plan to leave an inch or two of rim at the top, subtract that from the depth.
2. Convert inches to feet: divide inches by 12. Common depths: 6 in = 0.5 ft; 8 in = 0.67 ft; 10 in = 0.83 ft; 11 in = 0.92 ft; 12 in = 1 ft; 17 in = 1.42 ft; 18 in = 1.5 ft; 24 in = 2 ft.
3. Multiply: length x width x depth = cubic feet. Divide by 27 for cubic yards.

Then convert to bags by dividing the cubic feet by the bag size printed on the label and rounding up. If a bag is labeled in quarts or liters instead, use these conversions: 1 cubic foot is about 25.7 dry quarts or about 28.3 liters.

Worked example: 4 x 8 x 12-inch bed

• 4 ft x 8 ft x 1 ft = 32 cubic feet
• 32 / 27 = 1.19 cubic yards
• 32 / 2 = 16 bags of 2 cubic feet, or 32 / 1.5 = 21.3, so 22 bags of 1.5 cubic feet
• Add 15 percent for settling: about 37 cubic feet, roughly 1.4 cubic yards or 19 two-cubic-foot bags

Worked example: 3 x 6 x 10-inch bed

• 10 inches / 12 = 0.83 ft
• 3 x 6 x 0.83 = 15 cubic feet
• 15 / 27 = 0.56 cubic yards
• 15 / 2 = 7.5, so 8 bags of 2 cubic feet, or 10 bags of 1.5 cubic feet

Raised Bed Soil Chart for Common Sizes

Numbers below are the exact fill volume with bag counts rounded up; they do not include the extra for settling.

Bed size (L x W x depth)	Cubic feet	Cubic yards	1.5 cu ft bags	2 cu ft bags
2 x 4 ft x 10 in	6.7	0.25	5	4
3 x 6 ft x 12 in	18	0.67	12	9
4 x 4 ft x 10 in	13.3	0.49	9	7
4 x 4 ft x 12 in	16	0.59	11	8
4 x 8 ft x 6 in	16	0.59	11	8
4 x 8 ft x 10 in	26.7	0.99	18	14
4 x 8 ft x 12 in	32	1.19	22	16
4 x 8 ft x 18 in	48	1.78	32	24
4 x 12 ft x 12 in	48	1.78	32	24
2 x 8 ft x 24 in	32	1.19	22	16

For several beds, add up the cubic feet of each before converting to yards or bags; that avoids rounding errors that leave you a bag or two short.

Round, Oval and Odd-Shaped Beds

Corrugated metal beds often come in round and oval shapes. Use these formulas, with all measurements in feet:

• Round bed: 3.14 x radius x radius x depth. Radius is half the diameter. A 4-foot-diameter bed that is 17 inches deep: 3.14 x 2 x 2 x 1.42 = about 17.8 cubic feet, or 9 bags of 2 cubic feet.
• Oval bed with rounded ends: treat it as a rectangle plus a circle. For an 8 x 2 foot oval, that is a 6 x 2 rectangle plus a 2-foot circle; calculate each and add.
• L-shaped or irregular beds: split into rectangles, calculate each, then add.
• Keyhole beds: calculate the full circle, then subtract the access path and compost basket area.

Many manufacturers list the fill volume on the product page; it is worth checking against your own calculation. Our roundups of metal raised garden beds and cedar raised garden beds include dimensions for popular models.

How Deep Should the Soil Be?

Depth drives most of the volume, so decide it first.

• 6 to 8 inches: enough for lettuce, spinach, radishes, herbs and strawberries, especially over loosened native soil that roots can grow into.
• 10 to 12 inches: the practical standard for most vegetables, including tomatoes, peppers, beans, squash and onions.
• 18 to 24 inches: for long carrots and parsnips, potatoes, and beds over concrete, gravel or very poor soil where roots cannot go deeper.
• 24 inches and up: mainly for accessibility, so gardeners can work without bending. The plants rarely need all that soil.

If the bed sits directly on loosened ground, roots will grow past the bottom of the frame, so a 10 or 12-inch fill is usually plenty.

Filling Deep Beds for Less

A 4 x 8 bed that is 24 inches deep holds 64 cubic feet, more than 2.3 cubic yards. Most vegetables only need the top 12 to 18 inches to be good soil. You can fill the bottom with cheaper organic material:

• Logs and thick branches in the bottom third, hugelkultur style. They break down slowly and hold moisture. Avoid black walnut, cedar and treated wood. See hugelkultur vs. raised beds for pros and cons.
• Smaller branches, wood chips, leaves and straw in the next layer.
• Plain cardboard on the bottom to smother grass.
• Good soil mix in the top 12 to 18 inches.

Expect deeper settling with this method as the organic layers break down, so top up with compost each spring. Avoid filling the bottom with gravel or rubble “for drainage”; it creates a perched water table that keeps the soil above wetter, not drier.

What to Fill a Raised Bed With

Once you know the volume, decide the mix. A common, reliable blend is about 60 percent screened topsoil and 40 percent compost by volume. Another popular approach is roughly equal parts topsoil, compost and a coarse material such as aged bark fines or perlite for drainage. Pure compost is too rich and settles heavily; pure topsoil can compact. Our guide to raised bed soil mix covers recipes, and our picks for the best raised bed soil cover bagged options.

To split your total into components, multiply the cubic feet by each percentage. For a 4 x 8 x 12-inch bed (32 cubic feet) at 60/40: about 19 cubic feet of topsoil and 13 cubic feet of compost.

Bags vs. Bulk

Bags are convenient for one or two small beds, for hauling in a car, and for reaching a backyard without a wheelbarrow route. Bulk soil, sold by the cubic yard from landscape suppliers, usually costs considerably less per cubic foot once you need more than about a cubic yard, but adds a delivery fee and a pile on the driveway to move. One cubic yard is 27 cubic feet, the equivalent of 13 to 14 two-cubic-foot bags. Our comparison of bagged vs. bulk soil covers quality checks for bulk mixes, and for filling larger landscape areas, see how much topsoil you need.

Common Measuring Mistakes

• Using outside dimensions. A bed built from 2-inch-thick lumber that measures 4 x 8 feet outside is closer to 3.7 x 7.7 feet inside. That small difference adds up to a few cubic feet you do not need to buy.
• Mixing inches and feet. Multiplying 4 ft x 8 ft x 12 in gives 384, a meaningless number that would have you buying twelve times too much soil. Convert the depth to feet first: 4 x 8 x 1 = 32 cubic feet.
• Forgetting to round up bags. Always round each bed’s bag count up, not down, and buy one or two spare bags for top-ups.
• Ignoring the rim. Most gardeners leave an inch or two below the top edge so water and mulch stay in the bed. Measure depth to your intended soil line, not the top board.
• Counting dry bags as settled volume. Bagged mixes are often fluffy and compress once wet, which is one more reason to buy extra.

Plan for Settling and Top-Ups

Freshly filled beds commonly settle by an inch or more in the first months as air pockets collapse and organic matter decomposes. Order 10 to 20 percent extra, water the bed thoroughly after filling, and top it up before planting. Each following year, plan to add 1 to 3 inches of compost in spring or fall to replace what breaks down; our guide on adding compost to raised beds explains how much and when. For more soil-building basics, browse the composting and soil hub.