Raised Bed Soil Calculator

1. Enter the bed length, width and fill depth.
2. Enter the number of beds if you have more than one of that size.
3. Pick a soil mix, which splits the volume into components.
4. Pick a bag size and read the cubic yards, bags and mix breakdown.

Here is what each result line means:

Buying the parts separately? Size them with the topsoil and compost calculators.

A raised bed soil calculator is a garden tool that works out how much soil fills a raised bed, and splits that volume into the parts of your chosen mix. It turns the bed dimensions into cubic yards, cubic feet and bags.

Raised beds are filled with a blend, not plain dirt: usually topsoil loosened with compost, or a soilless mix like Mel’s Mix for square-foot gardening. The calculator sizes the whole fill and each ingredient.

It uses the same volume math as the topsoil and mulch calculators.

Length times width times the depth in feet gives the cubic feet in a bed. Here the depth is in inches, so it is divided by 12, and the total is divided by 27 for cubic yards.

1. Per bed = length times width times depth in inches, divided by 12.
2. Total = per bed times the number of beds; cubic yards = total divided by 27.
3. Each mix component = the total volume times its fraction.

Bags are the total cubic feet divided by the bag size. A cubic yard is 18 of the 1.5-cubic-foot bags, so bulk delivery is cheaper for several beds.

One 4 x 8 ft bed filled 10 in deep with a 70/30 topsoil and compost mix.

Calculation: Volume = 4 x 8 x 10 / 12 = 26.67 cubic feet = 0.99 cubic yards, or 18 bags of 1.5 cubic feet. The mix is about 0.69 cubic yards topsoil and 0.30 cubic yards compost.

A 4 x 4 ft bed at 12 in in Mel’s Mix is 16 cubic feet, about 5.3 cubic feet each of compost, peat and vermiculite.

Bed Size and Depth

Volume scales with length, width and depth. A deeper bed for root crops needs proportionally more soil; most beds are 6 to 12 inches.

Number of Beds

Several identical beds multiply the volume fast, which usually tips the order from bags to bulk.

Mix Ratio

The blend sets how much of each ingredient you buy. A 70/30 topsoil-compost mix is common; Mel’s Mix uses equal thirds of compost, peat and vermiculite.

Settling

Fresh soil settles after watering, so fill to the rim and expect to top up, or fill to 1 to 2 inches below and let it settle level.

Bags vs Bulk

A cubic yard is 18 of the 1.5-cubic-foot bags. For several beds, bulk soil and compost by the yard are far cheaper.

Common raised-bed mixes and what each volume splits into.

These are starting points from extension guidance and the Square Foot Gardening recipe. Adjust the blend to your soil, crop and drainage; a little coarse sand or perlite helps heavy mixes drain.

Filling New Beds

Size the soil to fill one or several new raised beds to depth.

Square-foot Gardening

Work out Mel’s Mix thirds for a classic square-foot bed.

Topping Up

Figure the single-material volume to top up a bed that has settled over a season.

Buying the Parts

Split the order into topsoil and compost, then price each with its own calculator.

Planning a Kitchen Garden

Scale the soil for a whole bank of beds before ordering bulk.

1. Filling with Plain Topsoil

Straight topsoil compacts and drains poorly in a bed. Blend in compost, about 30 to 50 percent.

2. Using the /27 Shortcut with Inches

The 27 divisor needs depth in feet. With inches, divide by 12 for cubic feet first, then by 27.

3. Buying Bags for Several Beds

A cubic yard is 18 bags of 1.5 cubic feet. For a few beds, bulk is far cheaper and quicker.

4. Forgetting Settling

Soil drops after watering. Fill a little proud or plan to top up so plants are not left in a sunken bed.

5. Too Shallow for Roots

Root crops want 10 to 12 inches. A shallow bed over hard ground limits what you can grow.

6. Over-rich Mix

All compost holds too much water and can burn seedlings. Keep compost to a third to half of the blend.

The calculator estimates fill volume and a mix split from the bed dimensions.

Multiply length by width by depth in feet. A 4 by 8 ft bed 10 in deep is 4 x 8 x 10/12 = 26.67 cubic feet, about 1 cubic yard, or 18 bags of 1.5 cubic feet.

A common blend is about 70 percent topsoil and 30 percent compost. For square-foot gardening, Mel’s Mix is equal thirds of compost, peat moss and coarse vermiculite.

Most beds are 6 to 12 inches. Leafy greens are fine at 6 inches; root crops and tomatoes prefer 10 to 12. Deeper beds need proportionally more soil.

About 18 of the 1.5-cubic-foot bags, 27 of the 1-cubic-foot bags, or 14 of the 2-cubic-foot bags. A cubic yard is 27 cubic feet.

The square-foot gardening recipe: equal parts by volume of blended compost, peat moss and coarse vermiculite. It is a light, soilless mix that drains well and holds moisture.

Bags suit one small bed. For several beds or more than a cubic yard, bulk topsoil and compost by the yard are far cheaper than bags.

About 30 to 50 percent of the blend. All compost holds too much water and can burn seedlings, so keep it to a third or half with topsoil.

Yes. Fresh soil settles after watering, so fill a little proud of the rim or plan to top up, and avoid a sunken bed.

At 12 inches deep, 4 x 4 x 1 = 16 cubic feet, about 0.59 cubic yards or 11 bags of 1.5 cubic feet.

No. The calculator runs entirely in your browser and nothing is stored or sent.

• Iowa State University Extension – Raised Bed Soil Mix (One-third mixes and 70/30 topsoil-compost blend.)
• Square Foot Gardening – Mel’s Mix (Equal thirds compost, peat and vermiculite recipe.)
• University of Maryland Extension – Raised Bed Soil (Topsoil-to-compost ratios for raised beds.)