Soil calculator for Iowa

A standard 4 x 8 raised bed at 12 inches of fill requires 32 cubic feet of soil. That converts to 1.2 cubic yards, often more material than new bed builders expect when planning around bags. Use this calculator to size any bed, planter, or landscape area for the three available soil types. Blend weight varies: garden soil weighs about 2,000 lb per cubic yard, screened topsoil weighs 2,200, and fill dirt weighs 2,400. Select the soil being ordered so the tonnage calculation reflects its weight.

Three-step bed calculation

1. Enter the length and width of every bed or planter in feet. Use multiple-areas mode to add all bed measurements together.
2. Use 4-6 inches when refreshing an established bed, 6 inches to build a new in-ground bed, and 12 inches to fill an empty raised planter.
3. To convert volume, divide cubic feet by 27 for cubic yards. For tonnage, multiply yards x 1.0 to 1.2 according to soil type.

Coverage and soil weights

Choose soil by application

• For vegetable plots, flower borders, and containers, use garden soil blended with compost for planting.
• Screened topsoil suits lawn finishing and final grading. The Topsoil Calculator provides lawn-depth guidance and examples.
• Use fill dirt as the lower layer for filling holes or changing grade, and reserve planting soil for the surface growing layer.
• For worn native ground, mix 2-3 inches of garden soil into the upper 6 inches rather than removing and replacing the full bed. It uses less material and gives roots a more workable layer.

Lower-cost raised-bed filling

For a planter taller than 18 inches, the full depth does not need garden blend. Build the lower third with fill dirt or coarse organic material that will break down in place, then reserve 8-12 inches of garden soil for the root zone. Calculate the two layers separately. Treat 12 inches as a measured depth, not an estimate.

Example: three 4 x 8 beds

For a 96-square-foot bed filled to 12 inches, the volume comes to 96 cubic feet, or 3.6 cubic yards and about 3.6 tons of garden soil. Plan on 4 yards. In the first weeks, blended soil drops roughly 10% as the compost portion settles; use the remaining material to refill the beds afterward.

Amending clay or sand in place

Dense clay sheds water slowly, while sandy ground drains quickly and leaves plants short of moisture between watering. For either condition, work 2-3 inches of compost-blended garden soil into the upper 6 inches and continue this for a couple of seasons. This requires a third of the material and lifting compared with complete replacement, while roots and worms gradually carry the amendment farther down. Replace soil to full depth only where debris or contamination makes the existing ground unsuitable.

Bulk soil compared with bags

At 40 lb per bag, a cubic yard is about 36 bags. One raised bed can mean 43 bags, with 43 bags to move again after loading. Bulk soil costs less per cubic foot and arrives in one tip with free delivery. Bags remain useful for topping up a single planter. Place the load on a tarp because soil can mark concrete, then fold and pull the tarp for cleanup.

Soil math for beds and bags

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