Soil calculator for Montana

A 4 x 8 raised bed filled to 12 inches takes 32 cubic feet of soil. That works out to 1.2 cubic yards, more than many first-time bed builders expect when they plan to pick up a few bags. Use this calculator for any bed, planter, or yard job and select from the three soil types. Material changes the weight: garden soil runs about 2,000 lb per cubic yard, screened topsoil is 2,200, and fill dirt is 2,400. Choose the soil being ordered so the tonnage is calculated correctly.

Three steps for bed calculations

1. Take the length and width of every bed or planter in feet. Multiple-areas mode combines all beds into a running total.
2. For existing beds, use 4-6 inches of improvement material. New in-ground beds need 6 inches, while an empty raised planter takes 12 inches.
3. Divide cubic feet by 27 for cubic yards. Then multiply yards by 1.0 to 1.2 for tons, based on the soil selected.

Coverage and material weight

Choose soil for the work

• Garden soil is compost-mixed planting material for vegetable gardens, flower beds, and planters.
• For final lawn grading and lawn cover, use screened topsoil. Find recommended lawn depths and calculation examples in the Topsoil Calculator.
• Put fill dirt below for hole filling or grade changes, then use good soil only for the planting layer.
• Instead of swapping out the whole bed, mix 2-3 inches of garden soil through the upper 6 inches of worn native ground. It uses less material, and roots establish more quickly.

Build up raised beds without wasting blend

There is no need to run costly garden blend to the bottom of a deep bed. For beds taller than 18 inches, use a lower third of less costly material, such as fill dirt or coarse organic matter that breaks down in place. Put 8-12 inches of garden soil above it, where the feeding roots are. Calculate each layer separately with two quick entries. In Montana, raised-bed work often starts in April, and measuring the full 12 inches keeps an order from coming up short.

Example: three 4 x 8 beds

A 96-square-foot bed filled 12 inches deep takes 96 cubic feet, or 3.6 cubic yards, weighing about 3.6 tons for garden soil. Plan on 4 yards. As compost settles in during the first weeks, blended soil drops roughly 10%; use the extra to bring the beds back to grade after that initial settling.

Improve clay and sand in place

Clay stays wet too long, and sandy ground loses moisture too quickly between waterings. For either, till 2-3 inches of compost-blended garden soil through the upper 6 inches and do it again for a couple of seasons. Compared with replacing soil to full depth, that is a third of the material and lifting. Roots and worms keep carrying the blend farther down over time. Reserve full-depth replacement for beds where construction debris or contamination makes native ground unsuitable.

Bulk soil compared with bags

At 40 lb per bag, a cubic yard comes to about 36 bags. One raised bed can be a 43-bag job, and loading 43 bags twice is no fun. Bulk soil costs less per cubic foot, comes in one tipped load, and free delivery means no truck fee to recover. Bags are best saved for topping off one planter. Ask for the pile on a tarp: soil can mark concrete, and the tarp makes cleanup a matter of dragging and folding.

Soil math for beds and bags

Other calculators