Soil calculator for Hawaii

Filling a 4 x 8 raised bed to 12 inches takes 32 cubic feet of soil. That comes to 1.2 cubic yards, often more than someone planning to use a few bags expects. Enter measurements for any planter, bed, or yard area, then choose among the available soil types. Material weights vary: garden soil is about 2,000 lb per cubic yard, screened topsoil is 2,200, and fill dirt is 2,400. Selecting the intended material gives the correct ton estimate.

Three steps for bed calculations

1. Start by measuring every bed or planter in feet. In multiple-areas mode, each entry is added to the overall total.
2. Match depth to the job: use 4-6 inches for existing-bed improvement, 6 inches for a new ground-level bed, and 12 inches to fill an empty raised planter.
3. To find cubic yards, take cubic feet / 27. For tons, use yards x 1.0 to 1.2 according to soil type.

Coverage and soil weights

Choose soil for the job

• Vegetable patches, flower beds, and planters use garden soil, with compost mixed in and ready for planting.
• For lawn areas and final grade work, select screened topsoil. Find lawn-depth recommendations and examples in the Topsoil Calculator.
• Put fill dirt below for filling holes or changing grade, then reserve good soil for the planting layer.
• Because replacing an entire worn bed takes more material, mix 2-3 inches of garden soil into the upper 6 inches instead. This uses less material and gives roots a gentler transition.

Fill raised beds with less blend

Because roots use the upper layer, a deep bed does not need costly garden mix from top to bottom. For beds taller than 18 inches, use fill dirt or coarse organic material that breaks down in the lower third, then place 8-12 inches of garden soil above it. Run the calculator once for each layer. On rainy windward lots, measure the 12 inches rather than estimating it so the bed is built to the planned depth.

Example: three 4 x 8 beds

For Ninety-six square feet at 12 inches deep, the volume is 96 cubic feet, equal to 3.6 cubic yards and about 3.6 tons of garden soil. Order 4 yards. As compost consolidates, blended soil settles roughly 10% in the first weeks, leaving material for topping off the beds.

Improve clay and sand in place

Heavy clay holds water, while sandy soil loses it quickly and can leave plants dry between waterings. For either condition, work 2-3 inches of compost-blended garden soil into the upper 6 inches and repeat for a couple of seasons. This requires a third of the material and lifting of full replacement, while roots and worms continue carrying the blend deeper. Reserve full-depth replacement for beds with construction debris or contamination in the native soil.

Bulk soil or bags

At 40 lb each, about 36 bags make a cubic yard. A single raised bed can amount to 43 bags, which means handling 43 bags more than once. Bulk material costs less by cubic foot and comes in one tip, with free delivery and no truck fee to recoup. Bags work well for topping off one planter. Have the load placed on a tarp, since soil can mark concrete and the tarp makes cleanup a matter of dragging and folding it up.

Soil math for beds and bags

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