Calculate how much compost, peat moss, or fertilizer to add based on bed size and soil type.
This calculator turns bed dimensions into a shopping list: you enter length and width, choose an amendment and a soil type, and it returns the volume to add in cubic feet, cubic yards, and bags. The geometry is straightforward - area times depth - but soil type changes the recommended depth, since sandy soil drinks up more organic matter than heavy clay and a brand-new raised bed takes a far larger share of compost than an established bed. The surprise is how small a modest layer is: a 2-inch dressing on a 4-by-8 bed is only about 5.3 cubic feet.
A concrete input and expected output from the current implementation.
Input
Bed: 4 ft x 8 ft; amendment: compost; depth: 2 in
Expected output
5.3 cubic feet, which is about 0.2 cubic yard. Buy 6 one-cubic-foot bags, or about one-fifth of a yard in bulk.
Volume is length x width x depth: 4 x 8 x (2/12) = 5.33 cubic feet. Dividing by 27 converts to cubic yards (0.198), and whole bags round the 5.33 up to 6.
The heavy lifting is arithmetic, not agronomy: length times width gives square feet, depth in inches divided by 12 gives feet, and the product is cubic feet. Dividing by 27 converts to cubic yards, the unit bulk suppliers quote. Bag counts divide that volume by the bag size and round up, so a partial bag always becomes a full one.
The soil type you select changes the application depth the calculator uses, and the depth drives everything: double it and the volume doubles. Sand loses water and nutrients fast and justifies a heavier organic layer, clay benefits from amendment to open pore space, and a new raised bed filled from scratch needs a much larger compost share than the thin top dressing an established bed wants.
For an established bed, 1 to 2 inches of compost as a top dressing is the common range - enough to feed the soil without smothering plants. A brand-new raised bed filled from scratch is different: compost is commonly a quarter to a third of the total mix there, which calls for a much deeper entry. The calculator converts whatever depth you choose; choosing the depth is the horticultural part.
Bags, at that size. Six 1-cubic-foot bags covers it, while bulk deliveries typically add minimums and delivery fees that dwarf the material cost of a fifth of a yard. One cubic yard covers about 324 square feet at 1 inch deep, so below a quarter yard, bagged product is simpler and usually cheaper; above a yard or two, bulk wins.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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