Compost Calculator

The mathematical formula is: Volume (Cubic Yards) = [Area (sq ft) × Depth (inches)] ÷ 324. To derive this: multiplying area in square feet by depth in feet (depth in inches ÷ 12) yields cubic feet. Dividing cubic feet by 27 (the number of cubic feet in one cubic yard) produces cubic yards. Because 12 × 27 = 324, dividing directly by 324 gives exact cubic yards.

A standard 4 ft × 8 ft bed is 32 square feet: 1) For a 1-inch annual spring top-dressing refresh: 32 × 1 ÷ 324 = 0.10 cubic yards (approx. 2.7 cubic feet, or two 1.5-cu-ft bags); 2) For a 2-inch deep soil conditioning layer: 5.3 cubic feet (approx. 3.5 bags); 3) To fill a 10-inch deep bed entirely with 50/50 compost-soil blend: requires 13.3 cubic feet of pure compost (approx. 9 bags of 1.5 cu ft).

Compost for lawn topdressing should be applied in a thin 1/4-inch to 1/2-inch layer so grass blades are not smothered: 1) At 1/4 inch deep: requires approximately 0.77 cubic yards (21 cubic feet) per 1,000 square feet of lawn; 2) At 1/2 inch deep: requires approximately 1.54 cubic yards (42 cubic feet) per 1,000 square feet. For a 5,000 sq ft lawn at 1/4 inch, order approx. 3.9 cubic yards of screened compost.

Because one cubic yard equals exactly 27 cubic feet: 1) Standard 1.5-cubic-foot bags: Exactly 18 bags equal one cubic yard (27 ÷ 1.5 = 18); 2) Large 2.0-cubic-foot bags: Exactly 13.5 bags equal one cubic yard (27 ÷ 2 = 13.5); 3) Compact 1.0-cubic-foot bags: Exactly 27 bags; 4) Small 0.75-cubic-foot bags: Exactly 36 bags.

Finished organic compost typically weighs between 1,200 and 1,400 pounds per cubic yard (approx. 45 to 50 pounds per cubic foot) at standard 40% to 50% moisture content. Heavily saturated wet compost can weigh up to 1,600 to 1,800 pounds per yard. A standard 1/2-ton pickup truck can safely haul 1.0 to 1.25 cubic yards of dry compost without exceeding payload capacity.

Bulk compost becomes significantly cheaper once your project requires more than 1.5 to 2 cubic yards (approx. 27 to 36 bags). Bagged organic compost costs $6 to $10 per 1.5-cu-ft bag ($108 to $180 per cu yd). Bulk screened compost from local landscape supply yards costs $30 to $50 per cubic yard, plus a $50 to $75 local delivery fee. For 3+ cubic yards, bulk delivery saves $150 to $300.

The ideal biological C:N ratio for rapid, odor-free composting is approximately 30:1 (30 parts carbon to 1 part nitrogen by weight). In practice, this is achieved by combining roughly 2 to 3 parts dry 'Brown' materials (dry leaves, straw, shredded cardboard with high C:N of 60:1–400:1) with 1 part moist 'Green' materials (kitchen scraps, fresh grass clippings, coffee grounds with low C:N of 15:1–25:1).

If the C:N ratio drops below 20:1 (too many wet food scraps or fresh grass clippings), composting microbes cannot consume the surplus nitrogen. Excess nitrogen volatilizes into pungent ammonia gas, making the pile smell like rotten eggs. The excess moisture also displaces air pockets, causing the pile to turn slimy, anaerobic, and cold. Remediate by turning in dry shredded leaves or sawdust.

If the C:N ratio exceeds 40:1 (excessive wood chips, straw, or autumn leaves without nitrogen sources), decomposition slows to a near standstill because bacteria lack the nitrogen protein building blocks needed to multiply. The pile remains cold and dry. Remediate by adding fresh grass clippings, coffee grounds, chicken manure, or blood meal and watering thoroughly.

While plants can survive in well-aged, mature compost, 100% compost is not recommended as a long-term growing medium. Pure compost has high soluble salt levels that can stunt seedlings, lacks structural mineral body (anchoring clay and sand), and will shrink by 30% to 50% over a single season as microbes break down organic carbon. Compost is best blended into raised bed mixes at 30% to 50% volume with topsoil, peat moss, and vermiculite.

Finished compost has five distinct characteristics: 1) Dark, rich chocolate brown or black crumbly texture; 2) Sweet, earthy forest floor aroma with zero foul or sour odors; 3) No recognizable original food scraps or leaves; 4) Ambient pile temperature (it no longer heats up after being turned); 5) Passes the baggie test: sealed in a plastic bag for 48 hours, it maintains a clean earthy smell without puffing up with anaerobic gas.

Compost is fully decomposed organic matter rich in microbial biology and balanced plant nutrients, designed to be integrated into the soil or applied as a nutrient-dense topdressing. Mulch consists of coarse, un-decomposed materials (wood chips, shredded bark, straw, pine needles) laid on top of the soil surface primarily to suppress weeds, retain moisture, and moderate temperature.