Compost Calculator
Work out how much compost a garden needs and how long a pile takes to produce it.
Last updated · View algorithm, constants and default inputs (JSON)
About this calculator
Estimate the volume needed for a bed and how many starting piles that volume represents after the selected shrinkage. Bag count is a 1 cu ft equivalent. The entered depth and shrinkage are planning assumptions; finished volume varies with materials and composting conditions.
Worked example
calculated with the default values below
Garden bed area 300 sq ft · Compost depth 1.5 in · Pile shrinkage 66 % · Starting pile volume 27 cu ft Compost needed 37.5 cu ft · Compost needed 1.39 cu yd · 1 cu ft bags equivalent 38 · Finished compost from your pile 9.2 cu ft · Whole starting piles needed 5.0
General background references
Disclaimer
These are general gardening estimates. Planting windows, growth, water and nutrient needs vary by place and plant. Check local extension guidance and product labels before acting on an estimate.
Implementation source
This record describes the implementation used by this page. It is not independent validation. External references, when available, support only the stated formula or definition; estimates and custom rules retain their own assumptions.
Formula and variables
V=A×d÷12; V_f=P×(1−s÷100); piles=ceil(V÷V_f)
- A
- bed area (sq ft)
- d
- depth (in)
- V
- volume needed (cu ft)
- P
- starting pile volume (cu ft)
- s
- shrinkage (%)
- V_f
- finished volume from one pile
1 yd³ = 27 ft³; bags are 1-ft³ equivalents and round up; whole starting piles round up. Actual shrinkage varies.
Frequently asked questions
How many whole piles does the example need?
At 300 sq ft and 1.5 in deep, the bed needs 37.5 cu ft. A 27 cu ft starting pile at 66% volume shrinkage yields 9.18 cu ft, so the calculator rounds 37.5 ÷ 9.18 up to 5 whole piles. Actual finished volume varies with materials and process.