Compost Calculator — Top-Dressing and Bed Prep
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GardenSays
Buy 3 bags of compost (top-dressing) (1 cu ft) — or 0.11 cubic yards in bulk.
- Needed
- 2.7 cu ft
- Buy
- 3 × 1 cu ft
- Bulk
- 0.11 cu yd
Your 8×4 ft needs 2.7 cu ft of compost (top-dressing) (75.5 litres); with the 10% settling allowance, plan for 2.9 cu ft (83.1 L). At under a cubic yard, bags are the practical choice: 3 × 1 cu ft bags.
Do this this week
- 1.Buy 3 × 1 cu ft bags — or 0.11 yd³ in bulk
- 2.Expect the fill to settle 10-20% the first season — that's what the buffer covers
- 3.Fill level and even — the math assumes interior dimensions and uniform depth
Typical estimate · How we decide
Why
For established beds, a 1-2 inch layer of compost worked into the top few inches each season maintains organic matter.
- The exact math says 2.7 bags — the recommendation rounds up and adds 10% because soil settles and measurements are never perfect.
- For small quantities, compare bag price against bulk WITH the delivery fee included — there is no universal crossover point.
When this doesn't apply
- Material settles: a raised bed fill can drop 10-20% in the first season — that's what the buffer covers.
- L-shaped or irregular areas: split into rectangles, calculate each, and add the results (multi-area support is coming).
- Assumptions: interior dimensions, uniform depth, level fill; cost excludes tax and delivery.
How this was calculated
- 1. Area (rectangle)32 sq ft
8 ft × 4 ft
- 2. Volume2.7 cu ft
32 sq ft × 1/12 ft deep
- 3. Purchase buffer× 1.1 → 2.9 cu ftGardenSays estimate
Soil settles and beds are never perfectly measured — a 10% allowance avoids a second trip (our estimate; adjustable above)
- 4. In bags3 bagsCalculated result
2.9 cu ft ÷ 1 cu ft per bag = 2.9, rounded up to whole bags
- 5. In bulk0.11 cu ydCalculated result
2.9 cu ft ÷ 27 (bulk is sold by the cubic yard)
Data sources
- University of Minnesota Extension — Composting in home gardens (checked 2026-07-15)
What changes this answer
Area shape
Length (ft)
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Width (ft)
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Depth (in)
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Bag size
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Bag price (optional) ($)
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Every change updates the answer, the math, the weekly steps and the share link instantly.
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