Compost amendment and mixed depth
The result is compost added to an existing layer, not the total fill for an empty bed. Mixed depth is a mathematical volume reference. Actual soil structure, incorporation, and settling can produce a different final surface level.
For total soil fill in an empty bed rather than amendment of existing soil, use the Raised Bed Soil Calculator with the bed dimensions.
The formula
The target fraction applies to the final combined soil and compost volume. Solving compost ÷ (soil + compost) = f gives the required amendment. The mixed depth assumes additive volumes without settling; bags round the amendment upward.
Worked example
A 100 ft² garden with 6 in of existing soil contains 50 ft³. A target compost share of 25% requires 50 × 0.25/0.75 = 16.667 ft³ of compost, or 0.6173 yd³. At 2 ft³ per bag, purchase nine bags. Ideal mixed depth becomes 8 in.
How to use this calculator
- Enter garden area, existing soil layer depth using the units shown.
- Set compost share after mixing, volume per bag.
- Calculate and review compost amendment volume alongside the supporting quantities.
Choose the amendment fraction from the soil task
The default percentage demonstrates the mixture calculation; it is not a recommended compost rate for every garden. Organic matter, fertility, salinity, texture, and existing soil condition should guide amendment decisions. Use a soil test and local extension advice when choosing a proportion. Compost nutrient content varies with its ingredients and maturity.
Final percentage differs from an addition percentage
Adding compost equal to 25% of the original soil volume produces a final fraction of only 20%. This tool instead solves for compost as a share of the completed blend. It assumes volumes add directly, which is useful for ordering but not a detailed model of pores, moisture, compaction, or decomposition over time.
Assumptions & limitations
What this calculation assumes
- Soil and compost volumes are treated as additive before settling.
What to keep in mind
- Does not prescribe an agronomic compost rate or predict nutrient supply.
Common questions
Is this the same as filling a raised bed?
No. It estimates amendment added to existing soil. An empty bed needs a total-fill calculation.
Why do I need more than 25% of the original volume?
The added compost also increases the total volume. A final 25% share requires one volume of compost for three volumes of existing soil.
Does the default fraction recommend an application rate?
No. It demonstrates the blend calculation. Select a compost rate from soil conditions, testing, and appropriate gardening guidance.
Sources & further reading
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