Raised Bed Soil Calculator Guide
The right raised-bed fill isn't just topsoil — it's a blend that drains, holds moisture, and doesn't collapse in year two.
A raised bed is only as good as what fills it. Pure topsoil compacts. Pure compost burns roots and shrinks by 30% in a year. The right raised-bed mix is a blend engineered for drainage, nutrition, and long-term stability.
This guide covers the volume math, the classic mixes that work, and how to avoid the sinking-soil problem every raised-bed gardener eventually hits.
How much soil for a raised bed?
Cubic feet = length × width × depth (all in feet). Cubic yards = cubic feet ÷ 27.
| Bed size | Depth | Cubic feet | Cubic yards |
|---|---|---|---|
| 4 × 4 ft | 12" (1 ft) | 16 | 0.6 |
| 4 × 8 ft | 12" (1 ft) | 32 | 1.2 |
| 4 × 8 ft | 18" (1.5 ft) | 48 | 1.8 |
| 6 × 12 ft | 18" (1.5 ft) | 108 | 4.0 |
The classic raised-bed mix (Mel's Mix and variants)
The most-referenced raised-bed recipe, popularized by Mel Bartholomew:
- 1/3 blended compost (from 3+ sources — not just one).
- 1/3 peat moss OR coco coir.
- 1/3 coarse vermiculite.
This mix is expensive but exceptional for vegetables. A budget alternative most landscapers actually use:
- 50% quality topsoil.
- 30% compost.
- 20% coarse sand or perlite.
The hugelkultur / lasagna alternative
For beds deeper than 18 inches, filling with pure soil mix gets expensive. Instead, build in layers:
- Bottom third: rotted logs, branches, and coarse organic material.
- Middle third: leaves, cardboard, grass clippings.
- Top third (planting zone): quality soil mix.
The bottom decomposes over years, feeding the bed and reducing soil cost by up to two thirds.
The sinking-soil problem
Every raised bed drops 1–2 inches per year for the first few years. This is normal. It happens because:
- Organic material breaks down and loses volume.
- Rainwater compacts loose fill.
- Plant roots dying back leave voids.
Common mistakes
- Filling with pure topsoil — it compacts and starves roots.
- Filling with pure compost — too hot for seedlings and shrinks fast.
- Skipping the underlayer on beds deeper than 18" — wastes soil budget.
- Not lining wooden beds with landscape fabric on the inside — beds rot from the inside out.
- Forgetting drainage holes on solid-bottom beds.
Frequently asked questions
- How much soil does a 4x8 raised bed need?
- At 12 inches deep: 32 cubic feet or 1.2 cubic yards. At 18 inches: 48 cu ft or 1.8 cu yd. Add 10% for settling.
- Can I use garden soil to fill a raised bed?
- Bagged garden soil is fine but expensive at scale. Above 1 cubic yard, mix bulk topsoil with compost — much cheaper for the same result.
- How deep does a raised bed need to be?
- 6–8 inches for lettuce and herbs, 12 inches for most vegetables, 18–24 inches for tomatoes, root crops, and deep-rooted perennials.
- Do I need to line the bottom of a raised bed?
- Only if it's built directly on hard surfaces like concrete. For beds on soil, remove the sod, loosen the subsoil, and skip a bottom liner.
Summary
Raised-bed soil is a mix, not a single ingredient. Order 10% extra to allow for settling, and use a hugelkultur underlayer to keep costs down on deep beds.
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These results are estimates only. Confirm quantities, compaction, waste, and delivery requirements with your supplier or project professional before ordering materials.