Raised Garden Bed Soil Volume & Mix Ratio Calculator (Cubic Feet & Bags)
Guessing at raised bed soil quantity leads to either an expensive surplus of bagged compost or a frustrating mid-project run back to the garden center. This calculator converts your bed's exact dimensions into cubic feet and splits the total into the classic Mel's Mix or a topsoil-based recipe.
Your 8.0×4.0ft bed at 12in deep needs 35.2 cu ft (1.30 cu yd) of soil mix.
| Component | Volume (cu ft) | Bags |
|---|---|---|
| Compost | 11.7 | 8 |
| Peat Moss / Coco Coir | 11.7 | 8 |
| Coarse Vermiculite | 11.7 | 8 |
| Total (10% compaction buffer included) | 35.2 | 24 |
- 1.
TotalCuFt = Length_ft × Width_ft × (Depth_in / 12) × 1.10 - 2.
ComponentVolume = TotalCuFt / 3 (Mel's Mix, equal thirds) - 3.
Bags = ceil(ComponentVolume / BagSize_cuft)
How to Use This Calculator
1. Enter your details
Use the labelled fields and units that match your situation.
2. Calculate instantly
The result, visual comparison, and working update as values change.
3. Review your result
Check assumptions, then copy, print, or share the calculation.
Formula & step-by-step maths
- 1.10
- 10% compaction buffer
- BagSize
- Cubic feet per bag purchased
Why the 10% compaction buffer matters
Loose bagged soil settles significantly once watered and walked around, often dropping 5-10% in volume within the first season. Building in that buffer up front means you won't be topping off the bed every few weeks.
Mel's Mix vs topsoil-based recipes
Mel's Mix (equal parts compost, peat moss, and vermiculite) is lightweight, drains well, and is the Square Foot Gardening standard. A 50/30/20 topsoil blend is cheaper at scale and holds moisture longer, better suited to larger beds and drought-prone climates.
Converting cubic feet to bags
Most compost, peat, and vermiculite bags are sold in 1.5, 2, or 3 cubic foot sizes — always check the bag label and adjust the bag-size input so the bag counts match what's actually on the shelf.
Deeper beds for root crops
Standard leafy greens and herbs are fine at 8-10 inches, but carrots, potatoes, and tomatoes benefit from 12-18 inches of depth — increase the depth input accordingly for a more accurate volume.
Common raised bed sizes and soil needs
| Bed size | Depth | Approx. cu ft (with buffer) |
|---|---|---|
| 4x4 ft | 8 in | ~11.7 |
| 4x8 ft | 12 in | ~35.2 |
| 3x6 ft | 10 in | ~16.5 |
| 2x8 ft | 8 in | ~11.7 |
| 8x8 ft | 12 in | ~70.4 |
| 4x4 ft | 12 in | ~17.6 |
Frequently Asked Questions
Three worked examples
Same engine, three different starting points — useful if you want to see how sensitive the answer is before you type your own numbers in.
Example 1: bed length 6.24 ft, recipe "Mel's Mix (1/3 compost, 1/3 peat, 1/3 vermiculite)"
On the lower / more conservative end. Your 6.2×4.0ft bed at 12in deep needs 27.5 cu ft (1.02 cu yd) of soil mix.
Example 2: bed length 8 ft, recipe "Mel's Mix (1/3 compost, 1/3 peat, 1/3 vermiculite)"
A typical middle-of-the-road setup. Your 8.0×4.0ft bed at 12in deep needs 35.2 cu ft (1.30 cu yd) of soil mix.
Example 3: bed length 10.4 ft, recipe "50/30/20 Topsoil / Compost / Amendment"
On the higher / more demanding end. Your 10.4×4.0ft bed at 12in deep needs 45.8 cu ft (1.69 cu yd) of soil mix.
Quick answers about the Raised Bed Soil Blend
What exactly does the Raised Bed Soil Blend work out?
Guessing at raised bed soil quantity leads to either an expensive surplus of bagged compost or a frustrating mid-project run back to the garden center. You enter bed length, bed width, soil depth and recipe (plus 1 more optional details) and the result panel updates straight away, so you can compare two or three versions of the same question in a few seconds.
What do I need before I start?
Only 5 fields: bed length, bed width, soil depth, recipe and bag size for compost/peat/vermiculite. Nothing else is needed and nothing is stored.
How is it calculated — why the 10% compaction buffer matters?
Loose bagged soil settles significantly once watered and walked around, often dropping 5-10% in volume within the first season. The same maths runs inside this page, so hand-checking the result on paper gives you the identical figure.
Why do two calculators give me different answers for raised Bed Soil Blend?
Mel's Mix (equal parts compost, peat moss, and vermiculite) is lightweight, drains well, and is the Square Foot Gardening standard. Different sites pick different assumptions, so always check which method a calculator states before you trust the gap between two numbers.
What does the "Common raised bed sizes and soil needs" table on this page tell me?
It is the reference range this tool works against — 6 rows from "4x4 ft" (8 in) up to "4x4 ft" (12 in). Use it to sanity-check whether the number you just calculated sits where you expected it to.
Which recipe should I pick?
The dropdown offers 2 choices — Mel's Mix (1/3 compost, 1/3 peat, 1/3 vermiculite) and 50/30/20 Topsoil / Compost / Amendment. Pick the one that matches your real situation rather than the one you would like to be true; recipe usually moves the final figure more than any other single input, so it is worth running it twice with the option above and below your guess.
Do I have to press a button or reload the page to see the result?
No. Raised Bed Soil Blend runs completely inside your browser, so the moment you change a value the cards recalculate — there is no submit step, no page reload and no waiting for a server round trip. That also means it keeps working on a weak or intermittent mobile connection.
Is it free, and do you keep what I type?
It is free with no sign-up, no app install and no usage limit. Nothing you enter into Raised Bed Soil Blend leaves your device — the calculation is JavaScript running locally, so there is no upload of your figures to DrHint or anyone else.
Can I use it on a phone?
Yes — the layout stacks to a single column on small screens and the number fields open the numeric keypad on both Android and iOS. Many people bookmark this page or add it to their home screen and re-open it whenever the question comes up.
Anything to be careful about with the result?
At 12 inches deep with a 10% compaction buffer, a 4x8 bed needs roughly 35 cubic feet of soil mix, split into about 12 cubic feet each of compost, peat moss, and vermiculite for Mel's Mix. Treat the output as a well-grounded estimate for planning, not as a professional, legal or medical decision on its own.
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