TryBuildCalcSquare Foot Gardening Soil Calculator (4x4 ft Mel's Mix)
Size your Square Foot Gardening bed soil instantly.
🕒 Last updated: September 19, 2026
Inputs
Bed Dimensions
ℹ️Choose a common raised bed footprint, or pick Custom to enter your own length and width.
ℹ️Typical raised bed depth runs about 6-12 in (general practice, not a fixed standard) — shallow-rooted crops can work with less, deep-rooted vegetables benefit from more.
Soil Mix Recipe
ℹ️The classic Square Foot Gardening recipe: equal thirds of a peat moss/coco coir aeration source, vermiculite, and compost (a blend of several compost types is often recommended, treated here as one compost category).
ℹ️Peat moss is the classic Mel's Mix ingredient; coco coir is a peat-free alternative widely used as a more sustainable modern substitute — either fills the same equal-thirds share.
Split evenly into thirds — Coco Coir / Vermiculite / Compost — not editable, per the classic recipe.
Bags & Cost Estimation
ℹ️Used to compute each component's own bag count from its own apportioned volume — pick the closest common soil/amendment bag size you'd buy.
Total Soil Mix Needed
8.8 cu ft
0.326 cu yd / 0.249 m³ / 249.2 L
Bed Size
16 sq ft
1.49 m² at 6" (15.2 cm) deep
Mel's Mix (Square Foot Gardening)
The classic Square Foot Gardening recipe: equal thirds of a peat moss/coco coir aeration source, vermiculite, and compost (a blend of several compost types is often recommended, treated here as one compost category).
| Component | Share | Volume | Bags |
|---|---|---|---|
| Coco Coir (peat-free alternative) | 33.3% | 2.94 cu ft (0.109 cu yd / 0.083 m³) | 3 |
| Vermiculite | 33.3% | 2.93 cu ft (0.109 cu yd / 0.083 m³) | 3 |
| Compost (blend of compost types) | 33.3% | 2.93 cu ft (0.109 cu yd / 0.083 m³) | 3 |
| Total | 100% | 8.8 cu ft | 9 |
Bag size: 1 cu ft (28.3 L) per bag — shared across all components.
Volume
Bare volume: 8 cu ft (0.296 cu yd / 0.227 m³)
Order volume (+10% settling): 8.8 cu ft
Assumptions Used
Total soil mix volume (bed area × fill depth, plus a 10% settling allowance) is split across your selected recipe's components by percentage share, using largest-remainder rounding so each component's displayed cu ft figure sums exactly back to the total order volume. Each component's bag count uses the same shared bag size selected above, rounded up to the next whole bag. This calculator estimates material quantity only — it does not verify soil pH, nutrient testing, or structural bed-frame design.
Diagram simplified for clarity (not to scale) — bed footprint shown schematically, soil mix layers shown proportionally to each component's volume share.
Looking for the verification checklist, reference tables, tips, or common mistakes?See the complete Raised Bed Soil Calculator.
Square Foot Gardening soil volumes
The Square Foot Gardening method pairs a grid-divided 4x4 ft bed with the Mel's Mix soilless recipe, designed to support intensive close planting spacing without needing native soil.
This page defaults to a 4x4 ft bed at 6 in deep with coco coir — edit the inputs above to match your actual bed and peat/coir preference.
Raised Bed Soil Calculator Formula: How Is It Determined?
Bed area and depth combine into a bare volume, a settling allowance is added, and the order volume is split across your chosen recipe's components by percentage share before converting each to a bag count.
Step 1 — Bed Area
Preset Bed Size: Area = Preset Length x Preset Width
Custom: Area = entered Length x entered Width (converted to ft if entered in m)
Choose a common raised bed footprint, or switch to Custom to enter your own length and width.
Step 2 — Bare & Order Volume
Bare Volume = Area x Depth
Order Volume = Bare Volume x (1 + 10% settling)
The 10% allowance covers typical settling once a fresh loose mix is watered in and worked — lighter than the Topsoil Calculator's 15% open-fill allowance, since a boxed bed doesn't need a grading waste allowance on top.
Step 3 — Component Split
Component Volume (exact) = Order Volume x (Component % / 100)
Component Volumes are rounded using largest-remainder apportionment so they sum exactly to the displayed Order Volume
Each component's share comes from your selected recipe (Mel's Mix splits into exact equal thirds; the topsoil-based blends default to an editable percentage split).
Step 4 — Bags & Cost
Component Bags = ceil(Component Volume / Bag Volume)
Component Cost = Component Bags x Price per Bag (if cost estimation is enabled)
All components share one selected bag size, applied to each component's own apportioned volume — each gets its own bag count and, optionally, its own price per bag.
Worked Example
This example walks through your current inputs above, using the same steps as the Formula section.
Input Values Used
| Input | Value | Why it is used |
|---|---|---|
| Bed Size | 16 sq ft (1.49 m²) | Sets bare volume |
| Fill Depth | 6 in (15.2 cm) | Sets bare volume and depth-advisory check |
| Recipe | Mel's Mix (Square Foot Gardening) | Sets each component's percentage share |
| Bag Size | 1 cu ft (≈28 L) — small bag — 1 cu ft (28.3 L) | Sets each component's bag-count divisor |
Step 1 & 2 — Volume
| Calculation | Result |
|---|---|
| Bare Volume = 16 sq ft x (6/12) ft | 8 cu ft (0.296 cu yd / 0.227 m³) |
| Order Volume = Bare Volume x (1 + 10%) | 8.8 cu ft (0.326 cu yd / 0.249 m³) |
Step 3 & 4 — Component Split, Bags & Cost
| Component | Calculation | Volume | Bags |
|---|---|---|---|
| Coco Coir (peat-free alternative) | 8.8 cu ft x 33.3% | 2.94 cu ft | ceil(3 / 1) = 3 |
| Vermiculite | 8.8 cu ft x 33.3% | 2.93 cu ft | ceil(3 / 1) = 3 |
| Compost (blend of compost types) | 8.8 cu ft x 33.3% | 2.93 cu ft | ceil(3 / 1) = 3 |
The Volume column shows each component's apportioned display figure (so the column sums exactly to the total); the ceil() calculation uses each component's own full-precision volume, which can very occasionally round to a different-looking pair of numbers than the Volume column while still producing the correct bag count.
Therefore, a 16 sq ft raised bed at 6" deep with the Mel's Mix (Square Foot Gardening) recipe needs 2.94 cu ft (3 bags) of coco coir (peat-free alternative), 2.93 cu ft (3 bags) of vermiculite, and 2.93 cu ft (3 bags) of compost (blend of compost types).
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Disclaimer: This calculator provides approximate results for planning and estimation purposes only. Actual requirements may vary based on site conditions, materials, workmanship, and local building regulations. Always consult a qualified engineer, architect, or construction professional before making final decisions.