TryBuildCalcRaised Bed Soil Calculator (Mel's Mix, Topsoil-Compost & Custom Recipes)
Find how much of each soil-mix ingredient your raised bed needs 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 — Peat Moss / 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
23.47 cu ft
0.869 cu yd / 0.665 m³ / 664.5 L
Bed Size
32 sq ft
2.97 m² at 8" (20.3 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 |
|---|---|---|---|
| Peat Moss | 33.3% | 7.83 cu ft (0.29 cu yd / 0.222 m³) | 6 |
| Vermiculite | 33.3% | 7.82 cu ft (0.29 cu yd / 0.221 m³) | 6 |
| Compost (blend of compost types) | 33.3% | 7.82 cu ft (0.29 cu yd / 0.221 m³) | 6 |
| Total | 100% | 23.47 cu ft | 18 |
Bag size: 1.5 cu ft (42.5 L) per bag — shared across all components.
Volume
Bare volume: 21.33 cu ft (0.79 cu yd / 0.604 m³)
Order volume (+10% settling): 23.47 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.
What Is a Raised Bed Soil Calculator?
A raised bed soil calculator finds how much of EACH ingredient a soil-mix recipe needs to fill a raised garden bed — not just a single total volume. Pick a recipe (the classic soilless Mel's Mix, a simple topsoil-compost blend, a topsoil-compost-amendment blend, or your own custom ratio), enter your bed size and fill depth, and it splits the total volume across each component with its own volume, bag count, and optional cost.
It supports common raised bed size presets (4x4, 4x8, 3x6, 2x8 ft) alongside custom length x width entry, and always shows results in cu ft, cu yd, m³, and litres together, so it works whichever unit you shop in.
What this calculator does not do:
- It computes soil-mix material quantity only — it does not perform a soil test or verify bed-frame structural design
- It is not the Topsoil Calculator — that separate tool is for a single-material open-area volume fill (lawn topdressing, general grading), not a multi-ingredient bed recipe
- It does not include labor for blending, filling, or leveling the finished mix
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 | 32 sq ft (2.97 m²) | Sets bare volume |
| Fill Depth | 8 in (20.3 cm) | Sets bare volume and depth-advisory check |
| Recipe | Mel's Mix (Square Foot Gardening) | Sets each component's percentage share |
| Bag Size | 1.5 cu ft (≈42 L) — common bag — 1.5 cu ft (42.5 L) | Sets each component's bag-count divisor |
Step 1 & 2 — Volume
| Calculation | Result |
|---|---|
| Bare Volume = 32 sq ft x (8/12) ft | 21.33 cu ft (0.79 cu yd / 0.604 m³) |
| Order Volume = Bare Volume x (1 + 10%) | 23.47 cu ft (0.869 cu yd / 0.665 m³) |
Step 3 & 4 — Component Split, Bags & Cost
| Component | Calculation | Volume | Bags |
|---|---|---|---|
| Peat Moss | 23.47 cu ft x 33.3% | 7.83 cu ft | ceil(7.8 / 1.5) = 6 |
| Vermiculite | 23.47 cu ft x 33.3% | 7.82 cu ft | ceil(7.8 / 1.5) = 6 |
| Compost (blend of compost types) | 23.47 cu ft x 33.3% | 7.82 cu ft | ceil(7.8 / 1.5) = 6 |
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 32 sq ft raised bed at 8" deep with the Mel's Mix (Square Foot Gardening) recipe needs 7.83 cu ft (6 bags) of peat moss, 7.82 cu ft (6 bags) of vermiculite, and 7.82 cu ft (6 bags) of compost (blend of compost types).
Essential Checklist+−
Complete these critical checks before approving the work or proceeding to the next construction stage.
✓Bed Measurement & Recipe Selection+-
- Bed length, width, and fill depth confirmed from an actual measurement of the built (or planned) frame, not assumed from a preset alone
- If using Custom Ratio, entered percentages confirmed to sum to exactly 100% before ordering material — not left unresolved past a validation error
✓Material Sourcing+-
- Topsoil (if used) confirmed free of contamination — no construction debris, invasive weed seed, visible trash, or reported herbicide carryover
- Vermiculite or perlite grade confirmed suitable for horticultural use, not a construction-grade product
✓Filling / Mixing+-
- Components thoroughly blended together before or during filling, not layered and left unmixed inside the bed
- Bed drainage confirmed adequate before filling — open bottom (most common) or drainage holes if the bed has a solid base
✓Planting & Follow-Up+-
- Bed frame material confirmed safe for food contact if growing edibles — avoiding older CCA-treated lumber or similar treated wood against soil
Full QC Checklist+−
Verification checklist for raised bed soil-mix takeoffs — covering bed measurement and recipe selection, material sourcing, filling/mixing, and planting/follow-up. Use the Essential Checklist for critical checks before finalizing, expand to Full QC Checklist for complete verification.
✓Bed Measurement & Recipe Selection+-
- Bed length, width, and fill depth confirmed from an actual measurement of the built (or planned) frame, not assumed from a preset alone
- Chosen fill depth matches what's actually being grown — shallow-rooted crops need less, deep-rooted vegetables (tomatoes, carrots) benefit from a deeper fill
- Recipe selected matches the actual goal — Mel's Mix for a soilless intensive SFG bed, a topsoil-based blend for a more traditional, lower-cost fill
- If using Custom Ratio, entered percentages confirmed to sum to exactly 100% before ordering material — not left unresolved past a validation error
- Compost source/blend considered, not treated as an interchangeable generic bag — a blend of several compost types is often recommended over a single source
- Peat moss vs. coco coir choice made deliberately for Mel's Mix, not defaulted without considering sustainability or local availability
✓Material Sourcing+-
- Topsoil (if used) confirmed free of contamination — no construction debris, invasive weed seed, visible trash, or reported herbicide carryover
- Vermiculite or perlite grade confirmed suitable for horticultural use, not a construction-grade product
- A small buffer beyond the calculated order volume considered, since bagged materials rarely divide into the exact calculated total
- Bag labels checked against the volumes assumed in this calculator's bag size selection before ordering, especially for compost (often sold by weight, not volume)
- Organic certification or input restrictions considered if the bed is intended for certified-organic or food-safety-sensitive growing
✓Filling / Mixing+-
- Components thoroughly blended together before or during filling, not layered and left unmixed inside the bed
- Filled depth spot-checked at multiple points across the bed, not assumed uniform from the total order quantity alone
- A weed barrier or cardboard base layer considered (if placing the bed over existing grass/weeds), and accounted for separately from this calculator's fill volume
- Bed drainage confirmed adequate before filling — open bottom (most common) or drainage holes if the bed has a solid base
- Settling allowance in the order volume understood as an estimate, and the bed topped up after initial watering-in if it settles noticeably below the frame's top edge
✓Planting & Follow-Up+-
- A soil test (pH, nutrient screening) considered before planting, especially for a topsoil-based blend or if this is a food-growing bed
- Planting spacing/plan matches the recipe and bed type chosen — Mel's Mix beds are commonly planted at Square Foot Gardening's intensive spacing, not conventional row spacing
- A plan for annual/seasonal compost top-up considered, since a raised bed mix settles and its organic content depletes over multiple growing seasons
- Bed frame material confirmed safe for food contact if growing edibles — avoiding older CCA-treated lumber or similar treated wood against soil
Raised Bed Soil Mix Reference Tables
Common raised bed soil-mix recipe ratios (general practice guidance, not a fixed standard), and how far one cubic yard stretches at common raised-bed depths.
| Recipe | Typical Ratio | Notes |
|---|---|---|
| Mel's Mix | 1/3 : 1/3 : 1/3 | Peat moss (or coco coir) : vermiculite : compost — fully soilless, classic Square Foot Gardening recipe |
| Topsoil-Compost Blend | ~50/50 to 60/40 | Topsoil : compost — simple, common, lower-cost; adjustable in this calculator |
| Topsoil-Compost-Amendment Blend | ~40/40/20 | Topsoil : compost : perlite-or-vermiculite — adds drainage/aeration over a straight topsoil-compost blend |
| Custom Ratio | User-defined | Up to 3 components, must sum to 100% |
Coverage per Cubic Yard (Approximate)
| Depth | Coverage per Cubic Yard |
|---|---|
| 6 in | ~54 sq ft |
| 8 in | ~40.5 sq ft |
| 10 in | ~32 sq ft |
| 12 in | ~27 sq ft |
When should you use this raised bed soil calculator?
- Planning a new raised bed build and buying each soil-mix ingredient separately.
- Comparing Mel's Mix against a lower-cost topsoil-compost blend for the same bed size using Compare.
- Entering your own supplier's recommended ratio via Custom Ratio, validated to sum to 100%.
- Sizing multiple beds of different presets or custom sizes by running the calculator once per bed.
- Budgeting total soil-mix material cost for a season's garden build-out.
Quick Raised Bed Soil Tips
- Blend components thoroughly before or while filling — a designed recipe only works as intended when it's a homogeneous mix, not layered.
- Use a blend of several compost types where possible rather than relying on one single compost source, for broader nutrient diversity.
- Wet peat moss thoroughly before mixing — dry peat strongly resists rewetting once compacted into a bed.
- Leave the bed's top edge slightly below full to allow for a mulch layer and initial watering-in settlement.
- Plan for an annual compost top-up — organic matter breaks down and the fill level drops a bit each season.
- Confirm the bed has an open bottom or proper drainage before filling — a raised bed with no drainage can waterlog badly.
Common Mistakes
- Filling a bed with plain topsoil (or plain garden soil) alone, without any compost or aeration amendment mixed in.
- Leaving components layered instead of thoroughly blended, creating uneven drainage and rooting conditions.
- Entering a Custom Ratio that doesn't sum to 100% and not noticing the calculation is blocked until the split is corrected.
- Using construction-grade vermiculite or perlite instead of a horticultural-grade product.
- Skipping a soil test on a topsoil-based blend intended for food crops, missing a pH or nutrient issue before planting.
- Filling a bed with no drainage consideration, leading to waterlogging after heavy rain or irrigation.
Limitations
- Computes soil-mix material quantity only — it does not perform a soil test or verify bed-frame structural design.
- The built-in typical-depth and recipe-ratio guidance is general horticultural practice, not a cited numeric standard — actual needs vary by crop, climate, and local supplier guidance.
- Custom Ratio supports up to 3 components — a recipe genuinely needing more distinct ingredients isn't modeled directly (combine similar ingredients into one component instead).
- All components share one selected bag size for bag-count purposes — actual bag sizes for topsoil, compost, and amendments often differ in practice.
- The 10% settling allowance is a fixed assumption, not user-adjustable per project or mix composition.
- Cost estimation covers soil-mix material only — delivery fees, bed-frame material, and labor for blending/filling are not included.
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.