Know exactly how much soil to buy and what to mix. This chart covers soil volume for every common raised bed size, 4 proven soil recipes, bulk-vs-bag costs, and the filling tips that save money and grow better plants.
1. The Volume Formula
Volume (cu yd) = Volume (cu ft) ÷ 27
Volume (gallons) = Volume (cu ft) × 7.48
Bags (2 cu ft) = Volume (cu ft) ÷ 2 (round up)
2. Soil Volume for Common Bed Sizes (12-inch depth)
| Bed Size | Area (sq ft) | Volume (cu ft) | Volume (cu yd) | Volume (gal) | 2-cu-ft Bags |
|---|---|---|---|---|---|
| 2 × 4 ft | 8 | 8 | 0.3 | 60 | 4 |
| 3 × 6 ft | 18 | 18 | 0.7 | 135 | 9 |
| 4 × 4 ft | 16 | 16 | 0.6 | 120 | 8 |
| 4 × 6 ft | 24 | 24 | 0.9 | 180 | 12 |
| 4 × 8 ft | 32 | 32 | 1.2 | 239 | 16 |
| 4 × 10 ft | 40 | 40 | 1.5 | 299 | 20 |
| 4 × 12 ft | 48 | 48 | 1.8 | 359 | 24 |
| 2 × 8 ft | 16 | 16 | 0.6 | 120 | 8 |
| 2 × 12 ft | 24 | 24 | 0.9 | 180 | 12 |
For a 6-inch-deep bed, halve the volume. For an 18-inch bed, multiply by 1.5. Leave 1–2 inches of headspace at the top for watering and mulch.
3. Four Proven Soil Mix Recipes
🥇 Balanced Mix (All-Purpose) — 60/30/10
Best for most vegetables. Topsoil gives structure, compost feeds plants, perlite improves drainage and aeration.
🥈 Mel's Mix (Square Foot Gardening) — 1/3 each
Lightweight and nutrient-rich. Peat moss retains moisture but is not renewable — substitute coco coir for a sustainable option.
🌿 Organic Mix (Heavy Feeders) — 50/30/20
High compost content for heavy-feeding crops (tomatoes, corn, squash). Coir is a sustainable peat alternative that holds moisture well.
☁️ Lightweight Mix (Containers & Wet Climates) — 40/30/30
Extra perlite for fast drainage. Use for rooftop beds, containers, or gardens in rainy climates where waterlogging is a risk.
4. Component Breakdown for a 4×8 ft Bed (32 cu ft)
| Mix | Topsoil | Compost | Perlite/Coir/Peat |
|---|---|---|---|
| Balanced | 19.2 cu ft (10 bags) | 9.6 cu ft (5 bags) | 3.2 cu ft (2 bags perlite) |
| Mel's Mix | 10.7 cu ft (6 bags) | 10.7 cu ft (6 bags) | 10.7 cu ft (6 bags peat/coir) |
| Organic | 9.6 cu ft (5 bags) | 16.0 cu ft (8 bags) | 6.4 cu ft (4 bags coir) |
| Lightweight | 12.8 cu ft (7 bags) | 9.6 cu ft (5 bags) | 9.6 cu ft (5 bags perlite) |
Bag counts assume 2 cu ft per bag. Perlite and coir are often sold in smaller bags (4–8 qt) — check the label. For bulk orders, round up to the nearest ½ cubic yard.
5. Bulk vs Bag Cost Comparison
| Option | Volume | Typical Price | Per Cubic Foot | For 32 cu ft (4×8 bed) |
|---|---|---|---|---|
| Bulk topsoil (delivered) | 1 cu yd (27 cu ft) | $30–$50 | $1.10–$1.85 | $35–$60 |
| 2-cu-ft bag (garden soil) | 2 cu ft | $5–$10 | $2.50–$5.00 | $80–$160 |
| 1.5-cu-ft bag (compost) | 1.5 cu ft | $4–$8 | $2.67–$5.33 | — |
| Bulk compost | 1 cu yd | $25–$40 | $0.93–$1.48 | — |
Bulk soil costs roughly half to a third of bagged soil. For any bed over 20 cu ft, buying topsoil and compost in bulk and mixing yourself saves significant money. Most landscape suppliers deliver with a minimum of 1 cubic yard.
6. Recommended Depth by Crop
| Depth | Best For | Crops |
|---|---|---|
| 6 inches | Shallow-rooted | Lettuce, greens, radishes, herbs, strawberries, onions |
| 8–10 inches | Most vegetables | Peppers, beans, peas, squash, cucumbers, garlic |
| 12 inches | Deep-rooted & root crops | Tomatoes, carrots, potatoes, corn, melons, okra |
| 18+ inches | Maximum root depth | Tomatoes (best flavor), sweet potatoes, parsnips, perennial veggies |
7. Filling & Maintenance Tips
🚫 Never Use Garden Soil Alone
Garden soil compacts in raised beds, drains poorly, and may contain pests and weed seeds. Always mix with compost and a lightening agent (perlite, vermiculite, or coir).
🚛 Buy in Bulk for Big Projects
For more than ~20 cu ft, buy topsoil and compost by the cubic yard. One cubic yard fills about thirteen 2-cu-ft bags — at half the cost.
📏 Leave Headspace
Fill beds to within 1–2 inches of the rim. This leaves room for watering without runoff and space for a layer of mulch on top.
🔄 Mix Thoroughly
Blend all components together before filling, or layer and till. Avoid pockets of pure peat or perlite that throw off water and nutrient balance.
🌱 Top-Dress Annually
Soil settles and compost decomposes. Each spring, add 1–2 inches of compost to replenish nutrients and organic matter. No need to replace all soil.
🧱 Save on Fill
For deep beds, fill the bottom third with organic "browns" (straw, leaves, wood chips) to save soil. They decompose over time, adding organic matter.
💧 Water as You Fill
Moisten each layer as you build the bed. This settles the soil and prevents large air pockets that dry out roots.
🧪 Test Before Planting
Test pH and N-P-K of your finished mix. Adjust with lime, sulfur, or fertilizer based on what you're growing. Compost pH is usually near-neutral.
8. Quick Conversions
| Convert | Multiply By | Example |
|---|---|---|
| Cubic feet → cubic yards | ÷ 27 | 32 cu ft ÷ 27 = 1.19 cu yd |
| Cubic feet → gallons | × 7.48 | 32 cu ft × 7.48 = 239 gal |
| Cubic yards → cubic feet | × 27 | 1 cu yd = 27 cu ft |
| Inches depth → feet | ÷ 12 | 12 in ÷ 12 = 1 ft |
| Cubic feet → 2-cu-ft bags | ÷ 2 | 32 cu ft ÷ 2 = 16 bags |