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) รท 27
Volume (gallons) = Volume (cu ft) ร 7.48
Bags (2 cu ft) = Volume (cu ft) รท 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 ร 4 ft | 8 | 8 | 0.3 | 60 | 4 |
| 3 ร 6 ft | 18 | 18 | 0.7 | 135 | 9 |
| 4 ร 4 ft | 16 | 16 | 0.6 | 120 | 8 |
| 4 ร 6 ft | 24 | 24 | 0.9 | 180 | 12 |
| 4 ร 8 ft | 32 | 32 | 1.2 | 239 | 16 |
| 4 ร 10 ft | 40 | 40 | 1.5 | 299 | 20 |
| 4 ร 12 ft | 48 | 48 | 1.8 | 359 | 24 |
| 2 ร 8 ft | 16 | 16 | 0.6 | 120 | 8 |
| 2 ร 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 1โ2 inches of headspace at the top for watering and mulch.
3. Four Proven Soil Mix Recipes
๐ฅ Balanced Mix (All-Purpose) โ 60/30/10
Best for most vegetables. Topsoil gives structure, compost feeds plants, perlite improves drainage and aeration.
๐ฅ Mel's Mix (Square Foot Gardening) โ 1/3 each
Lightweight and nutrient-rich. Peat moss retains moisture but is not renewable โ substitute coco coir for a sustainable option.
๐ฟ Organic Mix (Heavy Feeders) โ 50/30/20
High compost content for heavy-feeding crops (tomatoes, corn, squash). Coir is a sustainable peat alternative that holds moisture well.
โ๏ธ Lightweight Mix (Containers & Wet Climates) โ 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 4ร8 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 (4โ8 qt) โ check the label. For bulk orders, round up to the nearest ยฝ cubic yard.
5. Bulk vs Bag Cost Comparison
| Option | Volume | Typical Price | Per Cubic Foot | For 32 cu ft (4ร8 bed) |
|---|---|---|---|---|
| Bulk topsoil (delivered) | 1 cu yd (27 cu ft) | $30โ$50 | $1.10โ$1.85 | $35โ$60 |
| 2-cu-ft bag (garden soil) | 2 cu ft | $5โ$10 | $2.50โ$5.00 | $80โ$160 |
| 1.5-cu-ft bag (compost) | 1.5 cu ft | $4โ$8 | $2.67โ$5.33 | โ |
| Bulk compost | 1 cu yd | $25โ$40 | $0.93โ$1.48 | โ |
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 |
| 8โ10 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
๐ซ 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).
๐ 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 โ at half the cost.
๐ Leave Headspace
Fill beds to within 1โ2 inches of the rim. This leaves room for watering without runoff and space for a layer of mulch on top.
๐ 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.
๐ฑ Top-Dress Annually
Soil settles and compost decomposes. Each spring, add 1โ2 inches of compost to replenish nutrients and organic matter. No need to replace all soil.
๐งฑ 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.
๐ง 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.
๐งช 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 โ cubic yards | รท 27 | 32 cu ft รท 27 = 1.19 cu yd |
| Cubic feet โ gallons | ร 7.48 | 32 cu ft ร 7.48 = 239 gal |
| Cubic yards โ cubic feet | ร 27 | 1 cu yd = 27 cu ft |
| Inches depth โ feet | รท 12 | 12 in รท 12 = 1 ft |
| Cubic feet โ 2-cu-ft bags | รท 2 | 32 cu ft รท 2 = 16 bags |