Quick Answer
Multiply length × width × depth (converting depth from inches to feet by dividing by 12). A 4×8 raised bed at 12" deep needs 32 cubic feet of soil — about 1.19 cubic yards, 11 bags (3 cu ft), or 16 bags (2 cu ft).
Need exact amounts for multiple beds, custom shapes, or specific soil mixes?
Open the Free CalculatorRaised beds are one of the best ways to grow vegetables, flowers, and herbs — but before you plant a single seed, you need to answer one question: how much soil do I actually need?
Buy too little and you'll make an extra trip to the garden center. Buy too much and you'll waste money on soil you can't use. This guide gives you the exact formula, pre-calculated charts for every common bed size, the best soil mix recipes, and a cost comparison between bags and bulk delivery.
If you just want a number, jump straight to the soil charts below or use our free raised bed soil calculator.
The Formula (With Step-by-Step Example)
The volume formula for a rectangular raised bed is:
Volume (cu ft) = Length (ft) × Width (ft) × Depth (in) ÷ 12
Example — 4×8 bed, 12 inches deep:
- Multiply length × width: 4 × 8 = 32 sq ft
- Convert depth to feet: 12 in ÷ 12 = 1 ft
- Multiply area × depth: 32 × 1 = 32 cubic feet
- Convert to cubic yards (optional): 32 ÷ 27 = 1.19 cu yd
For circular beds, use: π × (diameter ÷ 2)² × depth ÷ 12
For L-shaped, U-shaped, or keyhole beds, break the shape into simpler rectangles or circles, calculate each piece, and add them together — or use our calculator which handles all five shapes automatically.
Soil Chart: Rectangular Raised Beds
The table below shows soil volume for the most popular rectangular raised bed sizes at three common depths. All volumes are rounded up.
| Bed Size | 6" Deep | 12" Deep | 18" Deep | Cu Yd (12") |
|---|---|---|---|---|
| 2×4 ft | 4 cu ft | 8 cu ft | 12 cu ft | 0.30 |
| 2×8 ft | 8 cu ft | 16 cu ft | 24 cu ft | 0.59 |
| 3×3 ft | 4.5 cu ft | 9 cu ft | 13.5 cu ft | 0.33 |
| 3×6 ft | 9 cu ft | 18 cu ft | 27 cu ft | 0.67 |
| 4×4 ft | 8 cu ft | 16 cu ft | 24 cu ft | 0.59 |
| 4×6 ft | 12 cu ft | 24 cu ft | 36 cu ft | 0.89 |
| 4×8 ft | 16 cu ft | 32 cu ft | 48 cu ft | 1.19 |
| 4×10 ft | 20 cu ft | 40 cu ft | 60 cu ft | 1.48 |
| 4×12 ft | 24 cu ft | 48 cu ft | 72 cu ft | 1.78 |
| 5×10 ft | 25 cu ft | 50 cu ft | 75 cu ft | 1.85 |
| 6×6 ft | 18 cu ft | 36 cu ft | 54 cu ft | 1.33 |
| 6×8 ft | 24 cu ft | 48 cu ft | 72 cu ft | 1.78 |
| 8×8 ft | 32 cu ft | 64 cu ft | 96 cu ft | 2.37 |
| 8×12 ft | 48 cu ft | 96 cu ft | 144 cu ft | 3.56 |
12 inches is the most common raised bed depth and is highlighted. Cubic yards column is based on the 12-inch depth.
Soil Chart: Circular Raised Beds
Circular beds (stock tanks, galvanized rings, keyhole gardens) use the formula π × r² × depth.
| Diameter | 6" Deep | 12" Deep | 18" Deep | Cu Yd (12") |
|---|---|---|---|---|
| 3 ft | 3.5 cu ft | 7.1 cu ft | 10.6 cu ft | 0.26 |
| 4 ft | 6.3 cu ft | 12.6 cu ft | 18.8 cu ft | 0.47 |
| 5 ft | 9.8 cu ft | 19.6 cu ft | 29.5 cu ft | 0.73 |
| 6 ft | 14.1 cu ft | 28.3 cu ft | 42.4 cu ft | 1.05 |
| 8 ft | 25.1 cu ft | 50.3 cu ft | 75.4 cu ft | 1.86 |
| 10 ft | 39.3 cu ft | 78.5 cu ft | 117.8 cu ft | 2.91 |
How Many Bags of Soil Do I Need?
Once you know your total cubic feet, divide by the bag size and round up. Here's a reference chart for the most popular bed sizes at 12" depth:
| Bed (12" deep) | Cu Ft | 1 cu ft bags | 1.5 cu ft bags | 2 cu ft bags | 3 cu ft bags |
|---|---|---|---|---|---|
| 3×6 ft | 18 | 18 | 12 | 9 | 6 |
| 4×4 ft | 16 | 16 | 11 | 8 | 6 |
| 4×6 ft | 24 | 24 | 16 | 12 | 8 |
| 4×8 ft | 32 | 32 | 22 | 16 | 11 |
| 4×10 ft | 40 | 40 | 27 | 20 | 14 |
| 4×12 ft | 48 | 48 | 32 | 24 | 16 |
| 6×8 ft | 48 | 48 | 32 | 24 | 16 |
| 8×8 ft | 64 | 64 | 43 | 32 | 22 |
Tip: Always buy 10% extra to account for settling. A 4×8 bed at 32 cu ft should have about 35 cu ft of soil purchased.
Bags vs. Bulk: Cost Comparison
Should you buy individual bags from the garden center or order bulk soil by the cubic yard? Here's how the costs compare for common bed volumes:
| Volume | Bags (2 cu ft @ $9) | Bulk ($25–$50/yd³) | You Save |
|---|---|---|---|
| 16 cu ft (4×4) | $72 | $15–$30 | $42–$57 |
| 32 cu ft (4×8) | $144 | $30–$60 | $84–$114 |
| 48 cu ft (4×12) | $216 | $45–$89 | $127–$171 |
| 96 cu ft (8×12) | $432 | $89–$178 | $254–$343 |
| 128 cu ft (2× 4×8) | $576 | $119–$237 | $339–$457 |
When to buy bags: One small bed, no delivery available, or you need a specific brand/mix. Bags let you pick your exact ingredients and blend them yourself.
When to buy bulk: Two or more beds, or any bed that needs 2+ cubic yards. Bulk delivery eliminates hauling individual bags and can save 40–70%. Many garden centers deliver 1 cubic yard minimum.
The break-even point is usually around 1.5–2 cubic yards. Below that, bags are competitive after you factor in delivery fees ($30–$75 typical). Our calculator shows both options side by side so you can compare for your exact volume.
How Deep Should Raised Bed Soil Be?
Depth depends on what you're growing and whether your bed sits on native soil or a hard surface like concrete or a patio.
| Plant Type | Minimum Depth | Ideal Depth | Examples |
|---|---|---|---|
| Herbs & leafy greens | 6" | 8–10" | Basil, lettuce, spinach, cilantro |
| Bush beans, peppers | 8" | 10–12" | Bell peppers, bush beans, peas |
| Tomatoes, squash | 10" | 12–18" | Tomatoes, zucchini, cucumbers |
| Root vegetables | 12" | 18–24" | Carrots, potatoes, parsnips, beets |
| Flowers (annuals) | 6" | 8–12" | Marigolds, petunias, zinnias |
| Flowers (perennials) | 8" | 12–18" | Coneflowers, lavender, daylilies |
Pro tip: If your raised bed sits on loosened native soil (not a hard surface), roots can grow below the bed. A 6" bed on prepared ground gives plants an effective growing depth of 12–18 inches.
Best Soil Mix Recipes for Raised Beds
Don't just dump one type of soil into your bed. A blended mix gives your plants the best combination of structure, nutrients, drainage, and moisture retention. Here are four proven recipes:
1. Classic Raised Bed Mix (All-Purpose)
- 60% Topsoil — bulk structure and minerals
- 30% Compost — nutrients and beneficial microorganisms
- 10% Potting Mix — lightweight drainage and aeration
Best for: general gardening, mixed beds, first-time gardeners. This is the recipe used by Gardener's Supply Co. and recommended by most extension services.
2. Vegetable Garden Mix (Nutrient-Dense)
- 40% Topsoil
- 40% Compost
- 10% Perlite or Vermiculite
- 10% Peat Moss or Coco Coir
Best for: tomatoes, peppers, squash, root vegetables, herbs. Higher compost content provides more slowly available nutrients through the season.
3. Flower Bed Mix
- 50% Topsoil
- 30% Compost
- 10% Perlite
- 10% Peat Moss
Best for: perennials, annuals, and cut flower gardens. More topsoil for stability with good drainage.
4. Succulent & Cactus Mix (Fast-Draining)
- 40% Coarse Sand
- 30% Potting Mix
- 20% Perlite
- 10% Compost
Best for: succulents, cacti, Mediterranean herbs (lavender, rosemary). Drains very quickly — critical for plants that rot in wet soil.
Our raised bed soil calculator lets you pick any of these recipes (or create a custom mix) and shows you exactly how many cubic feet of each component you need.
Topsoil vs. Garden Soil vs. Potting Mix: What's the Difference?
Walk into any garden center and you'll find bags labeled “topsoil,” “garden soil,” and “potting mix.” They're not interchangeable. Here's how they compare:
| Type | What It Is | Best For | Drawbacks |
|---|---|---|---|
| Topsoil | The top layer of native earth, screened for debris | Bulk fill, structure foundation | Low nutrients, may contain weed seeds |
| Garden Soil | Topsoil + compost + added fertilizers | In-ground beds, amending existing soil | Can compact in raised beds, heavy |
| Potting Mix | Soilless blend of peat, perlite, bark, vermiculite | Containers, seed starting, drainage component | Too lightweight alone for raised beds, dries quickly |
| Compost | Decomposed organic matter (plant waste, manure) | Nutrient boost, soil biology, moisture retention | Settles quickly, too dense on its own |
The takeaway: Don't fill your raised bed with 100% of any one type. A blend (like the recipes above) gives you the best of all worlds — structure from topsoil, nutrients from compost, and drainage from potting mix or perlite.
Soil Settling & How to Maintain Your Beds
Freshly filled raised beds look full and beautiful — for about two months. Then the soil settles 10–20% as organic matter decomposes, rain compresses air pockets, and roots break down soil aggregates. That's completely normal.
Year-one settling plan
- Buy 10% more soil than calculated and mound it slightly above the rim
- Water the bed deeply before planting to accelerate initial settling
- Add 1–2 inches of compost mid-season if levels drop noticeably
Annual spring maintenance
- Add 2–3 inches of compost to the top each spring
- Mix in 0.5 cups of all-purpose fertilizer per 5 sq ft of bed surface
- Fork-mix the amendments into the top 4–6 inches of existing soil
- Soil level should reach within 1 inch of the bed rim
You should not need to completely replace your raised bed soil. With annual compost top-offs and occasional fertilizer, the same base soil can sustain productive gardens for many years.
7 Ways to Save Money on Raised Bed Soil
- 1Buy bulk for 2+ cubic yards
You can save 40–70% compared to individual bags. Split a delivery with a neighbor to reduce per-yard costs even further.
- 2Use the Hügelkultur method for deep beds
Fill the bottom third of a tall bed (18"+) with logs, branches, and leaves. As they decompose, they provide nutrients and moisture retention — and reduce the soil you need to buy by 30–40%.
- 3Make your own compost
Compost is 30–40% of most raised bed recipes. Starting a compost pile or tumbler saves hundreds of dollars per year on larger gardens.
- 4Check municipal compost programs
Many cities offer free or low-cost compost from their yard waste recycling programs. Some will even deliver for a nominal fee.
- 5Shop end-of-season sales
Garden centers heavily discount bagged soil in late fall. Stock up for next spring's beds at 30–50% off.
- 6Reuse last year's soil
Don't dump old bed soil. Top off with 2–3 inches of compost and fertilizer each spring. The existing base is still valuable.
- 7Loosen native soil underneath
Before placing your bed, fork-loosen the native soil 6–10 inches deep. Roots grow beyond the bed, and you can use a shallower soil depth — a 6-inch bed on loosened ground acts like 12+ inches.
Get Your Exact Soil Amount
Enter your bed dimensions, pick a soil mix, and get instant results — volume, bags needed, bulk cost, and a printable shopping list.
Open the Soil Calculator — It's FreeFrequently Asked Questions
A 4×8 raised bed needs 16 cubic feet of soil at 6 inches deep, 32 cubic feet at 12 inches deep, or 48 cubic feet at 18 inches deep. At the most common 12-inch depth, that's about 1.19 cubic yards — roughly 11 bags of 3 cu ft soil or 16 bags of 2 cu ft soil. Use our free raised bed soil calculator for exact amounts with your specific dimensions.
A 4×4 raised bed needs 8 cubic feet of soil at 6 inches deep, 16 cubic feet at 12 inches deep, or 24 cubic feet at 18 inches deep. At 12 inches deep, that's about 0.59 cubic yards — roughly 6 bags of 3 cu ft soil or 8 bags of 2 cu ft soil.
Divide your bed's total cubic feet by the bag size. For example, a 4×8 bed at 12 inches deep needs 32 cu ft of soil. That's 32 one-cubic-foot bags, 16 two-cubic-foot bags, or 11 three-cubic-foot bags. Always round up and buy 10% extra for settling.
The classic raised bed soil recipe is 60% topsoil, 30% compost, and 10% potting mix (soilless growing medium). For vegetable gardens, use a richer mix: 40% topsoil, 40% compost, 10% perlite, and 10% peat moss or coco coir. For succulents or herbs that need excellent drainage, add 20–40% coarse sand or extra perlite.
For a single small bed (under 1 cubic yard), bags are more convenient and you avoid delivery fees. Above 2 cubic yards (~54 cu ft), bulk delivery is almost always cheaper — typically $25–$50 per cubic yard vs. $80–$120 for the equivalent in bags. The break-even point is usually around 1–2 cubic yards depending on your local prices.
Most vegetables grow well in 8–12 inches of soil. Root vegetables like carrots and parsnips need 12–18 inches. Herbs and lettuce do fine in 6–8 inches. If your bed sits on loosened native soil, roots can extend below — so a 6-inch bed on prepared ground acts like 12+ inches of growing depth.
Multiply length (ft) × width (ft) × depth (inches ÷ 12). Example: a 4×8 bed at 12" deep = 4 × 8 × 1 = 32 cubic feet. For circular beds, use π × radius² × depth. To convert cubic feet to cubic yards, divide by 27.
A 4×8×12" bed costs roughly $144 using 2 cu ft bags (~$9 each) or $30–$60 using bulk delivery ($25–$50/cu yd). Premium organic mixes cost more. Multiple beds? Bulk delivery saves 40–70%. Our calculator breaks down exact costs for both options.
None alone is ideal. Topsoil gives structure but fewer nutrients. Garden soil is nutrient-rich but can be dense. Potting mix is lightweight but too fluffy for large raised beds. The best approach: blend topsoil (structure), compost (nutrients and biology), and potting mix or perlite (drainage and aeration).
Yes — expect 10–20% settling in the first growing season as organic matter decomposes and air pockets compress. Buy 10% extra soil upfront and plan to add 2–3 inches of compost each spring when topping off your beds.
Absolutely. Old raised bed soil is a valuable base. Refresh it each spring: add 2–3 inches of compost, a quarter cup of granular fertilizer per square foot, and mix the amendments into the top 4–6 inches. This restores nutrients and improves texture without replacing the entire volume.
A 3×6 raised bed needs 9 cubic feet at 6 inches deep, 18 cubic feet at 12 inches, or 27 cubic feet at 18 inches. The 12-inch depth requires about 0.67 cubic yards — roughly 6 bags of 3 cu ft soil. This is one of the most popular raised bed sizes for small gardens.