How do I calculate cubic feet of soil for a raised bed?

Multiply length × width × depth in feet. A 4 × 8 × 1 ft bed needs 32 ft³. Use 1 ft (12 in) of depth for vegetables; 1.5–2 ft for root crops or deep-rooted perennials.

How deep should a raised bed be?

Most leafy vegetables need 6–8 inches of soil; tomatoes, peppers, and squash do better with 12 inches; carrots, parsnips, and other root crops need 18+ inches.

What is the best soil mix for raised beds?

A common formula: 60% topsoil, 30% compost, 10% drainage material (perlite or vermiculite). For "Mel's Mix" (Square Foot Gardening): 1/3 compost, 1/3 peat moss, 1/3 vermiculite by volume.

How many bags of soil do I need for a 4 × 8 raised bed?

At 12 inches deep: 32 ft³. That's 32 of the 1 ft³ bags, 22 of the 1.5 ft³ bags, or 16 of the 2 ft³ bags.

Should I add gravel at the bottom of a raised bed?

No — gravel layers actually impede drainage by creating a perched water table. Skip the gravel; cardboard or landscape fabric on the soil contact surface is enough.

Raised Bed Cubic Feet Calculator — Soil Volume for 4 × 8, 4 × 4 & Custom Beds

How to calculate cubic feet for raised bed

Raised beds are easy: multiply length × width × depth (all in feet) for cubic feet of soil. The depth determines what you can grow:

6–8 inches: lettuces, herbs, radishes, scallions
10–12 inches: tomatoes, peppers, beans, squash, most flowers
18 inches: carrots, parsnips, potatoes, dwarf fruit shrubs
24+ inches: deep-rooted perennials, pollinator habitats

Worked examples

Example 1: Standard 4 × 8 ft raised bed at 12 inches deep

4 × 8 × 1 = 32 ft³. About 16 bags of 2 ft³ soil mix or 1.19 yd³ delivered.

Example 2: Two 4 × 4 ft beds at 12 inches deep

2 × (4 × 4 × 1) = 32 ft³. Same total as one 4 × 8 ft bed.

Example 3: A deep 4 × 8 × 18 in carrot bed

4 × 8 × 1.5 = 48 ft³ = 1.78 yd³.

Common raised bed sizes (12 in deep)

Bed dimensions	Conversion factor	Worked example
4 × 4 × 1 ft	16 ft³	~8 of the 2 ft³ bags
4 × 6 × 1 ft	24 ft³	~12 of the 2 ft³ bags
4 × 8 × 1 ft	32 ft³	~16 of the 2 ft³ bags
4 × 12 × 1 ft	48 ft³	~24 of the 2 ft³ bags
6 × 12 × 1 ft	72 ft³	~36 of the 2 ft³ bags
4 × 8 × 1.5 ft	48 ft³	~24 of the 2 ft³ bags

Tips and considerations

Don't fill with topsoil alone

Pure topsoil compacts and drains poorly in a raised bed. Mix with compost (30%) and either perlite or coarse sand (10%) for proper drainage and nutrition.

Account for settling

Fresh soil mix settles 10–15% in the first weeks. Over-fill the bed by an inch or two and plan to top up with compost the next spring.

Consider bottom-filling tricks

For very deep beds (over 18 in), the bottom 6 inches can be filled with woody debris (hugelkultur), straw, or yard waste — saves money and improves long-term drainage.

Common mistakes to avoid

Forgetting depth in feet. A 4 × 8 ft bed at 12-inch depth is 32 ft³, but at 6-inch depth it is only 16 ft³. Soil order is depth-sensitive.
Ordering full topsoil for a deep bed. Most raised beds use 50% topsoil + 30% compost + 20% other amendments. Pure topsoil is overkill and underperforms.
Forgetting settling. Loose-mix soil settles 15–25% in the first season. Order extra and top up next year.
Using outside dimensions. The 2-by lumber forming the bed walls takes up real volume. Use inside-of-walls dimensions for soil orders.