Raised Garden Bed Guide: Soil, Size & Materials Calculator

Optimal Raised Bed Dimensions

The most important dimension rule for raised beds is width: keep it no wider than 4 feet if you can reach it from both sides, or 2 feet if you can only reach from one side. This ensures you never have to step in the bed, which would compact the soil. Common widths are 2, 3, and 4 feet.

Width: 3–4 feet maximum for access from both sides
Length: Any length works; 8–12 feet is common. Longer beds are fine but longer lengths of lumber may sag without a center support post.
Depth: 6 inches minimum for lettuce, herbs, and shallow-rooted crops. 12 inches for most vegetables. 18–24 inches for deep-rooted plants like tomatoes, carrots, and potatoes. Deeper beds allow better root development and better drought resistance.

Calculating Soil Volume

To find how much soil you need:

Cubic feet = Length × Width × Depth (all in feet)

To convert to cubic yards (how bulk soil is sold): divide cubic feet by 27.

Example: A 4×8 ft bed, 12 inches (1 ft) deep: 4 × 8 × 1 = 32 cubic feet ÷ 27 = 1.19 cubic yards. For two such beds, order approximately 2.5 cubic yards to account for settling.

Soil in raised beds settles 15–20% over the first season as organic matter decomposes. Plan to top up annually.

The Best Soil Mix for Raised Beds

Never fill a raised bed with native soil — it compacts, drains poorly, and doesn't provide enough nutrition. The classic "Mel's Mix" and similar formulas use:

60% quality topsoil or loam — provides structure and minerals
30% compost — provides nutrients, beneficial microbes, and improves drainage
10% perlite or coarse horticultural sand — improves aeration and drainage

You can buy pre-blended "raised bed mix" or "garden mix" from landscape suppliers, which approximates this ratio. Avoid mixes that are primarily peat moss — they become hydrophobic when dry and are not sustainable.

Raised Bed Materials Comparison

Cedar: The gold standard for DIY beds. Naturally rot-resistant, lasts 10–20 years. More expensive than pine.
Redwood: Similar to cedar, very durable, but expensive and harder to find.
Pine (untreated): Cheap and easy to find, but rots in 3–7 years.
ACQ pressure-treated lumber: Modern pressure-treated wood uses copper-based preservatives that are considered safe for food gardens. Lasts 20+ years.
Galvanized steel: Modern corrugated steel beds look great, are extremely durable (20–30 years), and don't rot. Heat up faster in spring. Cost: $150–$400 for a kit.
Composite lumber (recycled plastic/wood): Very long-lasting, no rot, but expensive and heavier.

Pest Protection and Base Preparation

Lay hardware cloth (1/4 inch galvanized mesh) on the ground before filling with soil. This prevents voles, gophers, and other burrowing rodents from tunneling up into the bed from below. Staple it to the bottom of the frame or just lay it loose on the ground — roots will grow through the small openings but pests cannot enter.

If placing beds on a lawn, lay cardboard under the hardware cloth as a weed barrier. It will decompose in 1–2 seasons but gives you time to establish the bed without competition from grass.

Frequently Asked Questions

How deep should a raised garden bed be for vegetables?

12 inches is the practical minimum for most vegetables including tomatoes, peppers, squash, and beans. 18–24 inches is ideal and allows the best root development. Shallow-rooted crops like lettuce, spinach, herbs, and radishes can do fine in just 6 inches. If building over concrete or pavement, 18–24 inches is recommended to give roots room to spread.

What's the best soil mix for raised beds?

A blend of 60% topsoil, 30% compost, and 10% perlite provides excellent drainage, nutrition, and texture. Avoid using 100% native soil (too heavy, poor drainage) or 100% compost (too rich, can burn seedlings). Pre-blended 'raised bed mix' from a landscape supplier is a convenient and cost-effective option for large beds.

Can I use regular garden soil in a raised bed?

Native garden soil is not recommended for raised beds. It typically contains clay that compacts under the weight of the bed walls and watering, preventing drainage and root growth. If your native soil is very sandy loam, it can be used as the 'topsoil' portion of a blend, but it should be mixed with generous amounts of compost before using it in a raised bed.

How long do wood raised beds last?

Cedar and redwood raised beds last 10–20 years. Untreated pine lasts 3–7 years before rotting. Modern pressure-treated lumber (ACQ-treated) lasts 20+ years and is considered safe for food gardens by the EPA and most university extension programs. Galvanized steel beds can last 30+ years with no rotting concerns.

Do raised beds need drainage holes?

Raised beds with open bottoms (placed directly on ground) drain naturally and don't need holes. If you're building a raised bed on a deck, patio, or any sealed surface, you must provide drainage — either by leaving the bottom open with hardware cloth, drilling large holes in the bottom boards, or using a container specifically designed with drainage. Without drainage, roots will sit in waterlogged soil and rot.