How Much Soil for a Raised Garden Bed?

The Volume Calculation

Length times width times depth, all in feet. The depth is the one most people get wrong because beds are measured in inches but volume is calculated in feet. A 4x8 bed that's 12 inches deep: 4 x 8 x 1 = 32 cubic feet. At 10 inches deep: 4 x 8 x 0.83 = 26.6 cubic feet.

Add 10% for settling. Fresh soil compresses and decomposes over the first season. A bed filled to the brim in April will be 2 inches below the rim by September. Adding 10% upfront means you start slightly mounded and end at a good level.

Choosing a Soil Mix

The most common mistake is filling a raised bed with plain topsoil. Topsoil is heavy, compacts easily, drains poorly, and often contains weed seeds. It works as a base layer for deep beds (18 inches+) but should never be the primary growing medium.

The gold standard is the Mel's Mix from the Square Foot Gardening method: equal parts compost (blended from multiple sources), peat moss or coconut coir, and coarse vermiculite. This combination drains well, retains moisture, provides nutrients, and stays fluffy for root growth. It's more expensive than plain soil but produces dramatically better results.

A budget-friendly alternative is 60% topsoil and 40% compost. It works adequately, costs less, and improves over time as you add compost each season. It's denser and drains slower than Mel's Mix, which means you water less often but risk waterlogging in heavy rain.

Premium mixes add worm castings (15-20% of the mix) for slow-release nutrients, coconut coir instead of peat (more sustainable and rehydrates easier), and perlite for drainage. These mixes produce the best yields but cost 2-3x more than basic topsoil/compost.

Bags vs Bulk Delivery

A single 4x8x12" bed needs 32 cubic feet of soil. Bagged potting mix comes in 2-cubic-foot bags. That's 16 bags at roughly $8-$15 each, totaling $128-$240 for one bed. For a single bed, bags make sense because you can pick the exact products you want and transport them in a car.

For two or more beds, bulk delivery almost always wins. A cubic yard of quality garden soil mix delivered costs $35-$65 from a landscape supply company. One cubic yard = 27 cubic feet, nearly enough for one 4x8x12 bed. Three beds need about 3 cubic yards: $105-$195 delivered. The same three beds in bags would cost $384-$720.

The break-even point is typically around 1.5-2 cubic yards. Below that, the delivery fee ($30-$50) eats the savings. Above that, bulk saves significantly.

Bed Depth Guidelines

Six inches works for shallow-rooted crops: lettuce, herbs, radishes, strawberries. This is the minimum for any productive raised bed.

Ten to twelve inches handles most vegetables: tomatoes, peppers, beans, squash, cucumbers, most root vegetables. This is the standard depth and what most pre-made bed kits are designed for.

Eighteen inches or more is needed for deep root crops like full-size carrots, parsnips, and potatoes. It also works well for beds placed on concrete or very poor soil because the depth provides enough root space without relying on the ground beneath.

Anything over 12 inches can use a layering strategy to save money: fill the bottom third with a mix of leaves, straw, and wood chips (the hugelkultur principle), then top with quality soil mix. The bottom layer breaks down over 1-2 years and feeds the soil above it.

Annual Maintenance

Add 1-2 inches of compost to the top of each bed every spring. This replaces volume lost to decomposition and settling, replenishes nutrients consumed by last year's crops, and feeds soil organisms that keep the bed healthy. A 4x8 bed needs about 5-7 cubic feet of compost per year (3-4 bags). This is the only recurring cost.