Raised Garden Bed Soil Calculator + Soil Mix

Freshly laid compost and soil along the edge of a wooden raised garden bed, top-down view

Guessing how much soil to order is how you end up with three leftover bags or a bed that’s four inches short. Here’s the actual formula, a reference table for common bed sizes, and the soil mix ratio that keeps plants healthy instead of waterlogged.

BBy the The Brofessionals editors · Updated July 2026 · 7 min read

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Raised bed soil is sold and delivered by volume — cubic feet in bags, cubic yards in bulk — so getting the math right before you order saves you a second trip (or a pile of unused soil bags in the garage). The good news: the formula is simple, and once you know your bed’s dimensions you can calculate it in seconds.

The Formula

Cubic feet = Length (ft) × Width (ft) × Depth (ft). Convert inches of depth to feet by dividing by 12 (a 10″ deep bed = 0.83 ft). To get cubic yards for bulk soil orders, divide your cubic feet total by 27.

Example: a 4×8-foot bed filled 12 inches deep is 4 × 8 × 1 = 32 cubic feet, or 32 ÷ 27 ≈ 1.19 cubic yards. That’s the number you’d give a bulk soil supplier, or use to count bagged soil (most bags are 1.5 or 2 cubic feet).

Reference Table: Common Raised Bed Sizes

Bed Size	Depth	Cubic Feet	Cubic Yards
2′ × 4′	10″	~6.7 cu ft	~0.25 cu yd
3′ × 3′	10″	~7.5 cu ft	~0.28 cu yd
3′ × 6′	10″	~15 cu ft	~0.56 cu yd
4′ × 4′	10″	~13.3 cu ft	~0.49 cu yd
4′ × 4′	12″	16 cu ft	~0.59 cu yd
4′ × 8′	10″	~26.7 cu ft	~0.99 cu yd
4′ × 8′	12″	32 cu ft	~1.19 cu yd
4′ × 8′	18″	48 cu ft	~1.78 cu yd

Round up: fresh soil and compost settles 10–15% after the first few waterings as air pockets compress. Order 10–15% more than the calculated volume, or plan on a top-off bag later in the season.

The Best Soil Mix Ratio for Raised Beds

Never fill a raised bed with plain garden topsoil or fill dirt alone — in a contained, above-ground bed it compacts, drains poorly, and starves roots of oxygen. Raised beds need a blended mix of topsoil, compost, and an aeration material.

Mix Style	Ratio (by volume)	Best For
Budget-friendly	60% topsoil / 30% compost / 10% aeration (perlite or coarse sand)	Large beds where cost matters most; still drains and feeds reasonably well
Balanced (most recommended)	40% topsoil / 40% compost / 20% aeration (perlite or vermiculite)	Most vegetable and herb beds — the standard go-to ratio
Premium / “Mel’s Mix” style	⅓ compost blend / ⅓ peat moss or coco coir / ⅓ coarse vermiculite	Intensive square-foot gardening, containers, or beds with poor native drainage

Check Raised Bed Garden Soil on Amazon → Check Perlite Soil Aerator on Amazon →

Aeration Material: Perlite or Vermiculite?

Perlite

Excellent drainage, lightweight
Holds less water — better for wet climates or beds prone to sogginess
Doesn’t compact over time

Vermiculite

Holds more moisture than perlite
Better choice in hot, dry climates where beds dry out fast
Slightly better at retaining nutrients

How to Use the Formula for Any Bed

1. Measure your bed’s inside length, width, and depth in feet (convert inches by dividing by 12).
2. Multiply length × width × depth to get cubic feet.
3. Divide by 27 if you need cubic yards for a bulk delivery order.
4. Add 10–15% to account for settling.
5. Split that total across topsoil, compost, and aeration material using your chosen ratio above.

FAQ

How many bags of soil do I need for a 4×8 bed?At 12 inches deep, a 4×8 bed needs about 32 cubic feet. Using standard 1.5 cubic-foot bags, that’s roughly 21–22 bags before accounting for settling — closer to 24 bags once you round up.

Is bagged garden soil the same as raised bed soil?No. “Garden soil” is meant to be mixed into existing ground soil and is often too dense on its own for a raised bed. Look for bags labeled specifically “raised bed mix” or blend your own using the ratios above.

Do I need to refill my raised bed every year?Not entirely — expect to top off with 1–3 inches of fresh compost each spring to replace what settled and was used up by the previous season’s plants, rather than refilling the whole bed.

Can I skip the aeration material if I’m on a budget?You can reduce it, but don’t eliminate it entirely in a mix that’s more than half compost — compost alone compacts over a season and starts holding too much water, which can suffocate roots.

The Brofessionals Editors

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Raised Garden Bed Soil Calculator + Soil Mix

The Formula

Reference Table: Common Raised Bed Sizes

The Best Soil Mix Ratio for Raised Beds