How Much Concrete Do I Need for a Slab?

By RugCalc Editorial Team · Last updated: October 2026 · 9 min read

Quick answer: concrete = length × width × depth (in feet), then divide by 27 for cubic yards. A 10×10 ft slab at 4 inches deep needs about 1.23 cubic yards, or roughly 61 bags of 80 lb mix. Here's the full formula, depth guidance, and a worked example.

The concrete formula

Concrete is measured by volume. Work in feet so the numbers divide cleanly into cubic yards:

Length (ft) × Width (ft) × Depth (ft) = cubic feet, then cubic feet ÷ 27 = cubic yards.

Because depth is usually given in inches, convert first: 4 inches = 4 ÷ 12 = 0.333 ft; 6 inches = 0.5 ft. Add 5–10% waste to your final number.

Depth by use

Project	Recommended depth	Notes
Patio / walkway	4 in	Fine for foot traffic and light loads
Garden shed pad	4 in	Check local code for shed size limits
Driveway (cars)	5–6 in	Reinforce with rebar or wire mesh
Structural / footings	8+ in	Needs an engineer for load design

Cubic yards to bags

One cubic yard equals 27 cubic feet. If you're mixing bags yourself, standard bag sizes make these volumes:

• 80 lb bag → 0.60 cu ft → about 45 bags per cubic yard
• 60 lb bag → 0.45 cu ft → about 60 bags per cubic yard
• 40 lb bag → 0.30 cu ft → about 90 bags per cubic yard

Worked example: a 10×10 ft slab

• Area = 10 × 10 = 100 sq ft
• Depth = 4 in = 0.333 ft
• Volume = 100 × 0.333 = 33.3 cu ft
• Cubic yards = 33.3 ÷ 27 = 1.23 cu yd
• 80 lb bags = 33.3 ÷ 0.60 = ~56 bags, plus 10% waste → ~61 bags

Ready-mix vs bagged mix

For anything much bigger than a few bags' worth, ready-mix (delivered by truck) is cheaper per cubic yard and saves your back. A rule of thumb: up to about 1 cubic yard, bagged mix is practical; beyond that, get a quote for ready-mix. You'll also need forms, a gravel base, and often rebar — budget those before you order.

Quick lookup: common slab sizes

Here's what common projects need at a 4-inch depth (with ~10% waste added):

Slab	Volume	Cubic yards	80 lb bags
4×4 (small pad)	5.3 cu ft	0.2	~10
8×10 (shed)	26.7 cu ft	1.0	~49
10×10 (patio)	33.3 cu ft	1.2	~61
10×12 (patio)	40 cu ft	1.5	~73
12×12 (small driveway)	48 cu ft	1.8	~88
20×20 (driveway, 5 in)	166.7 cu ft	6.2	~306 (or ready-mix)

A 20×20 driveway is well into ready-mix territory — hauling 300 bags is impractical for most people.

Strength, reinforcement & prep

Bagged mixes are labeled by compressive strength (for example 3,000 or 4,000 psi). Use 3,000 psi for patios and walkways and 4,000+ psi for driveways and anything with vehicle loads. Add rebar or wire mesh for any slab that carries cars, and place it mid-depth so it's actually in tension. Always put a compacted 4-inch gravel base under the slab — concrete poured on soft dirt will crack as it settles.

What it costs

Planning budgets for 2026: ready-mix usually runs $100–$150 per cubic yard, plus a delivery fee. Bagged mix costs more per cubic yard — roughly $150–$250 when you buy bags — but avoids the delivery minimum. On top of either, add forms, gravel base, rebar, and often a finisher. These are estimates; get local quotes for your project.

Pour day checklist

• Gravel base compacted and level.
• Forms set and braced, top edge at finished height.
• Rebar/wire in place and raised to mid-depth.
• Water and finishing tools ready; weather dry.
• Mixer/truck scheduled so you don't run out mid-pour.

Common mistakes

• Mixing up inches and feet. A 4-inch depth written as "4 ft" gives a wildly wrong, oversized order. Convert depth to feet first.
• Forgetting waste. Subgrade is never perfectly level — add 5–10%.
• Ordering ready-mix too tight. A truck can't easily add a little more once it's on site. Round up to the next quarter yard.
• No base. Concrete over soft dirt cracks. Compact a 4-in gravel base first.

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