How many yards of concrete do I need for a 10×10 slab?

At 4 inches deep: 1.23 yd³ (about 56 × 80-lb bags). At 6 inches deep: 1.85 yd³ (about 84 bags). Always add 10% overage for spillage and uneven subgrade — round up to 1.36 yd³ (4") or 2.04 yd³ (6").

How many 80-lb bags of concrete make a yard?

One 80-lb bag of premixed concrete yields about 0.60 cubic feet cured. Since 1 yd³ = 27 ft³, you need 27 ÷ 0.60 = 45 bags per cubic yard. A 60-lb bag yields 0.45 ft³, so 60 bags per yard.

How do I calculate concrete yardage for a round slab?

For a circular slab, use Yards = π × (radius_ft)² × depth_ft ÷ 27. Example: a 12-ft-diameter patio (radius 6 ft) at 4 inches deep = π × 36 × 0.333 ÷ 27 = 1.40 yd³. Add 10% overage for 1.54 yd³.

How much extra concrete should I order?

Add 10% overage for rectangular slabs on a level subgrade. Bump to 15–20% for irregular shapes, hand-mixed bag projects, or uneven ground. Short-loading a ready-mix truck costs $150–$300 in extra delivery fees, so over-ordering is much cheaper than under-ordering.

Concrete Yard Formula: How to Calculate Cubic Yards of Concrete

Whether you are pouring a patio, a driveway, or a foundation footing, the formula is the same: cubic yards = (length × width × depth in feet) ÷ 27. This guide walks you through the math, the unit conversions, and the practical rules of thumb (like 10% overage) so you never short-load a ready-mix truck.

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Quick Answer: Concrete Yards for Common Slabs

Precomputed cubic yards using Yards = (L × W × D in ft) ÷ 27, plus a 10% overage figure. Use the calculator above for your exact dimensions:

Slab	Dimensions (ft)	Depth (in)	Volume (ft³)	Yards	+10% overage	80-lb bags
Patio slab	10 × 10	4	33.3	1.23	1.36	75
Shed floor	12 × 12	6	72.0	2.67	2.93	163
Driveway	20 × 20	4	133.3	4.94	5.43	302
Garage floor	24 × 24	6	288.0	10.67	11.73	653
Sidewalk (per 10 ft)	4 × 10	4	13.3	0.49	0.54	30
Footing (per 10 ft)	2 × 10	12	20.0	0.74	0.81	45

80-lb bag yields 0.60 ft³ cured. The 24×24 garage example needs at least 1 full ready-mix truck (typical 10 yd³ minimum).

The concrete yard formula

Yards = (Length × Width × Depth in feet) ÷ 27

Why 27? Because 1 cubic yard = 27 cubic feet (3 ft × 3 ft × 3 ft). The formula converts a volume in ft³ into a volume in yd³.

Worked example: 10 × 10 slab at 4 inches deep

Length = 10 ft, Width = 10 ft, Depth = 4 in = 0.333 ft
Volume in ft³ = 10 × 10 × 0.333 = 33.3 ft³
Yards = 33.3 ÷ 27 = 1.23 yd³
Add 10% overage → 1.36 yd³
At 0.60 ft³ per 80-lb bag → 56 bags exact, 62 with overage

Worked example: 20 × 20 driveway at 4 inches deep

Length = 20 ft, Width = 20 ft, Depth = 0.333 ft
Volume = 20 × 20 × 0.333 = 133.3 ft³
Yards = 133.3 ÷ 27 = 4.94 yd³
With 10% overage → 5.43 yd³ (still less than a full 8-yd³ truck, but ready-mix minimums usually trigger anyway)

Metric conversion

For metric slabs: cubic meters = (L × W × D in meters). To convert m³ to yd³, multiply by 1.308. A 3 m × 3 m × 0.10 m slab is 0.9 m³ = 1.18 yd³.

Circular slabs, footings, and columns

Round slab: Yards = π × (radius_ft)² × depth_ft ÷ 27. A 12-ft diameter circle, 4 in deep = π × 36 × 0.333 ÷ 27 = 1.40 yd³.
Sonotube / post hole: Yards = π × (radius_ft)² × depth_ft ÷ 27. A 12-in diameter tube, 4 ft deep = π × 0.25 × 4 ÷ 27 = 0.12 yd³ per hole.
Sonotube feet: Add 1 ft to depth for the bell at the bottom of the tube.

The 10% overage rule

Always add 10% to the calculated yardage. Concrete slabs lose volume to:

Subgrade unevenness (the ground is rarely perfectly flat).
Spillage and over-excavation.
Form deflection under wet-concrete pressure.
Short-load fees from the ready-mix company if you run out mid-pour.

For irregular shapes, broken-up slabs, or hand-mixed bag projects, bump overage to 15–20%.

Ready-mix vs bags

Bags are cheaper for small jobs (under 1 yd³) and avoid the ready-mix minimum delivery fee (typically $150–$300 plus a 1–3 yd³ minimum order). Ready-mix wins for anything over 1 yd³ because mixing 60+ bags by hand takes an entire weekend. Quick rule: 1 yd³ ≈ 60 × 80-lb bags, and a typical residential slab needs 2–10 yd³.

Concrete Mix and Yield References

ASTM C138 / C138M — Density, Yield, and Air Content of Concrete. astm.org
ACI 211.1 — Standard Practice for Selecting Proportions for Normal, Heavyweight, and Mass Concrete. concrete.org
PCA — Design and Control of Concrete Mixtures (15th ed.). cement.org

FAQ

Frequently asked questions

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Cubic Yards = (Length × Width × Depth in feet) ÷ 27. The 27 is because 1 cubic yard = 27 cubic feet (3 × 3 × 3). Convert depth in inches to feet by dividing by 12. For a 10×10 slab 4 inches deep: (10 × 10 × 0.333) ÷ 27 = 1.23 yd³.