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Concrete Calculator: Calculate Yards & 80lb/60lb Bags

Calculate required concrete in cubic yards and pre-mix bags (80lb, 60lb, 40lb) for slabs, footings, and columns with waste margin.

Quick Answer:To calculate concrete for a rectangular slab: Length (ft) × Width (ft) × (Thickness in inches / 12) ÷ 27 = Cubic Yards.

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How to Use the Concrete Calculator (Slabs, Footings & Bags)

1Select your concrete project shape: Slab / Patio, Footing / Trench, Column / Sonotube, or Wall.
2Enter your dimensions in feet (length, width, height) and thickness in inches.
3Choose an appropriate waste factor (10% is standard to account for subgrade spillage).
4Review the total cubic yards needed for ready-mix delivery, or the number of pre-mix bags to buy at the hardware store.

Formula & How It Works

Concrete Volumetric Formulas

Cu Yds = (Length × Width × (Thickness / 12)) / 27 × (1 + Waste %)

Converts slab dimensions into cubic feet, adds waste margin, and divides by 27 to obtain cubic yards.

Worked Examples

Standard 10ft × 20ft Patio Slab (4 inches thick)

Input: L = 20 ft, W = 10 ft, T = 4 in, Waste = 10%

Explanation: Raw volume = 20 × 10 × (4/12) = 66.67 cu ft = 2.47 cu yds. With 10% waste = 73.33 cu ft (2.72 cu yds), requiring 123 bags of 80-lb concrete.

Result: 2.716 Cu Yds (73.33 cu ft) | 123 × 80lb bags

Round Sonotube Column

Input: Diameter = 12 in, Height = 8 ft, Qty = 2, Waste = 10%

Explanation: Radius = 0.5 ft. Raw volume for 2 columns = 2 × (π × 0.5² × 8) = 12.57 cu ft. With 10% waste = 13.82 cu ft.

Result: 0.512 Cu Yds (13.82 cu ft) | 24 × 80lb bags

Small 4ft × 4ft Sidewalk Pad

Input: L = 4 ft, W = 4 ft, T = 4 in, Waste = 10%

Explanation: Raw volume = 4 × 4 × (4/12) = 5.33 cu ft. Requires 10 bags of 80-lb pre-mix.

Result: 0.217 Cu Yds (5.87 cu ft) | 10 × 80lb bags

Frequently Asked Questions

How many 80 lb bags of concrete make a cubic yard?▼

It takes exactly 45 bags of 80-lb concrete to make one cubic yard (27 cubic feet / 0.60 cu ft yield per bag = 45 bags).

How many 60 lb bags of concrete are in a yard?▼

It takes 60 bags of 60-lb concrete to equal one cubic yard (27 / 0.45 = 60 bags).

How thick should a concrete patio slab be?▼

Standard pedestrian walkways and patio slabs should be 4 inches thick. Driveways supporting vehicles should be at least 5 to 6 inches thick.

Why should I add a waste percentage?▼

Uneven subgrade excavations, grade variations, form deflection, and minor spillage routinely consume 5% to 10% more concrete than theoretical dimensions.

How long does concrete take to cure?▼

Concrete achieves approximately 70% of its compressive strength in 7 days, and reaches its full 28-day design strength after 28 days.

Is this calculation private?▼

Yes, all estimates run client-side in your web browser with zero tracking.

Quick Reference & Conversion Table

Bag Size	Yield in Cubic Feet	Bags per Cubic Yard (27 cu ft)	Typical Use
80 lb Bag	0.60 cu ft	45 bags	Major slabs & foundation pours
60 lb Bag	0.45 cu ft	60 bags	Light slabs, pathways & post holes
40 lb Bag	0.30 cu ft	90 bags	Small repairs & patch jobs

⚠️ Common Pitfalls to Avoid

•Entering slab thickness in feet instead of inches (e.g., entering 4 instead of 4/12 of a foot).
•Forgetting to add 5% to 10% waste margin for uneven subgrades and spillage.
•Ordering bag concrete for large pours over 2 cubic yards (ready-mix truck delivery is significantly faster and more economical).

💡 Practical Real-World Uses

•Pouring concrete driveways, garage foundations, and backyard patio slabs.
•Setting fence posts and deck foundation piers using round cardboard Sonotubes.
•Budgeting pre-mix bags vs ready-mix delivery trucks for home renovation projects.

Ready-Mix Delivery vs Pre-Mix Bags

✓Ready-Mix Trucks deliver fresh concrete pre-mixed by the cubic yard, ideal for projects over 1.5–2 cubic yards.
✓Pre-Mix Bags are purchased at home improvement centers for small pours, fence posts, and DIY patches.

Last reviewed & updated: 2026-11-01

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