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Concrete Slab Yardage & Bag Calculator

Compute cubic yards, cubic feet, and pre-mix concrete bags required for flatwork slabs.

Quick Answer:Multiply length (ft) by width (ft) by depth (ft) and divide by 27 to find cubic yards of concrete needed.

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How to Use the Concrete Slab Calculator: Yards & Pre-Mix Bags Needed

1Enter the length and width of your slab in feet.
2Input slab thickness / depth in inches (typically 4 inches for residential patios).
3Set your project waste factor percentage (10% recommended for uneven sub-base grades).
4Review your required cubic yards for commercial delivery or bag counts for home mixing.

Formula & How It Works

Concrete Slab Volume Formulas

Cubic Yards = [Length (ft) × Width (ft) × (Depth (in) / 12)] / 27 × (1 + Waste%)

Calculates dimensional cubic volume converted to commercial ready-mix units.

Worked Examples

12' × 10' Patio Slab at 4 Inches Thick

Input: Length: 12 ft, Width: 10 ft, Depth: 4 in, 10% Waste

Explanation: 120 sq ft at 4 inches thick yields 40 cu ft net; adding 10% waste requires 1.63 cubic yards or 74 pre-mixed 80-lb bags.

Result: 1.63 Cubic Yards (44.0 cu ft) | 74 of 80-lb Bags

Frequently Asked Questions

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

It takes forty-five (45) 80-lb bags of pre-mixed concrete to equal one cubic yard (27 cubic feet).

How thick should a concrete slab be for a shed or hot tub?▼

A standard storage shed or hot tub foundation should be poured at least 4 inches thick on top of a compacted 4-inch gravel base.

When is it cheaper to order a ready-mix truck instead of bags?▼

Ready-mix truck delivery is typically more cost-effective and physically practical for any project requiring more than 1.5 to 2.0 cubic yards (roughly 70+ bags).

How long does concrete take to cure before walking on it?▼

Concrete sets enough for light foot traffic in 24 to 48 hours, but requires 28 days to reach its full structural design compressive strength.

Quick Reference & Conversion Table

Slab Dimensions (4" Thick)	Square Footage	Cubic Yards (10% Waste)	80-lb Pre-Mix Bags
8 ft × 10 ft	80 sq ft	1.09 yd³	49 bags
10 ft × 10 ft	100 sq ft	1.36 yd³	62 bags
12 ft × 15 ft	180 sq ft	2.44 yd³	110 bags
20 ft × 20 ft	400 sq ft	5.43 yd³	245 bags

⚠️ Common Pitfalls to Avoid

•Ordering exact geometric volume without a 10% buffer, leaving the slab short due to sub-grade soil undulations and form deflection.
•Pouring a slab thinner than 4 inches for vehicular traffic (driveways require 5 to 6 inches with rebar reinforcement).

💡 Practical Real-World Uses

•Estimating transit-mix concrete truck orders for residential foundation and patio pours.
•Budgeting bagged concrete mix purchases from local hardware home improvement centers.

Last reviewed & updated: 2026-11-11

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