Concrete Slab Calculator: How Much Concrete Do You Need?
Pouring a concrete slab for a patio, driveway, garage floor, or garden shed is one of the most durable home improvements you can make. However, ordering concrete requires precision: order too little and your pour hardens with a cold joint; order too much and you waste hundreds of dollars on ready-mix truck overage fees.
In this guide, we'll walk through how to calculate concrete volume in cubic yards, convert dimensions accurately, estimate pre-mixed bag quantities (60lb vs 80lb), and use our Free Concrete Slab Calculator for instant results.
The Concrete Slab Formula (Cubic Yards)
Ready-mix concrete suppliers sell concrete by the cubic yard (cu. yd. or yd³). One cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft).
To calculate the cubic yards needed for a rectangular slab:
Since slab thickness (depth) is almost always measured in inches, convert depth to feet first by dividing by 12:
Step-by-Step Example Calculation
Suppose you are pouring a 10 ft × 20 ft patio slab that is 4 inches thick:
- Convert thickness to feet:
4 inches / 12 = 0.333 feet. - Calculate cubic feet:
10 ft × 20 ft × 0.333 ft = 66.67 cubic feet. - Convert to cubic yards:
66.67 cu. ft. / 27 = 2.47 cubic yards.
Adding the 10% Waste Factor
Subgrade excavation is rarely perfectly flat, and forms often flex under heavy wet concrete. Always add a 10% safety margin (waste factor):
How Many Pre-Mixed Concrete Bags Do You Need?
For smaller DIY jobs (like sidewalk repairs or small shed bases), buying pre-mixed concrete bags from a home improvement store is often cheaper than ordering a ready-mix truck (which usually requires a 3 to 4 cu. yd. minimum order).
| Bag Size (Weight) | Volume Yield per Bag | Bags per Cubic Yard (27 cu. ft.) |
|---|---|---|
| 40 lb Bag | 0.30 cu. ft. | 90 Bags |
| 60 lb Bag | 0.45 cu. ft. | 60 Bags |
| 80 lb Bag | 0.60 cu. ft. | 45 Bags |
Recommended Slab Thickness Guidelines
Choosing the right slab thickness ensures your concrete can handle intended weight loads without cracking:
- 4 Inches (Standard): Sidewalks, residential patios, garden sheds, and lightweight floors.
- 5 to 6 Inches (Heavy-Duty): Residential driveways for cars and SUVs, heavy equipment sheds, and hot tub pads.
- 6 to 8 Inches (Commercial): Commercial parking pads, heavy truck driveways, and industrial shop floors.
Calculate Your Concrete Job in Seconds
Planning a pour? Input your length, width, and thickness into our free online calculator to instantly get cubic yards, total weight, estimated cost, and required 60lb/80lb bag counts.
Use Free Concrete Slab Calculator →