What is the 10% waste buffer?

Concrete poured on-site always loses a small amount to spillage, uneven sub-base settlement, and overfill. A 10% buffer is the industry-standard minimum for DIY slabs. For rough or sloped ground, consider adding 15%.

Which bag size gives the best value?

80 lb bags yield the most concrete per bag (about 0.60 cu ft) and are the most economical per cubic foot. 60 lb bags are the most common choice for homeowners. 40 lb bags are best when carrying heavy bags is impractical.

When should I order ready-mix instead of bags?

If your slab exceeds 1 cubic yard (roughly 10 ft × 10 ft × 4 in), a ready-mix truck is usually cheaper and far less labor-intensive. Below 1 cubic yard, pre-mix bags are typically more economical.

What thickness is standard for a concrete slab?

For a residential patio or walkway, 4 inches is standard. A driveway or garage floor should be at least 6 inches. Structural slabs and heavy-load areas typically require 8 inches or more — consult a structural engineer for those.

How are cubic yards related to cubic feet?

One cubic yard equals exactly 27 cubic feet (3 ft × 3 ft × 3 ft). Ready-mix concrete is sold by the cubic yard, while pre-mix bags are rated in cubic feet, so this calculator shows both for convenience.

Concrete Slab Calculator

Enter slab length, width, and thickness to get concrete volume in cubic yards, cubic feet, and cubic meters — plus the bag count with a 10% waste buffer.

Slab Dimensions

Unit System

Length (ft)

Width (ft)

Thickness (in)

Common: 4 in (patio), 6 in (driveway), 8 in (heavy load).

Pre-mix Bag Size

Waste allowance added to the bag count

Enter slab dimensions on the left to calculate concrete volume.

Summary

This concrete slab calculator computes the exact volume of concrete required for any rectangular slab — patios, driveways, garage floors, sidewalks, and more. Enter your slab dimensions in feet (or meters) and thickness in inches (or millimeters), choose a pre-mix bag weight, and instantly see cubic yards, cubic feet, cubic meters, and the bag count with an industry-standard 10% waste buffer. All calculations run in your browser; no data is sent to any server.

How it works

Select your preferred unit system: Imperial (feet/inches) or Metric (meters/millimeters).
Enter the slab length and width in feet (or meters).
Enter the slab thickness in inches (or millimeters). Common values: 4 in for a patio, 6 in for a driveway.
Choose the pre-mix bag weight: 40 lb, 60 lb, or 80 lb.
Click Calculate to see volume in cubic yards, cubic feet, and cubic meters, plus the bag count.
A 10% waste buffer is added to the bag count by default; pick 0–20% in the waste allowance box.

Use cases

Estimating concrete bags for a backyard patio or garden path.
Planning material purchases for a driveway or garage floor slab.
Checking concrete quantities before ordering a ready-mix truck.
Converting slab dimensions between imperial and metric units.
Budgeting concrete costs for a DIY home-improvement project.
Verifying a contractor's concrete estimate for a residential slab.
Teaching students how slab volume and material quantities are calculated.
Quickly comparing bag counts for different slab thicknesses.

Frequently Asked Questions

Last updated: 2026-05-23 · Reviewed by Nham Vu

Slab dimensions	—
Exact volume	—
Volume + 10% waste	—
Bag yield	—
Bags needed	—