How to Calculate Concrete Needed
Concrete is ordered by volume (cubic yards in the US) or by number of bags for small jobs. This guide explains the formula, how to measure, and how to add waste so you order the right amount for slabs, footings, or steps.
What it's for
Use this when you're pouring a slab, footing, or other rectangular volume. You enter length, width, and depth; the calculator gives volume in cubic yards and the number of 60 lb or 80 lb bags if you're mixing by hand. For large pours, suppliers deliver by cubic yard; for small projects, bags are convenient.
Step by step
- Measure length and width in feet (the area you're covering).
- Choose depth in inches (e.g. 4" for a slab, 6" for a driveway). The calculator converts to feet.
- Enter dimensions in the Concrete Calculator: length, width, depth.
- Add waste % — 5–10% is common for spillage and uneven subgrade. For footings or complex shapes, use 10% or more.
- Read the result — Cubic yards for delivery, or number of 60 lb / 80 lb bags for small jobs.
Formula
- Volume (cubic feet) = Length (ft) × Width (ft) × (Depth (in) ÷ 12).
- Cubic yards = Cubic feet ÷ 27.
- Bags: One 60 lb bag yields about 0.45 ft³; one 80 lb bag about 0.6 ft³. The calculator divides the total volume by these yields and rounds up.
Example
10 ft × 10 ft slab at 4" depth:
- Volume = 10 × 10 × (4/12) = 33.33 ft³ ≈ 1.23 cubic yards. With 10% waste ≈ 1.35 cubic yards.
- In bags (60 lb): 33.33 ÷ 0.45 ≈ 75 bags (before waste).
Tips
- Order by cubic yard for anything over a few bags; delivery is more efficient.
- Depth is critical—double-check before ordering. Thicker slabs need proportionally more concrete.
- Use the Concrete Calculator for your exact dimensions and waste factor.