Figure My Build

Concrete Bag Calculator

Enter the size of your pour and get the exact number of concrete bags to buy. Not just a volume: a shopping list with waste allowance and estimated cost built in.

Diagram of a concrete slab showing its length and width from above, with the thickness marked on a side view
Measure the pour's length and width; thickness is how deep the concrete goes.

Your buy-list

Enter your measurements

How the bag count is calculated

The calculator multiplies length × width × thickness to get the pour volume in cubic feet. Thickness in inches is divided by 12 to convert it to feet.

Each bag size fills a fixed volume once mixed. Both Quikrete and Sakrete publish the same yields: an 80 lb bag makes 0.60 cu ft, a 60 lb bag makes 0.45 cu ft, and a 40 lb bag makes 0.30 cu ft. The volume, plus your waste allowance, is divided by the bag yield and rounded up, because you can't buy part of a bag.

One cubic yard (27 cu ft) takes 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags. For anything much over a cubic yard, ready-mix usually wins on effort. The Concrete Slab Calculator prices that option.

Sources

The figures behind this calculator come from published manufacturer data and industry standards:

Worked examples

10 × 10 ft slab, 4 in thick

33.3 cu ft of concrete. 80 lb bags, 10% waste allowance.

Buy 62 bags

8 × 4 ft walkway, 4 in thick

10.7 cu ft of concrete. 60 lb bags, 10% waste allowance.

Buy 27 bags

6 × 6 ft pad, 6 in thick

18 cu ft of concrete. 40 lb bags, 10% waste allowance.

Buy 66 bags

Frequently asked questions

How many bags of concrete do I need for a 10×10 slab?
At the standard 4 inches thick, a 10×10 slab is 33.3 cubic feet. That is 62 × 80 lb bags with a 10% waste allowance (56 bags with no allowance). At 6 inches thick it rises to 92 bags, which is why most people order ready-mix for a slab this size.
How much does an 80 lb bag of concrete cover?
An 80 lb bag yields 0.60 cubic feet of mixed concrete. That covers about 3.6 sq ft at 2 inches thick, 1.8 sq ft at 4 inches, or 1.2 sq ft at 6 inches.
Should I add extra bags for waste?
Yes. Forms are never perfectly level and some mix stays in the wheelbarrow. A 10% allowance is the standard planning figure for flatwork; use 15% for irregular shapes or rough subgrade.
Bags or ready-mix: which is cheaper?
For small jobs (under about half a cubic yard) bags are cheaper and simpler. Past one cubic yard, ready-mix is usually better value once your time and mixing effort count, and far less physical work.
How many bags are in a cubic yard?
45 bags at 80 lb, 60 bags at 60 lb, or 90 bags at 40 lb, before any waste allowance.

Related calculators

Concrete Slab Calculator

Cubic yards of ready-mix for a slab, plus the bag count if you mix it yourself.

Concrete Volume Calculator

Cubic yards and cubic feet for any pour shape.

Concrete Block (CMU) Calculator

Blocks and mortar bags for a block wall.

Rebar Calculator

Rebar pieces and grid layout for a slab.

Estimate disclaimer: results are planning estimates based on standard manufacturer yields and coverage figures. Site conditions, product choice and installation method change actual quantities. Verify before you buy, and follow local building code for structural work. This tool does not replace professional engineering advice.

Use this calculator on your own site

Free to embed. Paste this code where you want the calculator to appear. It carries a small "by Figure My Build" credit link, which is all we ask.