Concrete Calculator

How the concrete volume is calculated

Work in feet, so convert the thickness first. A 4 in slab is 4 ÷ 12 = 0.333 ft thick.

Worked example: a 10 × 10 ft slab, 4 in thick

1. Volume: 10 × 10 × 0.333 = 33.3 ft³, or 1.23 yd³.
2. With 10% waste: 33.3 × 1.10 = 36.7 ft³, or 1.36 yd³.
3. In 80 lb bags at 0.60 ft³ each: 36.7 ÷ 0.60 = 61.1, so buy 62 bags.
4. In 60 lb bags at 0.45 ft³ each: 36.7 ÷ 0.45 = 81.5, so buy 82 bags.

Estimating the cost of bagged concrete

Bag prices vary by store, region and mix, so enter the price you're actually seeing. As an illustration only, at $7 a bag the 62 bags in this example would cost 62 × $7 = $434. That covers the mix alone, not forms, reinforcement, delivery or tool rental. Use it to compare against a ready-mix quote.

Typical bag yields

These are typical yields for standard mix. Specialty blends can differ, so go by the yield printed on your bag. Metric bag options use the same yield per unit of weight, which makes them approximate too.

Bags or ready-mix?

Weight is often what decides this. The 62 bags in the example above weigh about 5,000 lb in total, and each one has to be lifted and mixed before the first batch starts to set. Ask a local supplier for a ready-mix price once a job passes about a cubic yard, and ask about any minimum order or short-load fee.

Before you pour

• Measure the thickness at several points. Half an inch of extra depth on this slab adds about 4 ft³.
• Raise the waste allowance for rough or sloping ground, since low spots quietly use more concrete.
• Footings and structural slabs may have to meet local code, so check thickness and reinforcement rules first.
• Fence post holes have a post in the middle. The fence calculator subtracts it for you.