Concrete Calculator
Find the cubic yards, cubic feet and bag count for a slab, patio, pad or walkway, with extra for spillage. Handy on site for contractors confirming a ready-mix order, and at home for anyone planning a small pour.
Free, no sign-up. Runs in your browser; nothing you type is stored unless you choose to email yourself the results.
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Email me these results. We'll send your numbers, plus how to turn this calculator into a lead magnet on your own website.
Three steps, about a minute
Step 1
Enter the Length and Width of the pour in feet, measured inside the forms.
Step 2
Enter the Thickness in inches and the Extra for waste & spillage percentage.
Step 3
Use Cubic yards for a ready-mix order or the bag counts for small hand-mixed jobs.
How the concrete calculator works
Volume is length × width × thickness, with thickness converted from inches to feet by dividing by 12. Your extra percentage is added, and cubic feet ÷ 27 gives cubic yards, since 27 cubic feet make one yard. Bag counts use typical premix yields of about 0.6 cubic feet for an 80 lb bag and 0.45 for a 60 lb bag, each rounded up. Take the default 20 × 10 ft slab at 4 inches with 10% extra: that needs about 73.3 cubic feet, or 2.72 cubic yards. In bags, that's 123 at 80 lb or 163 at 60 lb, which shows why ready-mix is usually more economical above about 1 cubic yard. Subgrade dips, bowed forms and thickened edges use extra concrete. Footings, structural slabs and anything load-bearing should follow the engineer's specs and local code.
| Length | 20 ft |
|---|---|
| Width | 10 ft |
| Thickness | 4 in |
| Extra for waste & spillage | 10% |
| Cubic yards | 2.72 |
|---|---|
| Cubic feet | 73.3 |
| 80 lb bags (0.6 cu ft each) | 123 |
| 60 lb bags (0.45 cu ft each) | 163 |
Length × width × thickness ÷ 27. Ready-mix is usually more economical above about 1 cubic yard.
Using the numbers like a pro: tips for masonry and concrete contractors
Know your supplier's order increments and short-load charges, and round up accordingly. Running out mid-pour leaves a cold joint, and fixing that costs far more than the extra concrete.
Check grade before you order. A subgrade running half an inch low across a large slab adds real volume, so measure depth at several spots inside the forms, not only at the edges.
Break out base prep, reinforcement and finish on your quote, not just yards. When a customer sees a lower bid, they can tell what it leaves out.
Put a calculator like this on your own website
Someone searching for concrete yardage is planning a pour, often a patio, driveway or shed pad. Give them the number on your site, then offer a free on-site quote for the full job with a short form for measurements and photos. A prospect who already measured is further along than a cold inquiry. A version we build for you can ask for site photos along with the dimensions.
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Concrete Calculator questions
How many yards of concrete do I need for a 20x20 slab?
A 20 by 20 ft slab at 4 inches thick is 400 sq ft × 0.333 ft, or about 133 cubic feet. Divided by 27, that is about 4.94 cubic yards, or about 5.4 with 10% extra. At 6 inches thick, the same slab needs about 8.1 cubic yards with the extra included.
How many bags of concrete make a yard?
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so a yard takes about 45 bags. A 60 lb bag yields about 0.45 cubic feet, so a yard takes about 60. At that volume a ready-mix delivery is usually more economical and much less work.
How do you calculate cubic yards of concrete?
Multiply length by width in feet, then by thickness in feet (inches divided by 12). That gives cubic feet. Divide by 27 to get cubic yards, then add extra for spillage and uneven subgrade; this calculator defaults to 10%. A 10 by 10 ft slab at 4 inches is about 1.23 yards before extra.
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