Concrete Calculator: Slabs, Footings, Columns and Steps
Work out the concrete for a slab, footing, column or steps in cubic yards, the number of 60 and 80 lb bags and the cost, with a waste allowance. Use it for a slab or driveway quote; it is an estimate, not a substitute for measuring the forms.
Your numbers
4 in is common for patios and walks; driveways are often 4 to 6 in.
The NRMCA suggests 4 to 10% for spillage, uneven ground and form movement.
Optional. From your supplier's quote.
Concrete with 10% waste
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- In cubic feet
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- Before waste
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- 80 lb bags (0.60 ft³ each)
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- 60 lb bags (0.45 ft³ each)
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- 40 lb bags (0.30 ft³ each)
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- Cost in 80 lb bags
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- Ready-mix cost
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An estimate for quoting and ordering, not a substitute for measuring the forms on site.
Bag yields are QUIKRETE's figures for its Concrete Mix; other mixes differ, so check the bag.
The calculation runs in your browser and ParrotNotes does not store your numbers. They sit in the page address so you can bookmark or share the result.
How concrete volume is worked out
Concrete is ordered by volume, so every shape comes down to length × width × depth. Measure the long sides in feet and the thin one in inches, then divide the inches by 12 before you multiply: 4 in is 0.333 ft, 6 in is 0.5 ft. The result is in cubic feet; divide by 27 to get cubic yards, the unit ready-mix is sold in.
A round column or a cardboard form tube is π × radius² × height, with the radius being half the diameter in feet. Steps poured solid on the ground are a stack of blocks: the bottom step is one rise tall, the next two, and so on, so the total is width × run × rise × n(n + 1) ÷ 2 for n steps.
Add the waste allowance last. The number of identical pieces (four post footings, two pads) multiplies the volume before waste is added.
Worked examples: a patio slab and a driveway quote
Patio: 10 × 10 ft at 4 in thick is 10 × 10 × 0.333 = 33.33 cubic feet, or 1.23 cubic yards. With 10% waste that is 36.67 cubic feet, or 1.36 yards: 62 bags of 80 lb, 82 of 60 lb or 123 of 40 lb.
Driveway: a 20 × 24 ft pad at 4 in thick is 160 cubic feet, or 5.93 yards; with 10% waste, 6.52 yards. In 80 lb bags that is 294 bags, which is why a driveway quote is priced per yard of ready-mix. Put the supplier's price per yard in the calculator and it multiplies it out; add forming, base, reinforcement, finishing and labor on top in your quote.
How many bags make a yard
QUIKRETE's data sheet for its Concrete Mix gives about 0.30 cubic feet for a 40 lb bag, 0.45 for a 60 lb bag and 0.60 for an 80 lb bag. A cubic yard is 27 cubic feet, so it takes about 90, 60 or 45 bags. Sakrete lists the same 0.6 cubic feet for its 80 lb High-Strength Concrete Mix.
High-yield and fast-setting mixes differ, so read the yield on the bag you are buying and treat the counts here as an estimate. Bags are priced per bag, so the cost row uses the size you pick.
Why you order more than the plan says
The National Ready Mixed Concrete Association (NRMCA) advises ordering 4 to 10% over the plan volume. Forms bow under the weight, the subgrade is never perfectly flat, some concrete is spilled, and a slab poured a little thick adds up: NRMCA's example is that 1/8 in extra on a 4 in slab is a 3% shortage.
Running short in the middle of a pour is worse than a little left over, because a second truck or another run for bags leaves a cold joint. The calculator starts at 10%; lower it for simple, well-braced forms.
Tips for the site walk
Measure each area on its own and add them up; check the depth in several places, because low spots in the base take more concrete than the plan shows. Note where a truck can reach, whether it needs a pump or wheelbarrows, and what is under the slab.
Then write down what the customer told you: the finish they want, when they need it, what they expect to spend and who else is deciding. Those notes decide the quote as much as the yards do.
Sources, checked September 2026. All figures are estimates:
More for building-materials and contractor sales: Building materials sales, Construction daily report, Quote follow-up email.
The yards are only half the quote
The volume goes in the quote; what the customer said about access for the truck, the finish, the budget and the start date goes in ParrotNotes: record the site walk and get the summary and next steps.
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