Concrete calculator: how many cubic yards or bags?
Free concrete calculator: cubic yards from your sizes, then bags or a truck order. No sign-up.
CONCRETE TO ORDER
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Turn on JavaScript and this works out your concrete as you type. The formulas are below if you'd rather do it by hand.
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Two ways to buy the same concrete
Bags
Ready-mix
Still to confirm before you buy
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How the concrete calculator works
You type each part of the pour the way you measured it. A plain number means feet in the length and width boxes and inches in the thickness box, and you can type feet and inches together, like 12' 6". The line under each box says how we read it, and we read every section back to you before anything gets added up. That's where you catch a thickness typed as 4 feet instead of 4 inches. That slip buys twelve times the concrete.
The math is short. A slab, pad, footing or wall is length times width times thickness. A form tube or pier is a circle times its depth: pi times half the diameter, squared, times the depth. If you already know the area, say from our square footage calculator for an L-shaped patio, it's that area times the thickness. Anything inside the pour that concrete won't fill, like a tree ring you're leaving open, comes off before we multiply the section by how many you have. Everything runs in exact feet and inches, so a 9 by 9 foot square, 4 inches thick, comes out at exactly one cubic yard. Not a hair over.
Your allowance goes on top of the net volume, and we round the total up once, to something you can actually buy: whole bags, counted from the yield printed on your own bag, or a ready-mix order in your plant's steps. Say your plans call for a 12 by 14 foot patio, 4 inches thick. That's 56 cubic feet, or 2.07 cubic yards. Add 10% and you're at 2.28 yards. At 0.60 cubic feet a bag, it takes 103 bags. From a plant that sells in quarter yards, it's 2.50 yards, about a fifth more than the patio holds. Both show up as their own lines, so you can see what the rounding costs before you pay for it.
We never count bags from the weight printed on the front. Two 80-pound mixes can make different amounts. The 80 pounds tells you what you're lifting, not what it pours. Type what one bag makes instead, from the bag or the maker's data sheet, in cubic feet, liters or square feet at a stated depth.
Last, the line on QUIKRETE's sheet that matters more than any yield: wet concrete "causes severe skin burns and serious eye damage". Gloves and eye protection, every time.
Who this is for
Homeowners with a patio, a shed pad, a walkway or pergola footings staked out, and one purchase ahead: a cart of 80-lb bags from the home center, or a call to a ready-mix plant that sells by the yard.
Anyone holding a bag count from somewhere else who wants to check it against the yield on the bag they're actually buying.
Small contractors who measure in feet and inches and need a figure for the dispatcher before the cutoff, whether it's a slab, a run of footings or a row of form tubes.
Homeowner questions about concrete, yards and bags
How many 80 lb bags of concrete make a yard?
Forty-five, if your bag makes 0.60 cubic feet. That's the figure QUIKRETE's Concrete Mix sheet and Sakrete's High-Strength sheet both print for an 80-lb bag. A cubic yard is 27 cubic feet, and 27 divided by 0.6 is exactly 45, with nothing left for spills. Sixty-pound bags at 0.45 cubic feet take 60. Lightweight mixes make more per bag, and Sakrete's current Maximizer product page and linked data sheet give different coverage figures. Read the yield off the bag you're buying. The page and the sheet still disagreed when we checked on October 8, 2026.
How many bags of concrete for a 12x12 slab?
That depends on the thickness, and the thickness is your plans' call, not ours. If they say 4 inches, a 12 by 12 slab holds 48 cubic feet, or 1.78 cubic yards. At 0.60 cubic feet a bag that comes to exactly 80 bags, and "exactly" is the problem: one spill and you're short. With a 10% allowance it's 88. With 60-lb bags at 0.45 cubic feet, the bare slab takes 107.
How much concrete for a 12 inch Sonotube?
About 0.785 cubic feet for every foot of depth. A 12-inch tube is pi times 6 inches squared, so filled 4 feet deep it holds 3.14 cubic feet. At 0.60 cubic feet an 80-lb bag, that's 5.2 bags' worth, so 6 bags. Sonotube is a brand name, and any round cardboard form works the same way. Pick Round tube or pier, type the diameter and the depth, and set how many tubes you're filling.
How much does a yard of concrete cover?
It covers 81 square feet at 4 inches thick. A cubic yard is 27 cubic feet, so the shortcut is 324 divided by the thickness in inches: 108 square feet at 3 inches, 64.8 at 5, 54 at 6. That's before any allowance, and before the base turns out to be lower in the middle than it looked.
How much extra concrete should I order?
We don't pick that number for you, but the ready-mix trade has an opinion. NRMCA's guide on yield tells buyers to order 4 to 10 percent more than their estimate. Forms bulge, bases dip, and a 4-inch slab poured an eighth of an inch thicker than planned runs 3% short. The ASTM note it quotes is blunt about spills and over-digging: "none of which are the responsibility of the producer." Then rounding piles on. On a 12 by 14 foot patio, a 10% allowance becomes 20.5% extra once a plant rounds up to the next quarter yard.
How thick should a concrete slab be?
That's not a number we'll give you, and we'd be wary of any calculator that picks one from a dropdown. The right thickness depends on what sits on the slab, your soil and your local rules. That figure belongs to your plans, the kit instructions or your contractor. Footings and anything that carries a structure fall under the building code your town adopted. The IRC is a model code that cities and counties adopt and often amend, so your building department decides, frost depth included. Call 811 before you dig. Wait for the marks and confirm every utility has responded. We add up the sizes you give us and check none of that.
What is a short load fee for concrete?
It's a charge for a truck that leaves the plant with less than the plant's threshold on board. Those thresholds vary a lot. Gehring Construction in Columbus, Nebraska charges a small-load fee under 4 yards. Quick Mix Concrete in Douglasville, Georgia charges one at 6 yards or less. Townsend Concrete in Vacaville, California sets the threshold by delivery zone, from 3 to 6 yards. For a patio-sized pour, assume it applies until the plant says it doesn't. Ask on the phone, then put it in Add your own prices so it sits on a line of its own.
Is bagged concrete or ready-mix cheaper?
We put both side by side and don't pick one. Ask whether the plant has an order minimum or a minimum charge. Quick Mix Concrete, for one, sells "all yardage" in half-yard increments, and a small order usually carries a short-load fee. Bags cost you lifting, mixing and time instead: the 12 by 14 foot patio above takes 103 80-lb bags at 0.60 cubic feet, which is more than four tons through your own two hands. Put both quotes into Add your own prices and compare the totals yourself, before sales tax.
How many yards of concrete do I need?
Measure, multiply, then buy in whole units. Length times width times thickness gives cubic feet once everything is in feet, so 4 inches becomes a third of a foot. Divide by 27 for cubic yards. A 9 by 9 foot square, 4 inches thick, is exactly one yard, which makes a handy check on any figure you're handed.
That gives you the net volume. What you buy is bigger. Bagged concrete comes in whole bags, counted from the yield on the bag, and ready-mix follows the plant's ordering step and any minimum. Add your allowance, round up once at the very end, and you have a number you can say on the phone. Whether you came for a concrete slab calculator, a cubic yard calculator or a bag count, it all comes out of one set of measurements.
Sources and last checked
Checked October 8, 2026. Data version concrete-us-2026-10-07.
- Unit definitions: NIST Handbook 44 (2026), Appendix C, General Tables of Units of Measurement.
- Bag yields quoted in the answers above: QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022); Sakrete High-Strength Concrete Mix and Sakrete Maximizer Concrete Mix technical data sheets, and the current Maximizer product page.
- Ordering advice: NRMCA, Concrete in Practice 8, Discrepancies in Yield.
- Ready-mix ordering terms: Gehring Construction & Ready-Mix, 2026 pricing page; Quick Mix Concrete, pricing page; Townsend Concrete, price list updated August 22, 2026.
How we made it: the calculator is arithmetic over the numbers you type. It holds no bag yields, no thicknesses and no prices of its own, and the makers and plants above are named for what they publish. We aren't recommending any of them.
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