Concrete Calculator: Slabs, Footings, Posts, Bags

Estimate only. Check product labels and local building rules before you buy or build.

Quick answer

Multiply length × width × thickness in feet and divide by 27 to get cubic yards, then add 4 to 10% for waste. A 10 × 10 ft slab 4 in thick is 33.3 cu ft, or 1.23 cu yd; with 10% extra that is 1.36 cu yd, or 62 bags of 80 lb concrete mix at 0.60 cu ft per bag.

How to use the concrete calculator

Pick what you are pouring: a slab (patio, shed base, walkway), a footing (a trench or pad under a wall, deck or steps), or post holes and round columns.
Enter the size. Lengths are in feet; thickness, footing width and depth, and hole diameter and depth are in inches. Switch to metric to use meters and centimeters.
Read the concrete to order in cubic yards and the number of 40, 60 or 80 lb bags of premixed concrete. Bag counts include the extra for waste and are rounded up to whole bags.
Open "Waste allowance and bag yields" to change the extra from 10%, or to type the yield printed on the bag you plan to buy.

How do you calculate how much concrete you need?

Multiply length by width by thickness, with every measurement in feet, to get cubic feet, then divide by 27 to get cubic yards. A 10 × 10 ft slab 4 in thick is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 cu yd.

For a round hole or column, the cross-section is π × diameter² ÷ 4. Multiply it by the depth and the number of holes, after subtracting the part of each hole that a post fills. Then add the waste allowance and divide by the yield of one bag, rounding up to a whole bag.

slab volume = length × width × thicknessfooting volume = length × width × depth × number of footingshole volume = (π × diameter² ÷ 4 − post width²) × depth × number of holescubic yards = cubic feet ÷ 27concrete to order = volume × (1 + waste % ÷ 100)bags = concrete to order ÷ yield of one bag, rounded up

How much concrete for a 10 × 10 slab?

A 10 × 10 ft slab 4 in thick takes 1.36 cu yd of concrete with 10% extra, which is 62 bags of 80 lb mix. The table gives the concrete to order for the same 100 sq ft at common thicknesses. Bag counts use the yields on the QUIKRETE Concrete Mix data sheet: 0.60 cu ft for an 80 lb bag, 0.45 cu ft for 60 lb and 0.30 cu ft for 40 lb.

Cubic yards to order and number of 80, 60 and 40 lb bags for 100 sq ft of slab, with 10% extra
Slab	Cu yd	80 lb	60 lb	40 lb
3.5 in	1.19	54	72	107
4 in	1.36	62	82	123
5 in	1.70	77	102	153
6 in	2.04	92	123	184

For another area, multiply the cubic yards by the area in square feet ÷ 100. For bags, multiply that by 27 to get cubic feet, divide by the bag yield and round up.

Worked examples

Bags of concrete for a 10 × 10 ft patio, 4 in thick

The slab is 10 × 10 × (4 ÷ 12) = 33.3 cu ft, which is 33.3 ÷ 27 = 1.23 cu yd. With 10% extra, plan on 36.7 cu ft, or 1.36 cu yd.

An 80 lb bag yields 0.60 cu ft, so 36.7 ÷ 0.60 = 61.1, rounded up to 62 bags. That is 4,960 lb of dry mix to haul. With 60 lb bags it takes 82, and with 40 lb bags 123.

Concrete for six fence posts

The posts measure 3.5 in square and are 8 ft long. The QUIKRETE data sheet suggests a hole about three times the post width and one third of the post height deep, so the holes are 10.5 in across and 32 in deep.

Each hole has a cross-section of π × 10.5² ÷ 4 = 86.6 sq in, less 3.5 × 3.5 = 12.25 sq in for the post, which leaves 74.3 sq in. Times 32 in and six holes, that is 14,273 cu in, or 8.26 cu ft. With 10% extra it is 9.09 cu ft (0.34 cu yd): 16 bags of 80 lb, 21 of 60 lb or 31 of 40 lb.

How thick should a concrete slab be?

Section R506.1 of the International Residential Code sets a minimum of 3½ in (89 mm) for concrete floors on the ground, and the calculator flags a thinner slab. That rule is written for floors of houses and local codes adopt it in different editions, so for a patio or a walkway treat the flag as a reference point. QUIKRETE rates its Concrete Mix for pours 2 in thick or more.

Driveways, garage floors, footings and anything that carries a building or a vehicle need a design that meets your local code, so check with the building department or an engineer before you pour. Their requirements take priority over any figure on this page.

How much extra concrete should I order?

Order 4 to 10% more than the volume of the forms, as the National Ready Mixed Concrete Association advises; the calculator adds 10% unless you change it. Small errors add up: the association notes that pouring a 4 in slab 1/8 in thicker than planned uses 3% more concrete.

Ready-mix is sold by the cubic yard of fresh concrete as it leaves the truck, so give the supplier the "concrete to order" figure, not the volume of the forms. Once hardened, concrete can take up about 2% less volume than it did fresh.

Questions

How many bags of concrete make a yard?

It takes 45 bags of 80 lb concrete mix, 60 bags of 60 lb or 90 bags of 40 lb to make one cubic yard, using the yields QUIKRETE publishes: 0.60, 0.45 and 0.30 cu ft per bag. A cubic yard is 27 cu ft, so divide 27 by the yield of one bag. Add the waste allowance on top: with 10% extra, a cubic yard calls for 50 bags of 80 lb.

How many bags of concrete do I need per fence post?

A 3.5 in square post in a hole 10.5 in wide and 32 in deep takes 3 bags of 80 lb concrete mix or 4 bags of 60 lb, with 10% extra. That hole holds 1.38 cu ft of concrete once the post is subtracted. Wider holes add up quickly because the volume grows with the square of the diameter: a 12 in hole of the same depth holds 36% more.

How do I calculate concrete for a footing?

Multiply the length of the footing in feet by its width and depth in feet, then divide by 27 for cubic yards. A footing 20 ft long, 12 in wide and 8 in deep is 20 × 1 × 0.667 = 13.3 cu ft, or 0.49 cu yd. With 10% extra that is 0.54 cu yd, or 25 bags of 80 lb. For a footing around a building, measure along its center line so the corners are not counted twice.

How many square feet does a yard of concrete cover?

One cubic yard of concrete covers 81 sq ft at 4 in thick, 108 sq ft at 3 in and 54 sq ft at 6 in. To find the area for any thickness, divide 324 by the thickness in inches: a cubic yard is 27 cu ft, and 27 cu ft spread 1 in thick covers 324 sq ft. Order the extra for waste on top, because the ground and the forms are never perfectly flat.

How much does a cubic yard of concrete weigh?

A cubic yard of concrete made from QUIKRETE Concrete Mix weighs about 3,780 lb, close to 2 tons. The data sheet gives a unit weight of about 140 lb per cubic foot, and a cubic yard is 27 cu ft. Plan for the weight of the dry bags as well: the 45 bags of 80 lb that make a cubic yard weigh 3,600 lb before any water is added.

Sources

The QUIKRETE Companies, QUIKRETE Concrete Mix (No. 1101) data sheet (checked October 19, 2026)Bag yields of 0.30, 0.45 and 0.60 cu ft for 40, 60 and 80 lb, unit weight of about 140 lb per cu ft, 2 in minimum pour and post hole sizing.
National Ready Mixed Concrete Association, CIP 8: Discrepancies in Yield (checked October 19, 2026)Order 4 to 10% over plan dimensions; 1/8 in on a 4 in slab is 3%; ready-mix is sold by fresh volume; hardened volume about 2% less.
City of Richmond, Kentucky, Building Inspection, 2015 IRC Deck Section (includes R506.1 Concrete floors on ground) (checked October 19, 2026)International Residential Code, 2015 edition, R506.1: concrete slab-on-ground floors a minimum of 3 1/2 in (89 mm) thick. Local codes adopt different editions.

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