Concrete calculator

Enter what you are pouring: a slab, a wall footing, pad footings, post holes, form tubes, a column or a set of steps, one part or several. You get cubic yards to order from a ready-mix plant, cubic feet and cubic metres, and how many 40, 50, 60 or 80 lb bags, each from the maker's own yield. Open the IRC check to see the code's least footing and slab sizes.

Part 1

What you are pouring

Length (ft)

Width (ft)

Thickness (in)

4 in is common for a patio or walk.

How many

Extra for waste (%)

Ready-mix producers advise 4 to 10% over the plan; 0 for the exact volume.

One source: NRMCA, the ready-mixed concrete industry’s association (Concrete in Practice 8)

Bag you are buying

Check footings and slabs against the IRC

The house the footing carries (light frame)

Roof or snow load

Soil (IRC presumptive bearing value)

Building width across the footing (ft)

The table assumes 32 ft; wider or narrower houses change the footing.

Ready-mix to order

1.36 yd³

36.67 ft³ · 1.038 m³, with 10% extra on 33.33 ft³

Quikrete Concrete Mix, 80 lb

62 bags

One source: Quikrete, the maker of that bag

Bags by size
Bag size	Bags	Yield
40 lb	123	0.3 ft³ Only two sources publish this: Quikrete and Sakrete
50 lb	98 (97–98)	0.378 ft³ Only two sources publish this: Quikrete and Sakrete
60 lb	82 (74–82)	0.45 ft³ Median of 3 sources
80 lb	62	0.6 ft³ Median of 3 sources

Ready-mix is sold by the cubic yard. Ask the plant about its smallest load before you order a small pour; for a few yards or less, bags may be simpler.

Frost depth is set locally: footings go below your area’s frost line (IRC R403.1.4.1 points to the local table), so check your local building code. It also amends the IRC and may adopt a different edition.

Why: 33.33 ft³ to fill plus 10% for waste is 1.36 yd³ to order, or 62 bags of Quikrete Concrete Mix, 80 lb.

Every bag, by its maker’s yield

Bag	Yield	Bags
Quikrete Concrete Mix, 80 lb	0.6 ft³	62
Sakrete High Strength Concrete Mix, 80 lb	0.6 ft³	62
Roman High Strength Concrete Mix (American Dry Mix), 80 lb	0.6 ft³	62
Quikrete Concrete Mix, 60 lb	0.45 ft³	82
Sakrete High Strength Concrete Mix, 60 lb	0.45 ft³	82
Rapid Set Concrete Mix, 60 lb	0.5 ft³	74
Quikrete Concrete Mix, 50 lb	0.375 ft³	98
Quikrete Fast-Setting Concrete Mix, 50 lb	0.375 ft³	98
Sakrete Fast Setting Concrete Mix, 50 lb	0.38 ft³	97
Quikrete Concrete Mix, 40 lb	0.3 ft³	123
Sakrete High Strength Concrete Mix, 40 lb	0.3 ft³	123

Each yield is the maker’s own figure for its own bag, read from its data sheet. Makers say yields are approximate.

Why these numbers?

The situation
One part to pour: 33.333 ft³ = 33.333 ft³.
From your input: the shapes and their sizes
What that means
Ready-mix producers advise ordering 4 to 10 percent over the plan volume for spillage, forms that spread, an uneven base and over-digging; you chose 10%.
One source: National Ready Mixed Concrete Association, the ready-mixed concrete industry’s association (Concrete in Practice 8) , sources [1]
So
33.333 × 1.1 = 36.667 ft³; ÷ 27 = 1.358 yd³; × 0.3048³ = 1.038 m³.
Arithmetic: exact by definition
The number
Quikrete rates Quikrete Concrete Mix, 80 lb at 0.6 ft³ a bag: 36.667 ÷ 0.6 = 61.11, so 62 bags.
One source: Quikrete, the maker of that bag , sources [2]
The number
Frost depth is set locally: footings go below your area’s frost line (IRC R403.1.4.1 points to the local table), so check your local building code. It also amends the IRC and may adopt a different edition.
From your input: where you build

Sources

How we source these numbers

How the concrete is worked out

Every shape is a volume: length times width times thickness for a slab, a footing or a pad; the area of a circle times the depth for a post hole, a form tube or a round slab; and for steps, a block per step, each one riser taller than the last. Long sides are in feet and thicknesses in inches, so a slab's thickness is divided by 12 before it is multiplied. A cubic yard is 27 cubic feet, and a cubic metre is 35.3 cubic feet.

Order more than the plan volume. Forms bow, the ground is never flat, and a little spills. The National Ready Mixed Concrete Association tells buyers to allow 4 to 10 percent over the plan; the calculator starts at 10 and you can change it. Running short mid-pour leaves a seam where fresh concrete meets concrete that has begun to set.

A bag's yield is printed on its data sheet: an 80 lb bag of Quikrete, Sakrete or American Dry Mix's Roman mix makes about 0.60 cubic feet, a 60 lb bag 0.45 (Rapid Set's 60 lb bag makes 0.5), and a 40 lb bag 0.30. That is 45 of the 80 lb bags to a cubic yard. Bags suit post holes, steps and small slabs; for a few cubic yards or more, a ready-mix truck is usually simpler. Ask the plant about its smallest load: this page gives no prices.

Setting fence posts? The fence calculator works out the posts and their holes first. For a deck, the deck calculator sizes each footing from the American Wood Council's DCA 6 tables, and the stair calculator gives the rise and tread for concrete steps.

What the code says about footings and slabs

The International Residential Code sets the least size of a house's concrete footings in Table R403.1(1), by the number of stories, the roof or snow load and how much load the soil can bear. No footing is narrower than 12 inches or thinner than 6 (R403.1.1), each sits at least 12 inches below undisturbed ground (R403.1.4), and a concrete floor slab is at least 3 1/2 inches thick (R506.1). Patios and walks aren't floors, so the code doesn't set their thickness. Frost depth is local: your building department sets it, and it decides how deep footings go where the ground freezes.

Sources

How we source these numbers