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Concrete calculator

Cubic yards and 80, 60 or 40 lb bags for slabs, footings and post holes

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Shape

Patios and walkways are commonly 4 in thick.

Identical areas, beds or holes

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Enter your measurements and the depth. The result updates as you type.

Estimate only. Check quantities with your supplier before you buy.

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Estimates only. Densities, bag yields and ground conditions vary. Check quantities with your supplier, and follow local codes for structural work.

Concrete is the one material here where a shortage mid-pour is a real problem. Once you start, you cannot stop for a second trip to the store. The calculator gives a volume in cubic yards for ready-mix, and the number of 40, 50, 60 or 80 lb bags if you are mixing yourself.

Measuring a slab

Slabs are normally rectangles. Measure the finished size inside the forms, in feet. For a curved edge, cut the shape into a rectangle and a half-circle, or see measuring irregular areas. Quikrete's own patio guide points out that a 20 ft round patio has about 315 square feet of surface against 400 for a 20 ft square, so a round slab takes noticeably less.

Thickness is the lever. Measure from the top of the compacted base to the top of the forms. If the base is uneven, take several readings and use the average, then add the buffer below.

The formula

Length times width times thickness gives cubic feet, once the thickness is in feet (inches divided by 12). Divide by 27 for cubic yards.

For a 4 inch slab there is a shortcut: square feet divided by 81 equals cubic yards. For bags, divide cubic feet by the yield printed on the bag.

Manufacturer yields for pre-mixed concrete (Quikrete and Sakrete state the same figures):

BagYieldBags per cubic yard
40 lb0.30 cu ft90
50 lb0.375 cu ft72
60 lb0.45 cu ft60
80 lb0.60 cu ft45

Worked example: a 10 by 12 ft patio

  1. Area: 10 x 12 = 120 sq ft.
  2. Thickness: 4 inches is 1/3 foot.
  3. Volume: 120 x (4/12) = 40 cubic feet. Divided by 27, that is 1.48 cubic yards.
  4. Add 10 percent for spillage and an uneven base: 1.48 x 1.1 = 1.63 cubic yards. Ready-mix is usually ordered in quarter or half yards, so ask for 1.75.
  5. In 80 lb bags: 40 / 0.6 = 66.7, so 67 bags without any buffer, or 74 with it. That is 5,360 lb of dry bags to carry. The finished slab weighs more, roughly 5,600 to 6,000 lb at 140 to 150 lb per cubic foot, because of the added water.

Check by the 81 rule: 120 / 81 = 1.48. Same answer.

Metric: 3.05 m by 3.66 m, 10 cm thick, about 1.13 cubic metres, or 1.25 with the buffer. An 80 lb bag is 36.3 kg.

Choosing a thickness

Going from 4 to 6 inches multiplies the volume by 1.5. A square foot needs 0.0123 cubic yards at 4 inches and 0.0185 at 6.

One cubic yard of finished concrete covers 81 square feet at 4 inches, 65 at 5, 54 at 6 and 41 at 8.

Types of mix

General-purpose concrete mix is cement, sand and gravel, designed for about 4,000 psi after 28 days. It suits slabs, footings and steps.

High-early or 5,000 psi mix gains strength faster. Quikrete's data shows 3,500 psi at 7 days for its 5000 mix against 2,500 for the standard mix, which matters if you need to use the slab quickly.

Fast-setting mix is sold for setting posts; Quikrete's calculator sizes it by post-hole depth, not by slab area.

Lightweight mixes such as Sakrete Maximizer yield more per bag. Sakrete's page states 3 sq ft at 4 inches deep, which works out to 1 cubic foot a bag (check which bag size that refers to). Always use the yield printed on the bag you are buying.

Bags or ready-mix?

An 80 lb bag yields 0.6 cubic feet, so a yard needs 45 of them, about 3,600 lb of bagged powder. Mixing by the barrow or in a small mixer, with roughly 6 pints of water per bag (Quikrete's range is 6 to 9), is workable for a footing or a small pad. Concrete Network advises ordering larger jobs such as driveways by the yard instead of mixing bag after bag.

Ask the plant about minimum loads, short-load charges, and how far the truck can get onto your property. Those vary and are not on any published list we could cite.

Common mistakes

Values this calculator uses

Weights are loose, dry, typical figures in US tons (2,000 lb). Your supplier's numbers win if they differ.

ItemValueSource
Normal-weight concrete2 tons / cu yd (range 1.89 to 2.03)quikrete.com, fhwa.dot.gov, engineeringtoolbox.com; confidence: high
80 lb bag (0.6 cu ft)0.6 cu ftQuikrete Concrete Mix (No. 1101) data sheet yield
60 lb bag (0.45 cu ft)0.45 cu ftQuikrete Concrete Mix (No. 1101) data sheet yield
40 lb bag (0.3 cu ft)0.3 cu ftQuikrete Concrete Mix (No. 1101) data sheet yield
50 lb bag (0.375 cu ft)0.375 cu ftQuikrete Concrete Mix (No. 1101) data sheet yield
90 lb bag (0.675 cu ft)0.675 cu ftQuikrete Concrete Mix (No. 1101) data sheet yield

Sources

Questions

How many bags of concrete make a cubic yard?

Forty-five 80 lb bags, 60 of 60 lb, 72 of 50 lb or 90 of 40 lb. Those counts come from the manufacturer yields of 0.60, 0.45, 0.375 and 0.30 cubic feet per bag, set against the 27 cubic feet in a yard. Bagged concrete for a full yard is a lot of lifting, 3,600 lb of dry mix.

How many 80 lb bags do I need for a 10 x 10 slab?

At 4 inches thick, 56 bags. The slab is 100 x (4/12) = 33.3 cubic feet, and 33.3 / 0.6 = 55.6. At 6 inches thick the volume is 50 cubic feet, which needs 84 bags. These are bare minimums, so add 10 percent or more.

How much does a cubic yard of concrete weigh?

About 4,000 lb, give or take. Quikrete lists the unit weight of its bagged concrete mix at about 140 lb per cubic foot, which is 3,780 lb a yard. The Federal Highway Administration uses 145 lb per cubic foot for normal-weight concrete (3,915 lb a yard), and handbooks give 140 to 150 (3,780 to 4,050). So 4,000 is a fair round number, and plain mix lands a little below it.

How many square feet does a bag of concrete cover?

An 80 lb bag covers 1.8 square feet at 4 inches thick and 1.2 at 6 inches. A 60 lb bag, at 0.45 cubic feet, covers 1.35 sq ft at 4 inches. Per cubic yard, the coverage is 81 square feet at 4 inches, 65 at 5, 54 at 6 and 41 at 8.

How thick should a concrete slab be?

For patios and walkways, 4 inches. Quikrete describes a 4 inch slab set about 2 inches above grade as suitable for a patio. Driveways start at 4 inches for cars and go to 5 or 6 for heavier vehicles, and garage floors usually get 5 to 6. Below 2 inches, a standard concrete mix is not the right product.

How much extra concrete should I order?

Ten percent as a minimum. Concrete Network recommends 10 percent on a driveway and 10 to 15 percent for a patio, because of spillage and uneven ground. Ready-mix trucks usually round to the nearest quarter or half yard.

How much water goes into a bag?

For an 80 lb bag of Quikrete Concrete Mix, about 6 pints to start, and up to 9 pints at most if it is too stiff to work. A 40 lb bag starts at 3 pints and tops out at 4.5. Quikrete says to aim for a 2 to 3 inch slump when mixing in a mixer. Adding more water than the maximum makes it easier to place and weaker once it sets.

Is bagged mix strong enough for a patio or driveway?

Quikrete's standard Concrete Mix is rated at 4,000 psi after 28 days, with 2,500 psi at 7 days, and the company lists sidewalks, patios, steps and driveway repairs among its uses. Sakrete's high-strength mix is also rated at 4,000 psi. Strength depends on mixing it properly and curing it. Quikrete's data sheet asks for 5 days of curing in warm weather.