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Concrete Calculator

Work out the concrete for a slab, a footing, a set of post holes or a column, then how many bags of your size that is, or when a ready-mix truck is the better buy.

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Where are you?

AWG, NEC, 120/240 V, miles and gallons . We remember this on every calculator.

What are you pouring?

Post holes subtract the post itself. A column is a round form such as a cardboard tube.

Slab length
Slab width
Slab thickness

4 inches suits a patio or shed base. 5 to 6 inches is the figure for a driveway or anything a vehicle stands on.

Waste and spillage

Uneven subgrade, spillage and the bit left in the barrow. 10% is realistic for bags; ask for a little over on a truck delivery because a short pour is far worse than a long one.

%
Bag size

An 80 lb bag yields 0.6 cubic feet, and every other size scales from that. Smaller bags are easier to carry and cost more per yard.

Your result

80 lb bags to buy

62 bags

Concrete needed
1.36 cu yd
In cubic metres
1.04 m³
In cubic feet
36.7 cu ft
Before waste
1.23 cu yd
Each bag yields
0.6 cu ft
Better bought as
Bags

🪿 The goose says: You need 1.36 cu yd of concrete, which is 62 bags of 80 lb mix with 10% waste included.

  • Wet concrete is strongly alkaline and burns skin on long contact, through clothing and boots as well as on hands. Wear gloves and eye protection, keep it out of your boots, and rinse any splashes off straight away.
  • 62 bags is a realistic hand-mix job, though a mixer hired for the day turns it from an ordeal into an afternoon. Mix to a consistent wetness, because varying the water between batches varies the strength.

How this concrete calculator works

Concrete is volume, and the only awkward part is that the three numbers you measure are rarely in the same unit. A slab is feet by feet by inches, a footing is feet by inches by inches, and a post hole is a circle in inches sunk a depth in inches.

Everything here is converted to feet first, multiplied, and then divided into whichever unit you are buying in. Cubic yards for a truck, cubic metres in Europe, and bags for anything you can carry.

Bag counts come from one published figure rather than a table. An 80 lb bag of ready-mixed concrete yields 0.6 cubic feet, which is 0.0075 cubic feet for every pound of dry mix, and every other bag size follows from that.

  1. Step 1 Slab volume cubic_feet = length_ft × width_ft × (thickness_in ÷ 12)
  2. Step 2 Footing volume cubic_feet = length_ft × (width_in ÷ 12) × (depth_in ÷ 12)
  3. Step 3 Post holes, less the post per_hole = (hole_area − post_area) × depth_ft A 4 × 4 post really measures 3.5 in, and it displaces about 15% of a 10 in hole.
  4. Step 4 Add waste ordered = volume × (1 + waste%) Uneven subgrade, spillage and what stays in the barrow.
  5. Step 5 Into the units you buy in cubic_yards = cubic_feet ÷ 27 cubic_metres = cubic_feet × 0.0283
  6. Step 6 Bags bags = ceil(cubic_feet ÷ (bag_lb × 0.0075)) So 0.6 cu ft for an 80 lb bag, 0.45 for a 60, 0.3 for a 40.

For example, a 10 × 10 ft slab at 4 inches is 33.3 cubic feet, or 1.23 cubic yards. With 10% waste that is 1.36 cubic yards, and at 0.6 cubic feet a bag it takes 62 bags of 80 lb mix. That is also the point where a ready-mix truck starts to make more sense than a pallet of bags.

Concrete slab chart

How much concrete a slab takes at common thicknesses, with 10% waste, and how many 80 lb bags that is. Every figure comes from the same formula as the calculator above.

Concrete slab chart
Slab4 in thick5 in thick6 in thick
4 × 4 ft 0.22 cu yd10 bags 0.27 cu yd13 bags 0.33 cu yd15 bags
8 × 8 ft 0.87 cu yd40 bags 1.09 cu yd49 bags 1.3 cu yd59 bags
10 × 10 ft 1.36 cu yd62 bags 1.7 cu yd77 bags 2.04 cu yd92 bags
12 × 12 ft 1.96 cu yd89 bags 2.44 cu yd110 bags 2.93 cu yd132 bags
20 × 20 ft 5.43 cu yd245 bags 6.79 cu yd306 bags 8.15 cu yd367 bags

Volumes and bag counts both include 10% waste. An 80 lb bag yields 0.6 cubic feet. Past about 1.5 cubic yards, ready-mix delivered by truck is usually cheaper and far faster than mixing bags by hand.

Concrete slab chart showing cubic yards and 80 pound bags for 4 by 4 to 20 by 20 foot slabs at 4, 5 and 6 inches thick
The same chart as an image, to save or pin.

Assumptions and limits

Everything this calculator quietly assumes on your behalf. If one of these does not match your situation, the answer will be off.

  • Bag yield follows the published 0.6 cubic feet for an 80 lb bag, scaled by weight for every other size. Brands vary by a few percent, and the yield printed on your own bag is the one to trust.
  • Waste defaults to 10%, which covers spillage, an uneven subgrade and the concrete left in the barrow. A trench dug by hand swallows more than a formed slab does.
  • Volumes are the finished pour. Concrete does not compact or settle the way gravel does, so what you place is what you get.
  • Post holes subtract the post. Nominal lumber is smaller than its name, so a 4 × 4 is taken as 3.5 inches and a 6 × 6 as 5.5.
  • No reinforcement is allowed for. Rebar and mesh displace a negligible amount of concrete, but they change what the slab can carry, and that is a design question rather than a volume one.
  • Slab thickness is uniform. A thickened edge, a haunch around the perimeter or a fall for drainage all add concrete that is not counted here.
  • The ready-mix suggestion is a rule of thumb at about one and a half cubic yards. The real break-even depends on delivery charges, minimum loads and how far the truck has to come, and some suppliers charge a short-load fee well above that.
  • Nothing here covers mix strength. The bagged product you buy is a set strength, usually around 3,000 to 4,000 psi, and structural work has its own specification.

Frequently asked questions

How many bags of concrete do I need for a 10 × 10 slab?

About 62 bags of 80 lb mix at 4 inches thick, including 10% waste. That is 1.36 cubic yards. At that size a ready-mix truck is usually cheaper and much quicker than mixing 62 bags by hand.

How much does an 80 lb bag of concrete cover?

It yields 0.6 cubic feet, which covers about 1.8 square feet at 4 inches thick, or 3.6 square feet at 2 inches. A 60 lb bag yields 0.45 cubic feet and a 40 lb bag 0.3, so it takes 45 of the 80 lb bags to make a cubic yard.

How many bags of concrete per fence post?

About two 80 lb bags for a 4 × 4 post in a 10 inch hole 30 inches deep, once the post itself is subtracted. Wider holes and deeper frost lines push it to three. The fence calculator uses the same two-bag figure when it works out a whole run.

When should I use ready-mix instead of bags?

Past about one and a half cubic yards, which is roughly 60 bags of 80 lb mix. Beyond that the bags cost more per yard than the truck, and the real problem is time, because a large pour has to be placed and finished before the first of it sets.

How thick should a concrete slab be?

Four inches for a patio, a shed base or a footpath. Five to six inches for a driveway or anything a vehicle parks on, and more where the ground is poor. Under four inches a slab cracks under any load heavier than foot traffic.

How do I calculate concrete for post holes?

Work out the hole as a cylinder, subtract the post, and multiply by the number of holes. A 10 inch hole 30 inches deep is 1.36 cubic feet, and a 3.5 inch square post takes 0.21 of that back, leaving 1.15 cubic feet a hole.

How much concrete is in a cubic yard?

Twenty seven cubic feet, which is 45 bags of 80 lb mix, 60 bags of 60 lb or 90 bags of 40 lb. A cubic yard covers 81 square feet at 4 inches thick, and weighs roughly two tons once mixed.

Does concrete need to be ordered with extra?

Yes. Ten percent is the usual allowance for a bagged job and covers spillage and an uneven base. On a truck delivery, order slightly over rather than under, because a pour that runs short while the first half is setting leaves a cold joint you cannot undo.

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