How many bags of concrete do I need?

A bag of concrete mix promises a volume, not an area, and the promise changes with the size of the bag. This calculator takes your slab dimensions and a bag size and returns the number of bags, the volume they represent, and the total weight you will be lifting.

Length (ft)

Width (ft)

Thickness (in)

Bag size 40 lb bag (0.30 ft³)50 lb bag (0.375 ft³)60 lb bag (0.45 ft³)80 lb bag (0.60 ft³)

Waste allowance (%)

Bags needed

20bags

Volume 11.7 ft³Total weight 1,600 lb

Assumes standard concrete mix at the stated water content. Fast-setting, high-strength and post-hole mixes have different yields printed on their own bags.

The published yields used here are the standard ones: 40 lb = 0.30 ft³, 50 lb = 0.375 ft³, 60 lb = 0.45 ft³ and 80 lb = 0.60 ft³. Those figures assume the bag is mixed with the stated amount of water. Adding extra water to make it easier to work makes the mix weaker and reduces the yield, which is why the last bag often comes up short.

Bags make sense for fence posts, step repairs, a small pad or a shed base up to about one cubic yard. Beyond that the arithmetic stops favouring them: a cubic yard is 45 bags of 80 lb mix, which is 3,600 lb of material to mix by hand in a wheelbarrow, and ready-mix arrives in twenty minutes.

How the calculation works

Volume (ft³) = length × width × thickness ÷ 12, multiplied by (1 + waste ÷ 100). Bags = volume ÷ bag yield, rounded up. Total weight = bags × bag size.

Rounding up is the important step: bags cannot be bought in fractions, and a part-bag is a wasted bag unless you have another use for it.

• The slab is rectangular with uniform thickness; thickened edges and footings should be calculated separately.
• Bag yields are the manufacturer-published figures for standard concrete mix at the stated water content.
• Waste covers spillage, uneven subgrade and the loss from mixing in a wheelbarrow, which is more than people expect.
• Mixing by hand is assumed. A drum mixer wastes less, so the waste allowance can drop to 5%.

Sources: Bag yields: 40 lb 0.30 ft³, 50 lb 0.375 ft³, 60 lb 0.45 ft³, 80 lb 0.60 ft³ (manufacturer-published)

Worked examples

Questions

How many 80 lb bags of concrete make a cubic yard?

45, because each yields 0.6 ft³ and there are 27 ft³ in a yard. In 60 lb bags it is 60, and in 40 lb bags 90.

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

At 4 inches thick, 33.3 ft³ before waste: 56 bags of 80 lb mix, or 62 with a 10% allowance. Consider ready-mix at that size.

Do bigger bags give better value?

Usually yes per pound of mix, but the difference is small. The real saving at scale is ready-mix delivery, not bag size.

How much water goes in a bag?

About 3 quarts for an 80 lb bag, and less than the bag suggests if the sand is damp. Add most of it, mix, then add the rest — over-watering is the most common cause of weak concrete.

Can I mix partial batches?

Yes, but only mix what you can place and finish within about 30 minutes. A cold joint between batches is weaker than a continuous pour.

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Written and maintained by Sam Winters. Last updated 2026-11-04. Every figure is computed in your browser; nothing you enter is sent anywhere. Found an error? Tell me — accuracy reports get fixed first.