Concrete & Masonry
Concrete Calculator
Enter your slab or footing size and this tells you the bags to buy, or when to call a truck instead.
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| Volume of the pour | — |
|---|---|
| Plus the waste allowance | — |
| Concrete needed | — |
| In cubic yards | — |
| Bags, rounded up | — |
What to buy
Paid links. If you buy through one, we earn a commission. Your price is the same. Nothing is listed here unless the calculator above says you need it.
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Concrete mix, by the bag
The bag count above uses the yield for the size you picked. Check the yield printed on the bag, because fast-setting and crack-resistant mixes yield differently from standard mix.
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Mixing tub or wheelbarrow
Past a few bags you need somewhere to mix. A wheelbarrow doubles as the mixer and the transport.
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Concrete float and edger
Finishing tools. Not part of the volume math, but a slab you cannot float is a slab you cannot finish.
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Rebar or wire mesh
Anything you plan to drive on or park on needs reinforcement. It does not change the volume, it changes whether the slab cracks.
How to estimate concrete
Concrete is bought by volume and poured into a shape, so the work is finding the volume of that shape in cubic feet and dividing by what a bag yields. The one number you cannot derive yourself is the bag yield, because it depends on the density of the mix.
slab: cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
footing: cubic feet = 3.1416 × radius (ft)² × depth (ft) × how many
bags = cubic feet × (1 + waste) ÷ yield per bagYields per bag are Quikrete's stated figures: 0.60 cu ft for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb.
Where the bag yields come from
The QUIKRETE Concrete Mix #1101 SPEC-DATA sheet states it plainly: an 80 lb bag yields approximately 0.60 cubic feet, a 60 lb bag approximately 0.45, and a 40 lb bag approximately 0.30. Those are the numbers this calculator uses.
Notice the yields are not proportional to a round number of bags per yard by accident. A cubic yard is 27 cubic feet, so it takes 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags. Those three numbers are worth remembering, because they are how you sanity-check any concrete estimate in your head.
Check the bag you actually buy. Fast-setting mix, crack-resistant mix, and high-strength mix all have their own yields, and a fast-setting 50 lb bag yields about 0.375 cubic feet rather than the 0.45 you might assume from the weight.
Slabs
Length times width times thickness, with the thickness converted from inches to feet. A 10 by 10 foot slab at 4 inches is 10 × 10 × 0.333, which is 33.3 cubic feet, or 1.23 cubic yards before waste.
Quikrete's sheet says the mix is for pouring 2 inches thick or more. Four inches is the common thickness for patios, walkways, and shed floors. Thinner than 2 inches is outside what the product is specified for, and thin concrete cracks.
Round footings and post holes
A hole is a cylinder, so the volume is pi times the radius squared times the depth. The radius is half the diameter, and everything has to be in the same unit before it multiplies.
A 12 inch diameter hole 36 inches deep is a 0.5 foot radius and 3 feet deep, so 3.1416 × 0.25 × 3, which is 2.36 cubic feet per hole. Four of those is 9.4 cubic feet, or 16 bags at 80 lb.
This calculator gives you the full hole. If you are setting a post in it, the post displaces some of that volume, and most people put a few inches of gravel in the bottom for drainage. Our fence post calculator handles that subtraction properly.
Why the waste allowance
Ten percent by default, and it is not padding. The subgrade under a slab is never perfectly flat, so the average thickness is usually a little more than the number you measured. Forms bow outward under the weight of wet concrete. Some mix stays in the tub and on the tools.
The asymmetry matters more than the percentage. A spare bag costs a few dollars and can be returned. Running out in the middle of a pour means the first batch starts setting before the last one arrives, and you get a cold joint: a permanent plane of weakness right through your slab. Buy the extra bag.
When to stop bagging and call a truck
Around one cubic yard. That is 45 of the 80 lb bags, roughly 3,600 pounds of material to buy, get home, and mix in batches, all while the pour needs to stay continuous.
Ready-mix suppliers usually have a minimum load and charge a short-load fee below it, so a single yard is not automatically cheaper on paper. Price both. But past a yard the arithmetic and the labor both point the same direction, and past two yards it is not really a debate.
Common estimating mistakes
- Forgetting to convert thickness. A 4 inch slab is 0.333 feet, not 4. Multiplying by 4 overstates the pour by a factor of 12.
- Using diameter instead of radius. A cylinder uses the radius squared. Using the diameter gives four times too much concrete.
- Assuming yield scales with weight. Different mixes have different densities. Read the yield on the bag rather than scaling from a number you remember.
- Ordering exactly. Concrete is the material where running short is most expensive, because you cannot pause a pour and resume it tomorrow.
- Skipping reinforcement in the estimate. Rebar and mesh do not change the volume, but leaving them out of the plan changes whether the slab lasts.
When to call a pro
Two boundaries here, and both are real.
Structural work is the first: footings that carry a building, anything supporting a wall or a deck ledger, and anything a permit covers. Getting the volume right is the easy part of that job, and it is not the part that fails.
The second is simply time. Concrete does not wait. Past a yard poured by hand, you need enough people to keep mixing, placing, and finishing continuously, and a slab that sets while you are still mixing cannot be fixed afterward.
Questions
How many bags of concrete are in a cubic yard?
Using Quikrete's stated yields, it takes 45 of the 80 lb bags, 60 of the 60 lb bags, or 90 of the 40 lb bags to make one cubic yard. A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet, so 27 divided by 0.60 is 45.
How much does an 80 lb bag of concrete cover?
About 0.60 cubic feet, which is Quikrete's stated yield on the Concrete Mix 1101 data sheet. At a 4 inch slab thickness that spreads to roughly 1.8 square feet per bag. A 60 lb bag yields about 0.45 cubic feet and a 40 lb bag about 0.30.
At what point should I order ready-mix instead of bagging it?
Around a cubic yard, which is 45 of the 80 lb bags. That is roughly 3,600 pounds of material to buy, haul, and mix in batches, and the pour has to stay continuous to avoid cold joints. Most ready-mix suppliers have a minimum load and a short-load fee, so price both, but past a yard the truck usually wins on cost as well as on effort.
How thick does a slab need to be?
Quikrete's data sheet says its Concrete Mix is for pouring concrete 2 inches thick or more. Four inches is the common thickness for patios, walkways, and shed floors. Anything you will park a vehicle on generally wants more thickness and reinforcement, and that is a question for your local building department rather than a calculator.
Why add a waste allowance?
Because the hole is never the size you measured. Subgrade settles unevenly, forms bow slightly, and some mix stays in the tub. Ten percent is the usual allowance. Running out mid-pour is much worse than having a spare bag, since concrete that sets before the next batch arrives leaves a cold joint.
Every constant on this page is logged with its source and the date it was checked in data/sources.md.