Concrete Calculator
Use this concrete calculator to find out how many bags of concrete or cubic yards of ready-mix you need for a slab, footing or post hole. Enter your measurements and it rounds up to whole bags, with a waste allowance built in.
Result
62 bags of 80 lb concrete
| Volume (no waste) | 1.23 yd³ (33.3 ft³) |
|---|---|
| Volume + 10% waste | 1.36 yd³ (36.7 ft³) |
| Or 60 lb bags (27 kg) | 82 |
| Or 40 lb bags (18 kg) | 123 |
| Ready-mix order | 1.5 yd³ |
Results update as you type.
How Much Concrete Do I Need?
Concrete is sold by volume: cubic yards for ready-mix trucks, and a set yield per bag for bagged mix. Every concrete job comes down to the same three steps:
- Work out the volume of the space you’re filling, in cubic feet.
- Add a waste allowance, usually 10%.
- Divide by the yield of the bag you plan to buy, or by 27 to get cubic yards.
The calculator above does this for you. The rest of this page shows the math, so you can check the numbers or work them out on site with a tape measure and a phone.
How to Calculate Concrete by Hand
Slabs, patios and pads
Multiply length × width × thickness, with every measurement in feet. Slab thickness is usually given in inches, so divide it by 12 first.
Volume (ft³) = length (ft) × width (ft) × (thickness (in) ÷ 12)
Footings
A continuous footing is a long, narrow slab. Use the total run of footing as the length, and convert both the width and the depth from inches to feet:
Volume (ft³) = length (ft) × (width (in) ÷ 12) × (depth (in) ÷ 12)
Post holes and round columns
A round hole is a cylinder. Halve the diameter to get the radius, then convert inches to feet. That’s why the formula divides by 24:
Volume (ft³) = 3.1416 × (diameter (in) ÷ 24)² × depth (ft)
Multiply by the number of holes, and use the same formula for Sonotube forms.
Worked example: a 12 × 16 ft patio
Say you’re pouring a 12 by 16 foot patio, 4 inches thick:
- Volume: 12 × 16 × (4 ÷ 12) = 64 ft³
- In cubic yards: 64 ÷ 27 = 2.37 yd³
- Add 10% waste: 64 × 1.10 = 70.4 ft³ (2.61 yd³)
- 80 lb bags: 70.4 ÷ 0.6 = 117.3, rounded up to 118 bags
- Or ready-mix: 2.61 yd³, rounded up to the next quarter yard, is 2.75 yd³
118 bags weigh 9,440 lb, which is more than four tons to unload, carry and mix by hand. At that size, ready-mix is almost always the better choice (see below).
How Many Bags of Concrete Per Yard?
Bag yields come from manufacturer data sheets for standard concrete mix:
| Bag size | Yield per bag | Bags per cubic yard | Bags per 10 ft³ |
|---|---|---|---|
| 80 lb | 0.60 ft³ | 45 | 17 |
| 60 lb | 0.45 ft³ | 60 | 23 |
| 40 lb | 0.30 ft³ | 90 | 34 |
Per cubic foot, the 80 lb bag is usually the cheapest. The 60 lb and 40 lb sizes are easier to lift, which counts if you’re carrying bags across a yard or into a basement. Specialty mixes such as fast-setting, high-strength or crack-resistant yield different amounts, so check the bag and adjust.
Tips & Common Mistakes
Measure the actual depth, not the form height
Forms are often set on top of a base that isn’t perfectly level. Dips in the subgrade quietly add volume. On a 10 × 10 ft slab, an average of just half an inch of extra depth adds 4.2 ft³, which is seven more 80 lb bags. Measure depth at several points inside the forms and use the average.
Don’t skip the waste allowance
Some concrete always ends up in the wheelbarrow, on the mixer drum, or pushed out past the forms. The calculator defaults to 10%. Use 5% only for small, carefully formed pours on a compacted base. Use 15–20% for hand-dug footings and holes, because their sides are never as neat as the plan.
Running short costs more than leftover bags
If you run out mid-pour, the concrete you’ve already placed starts to set before the next batch arrives. That leaves a cold joint, a weak seam that tends to crack. Unopened bags can often be returned, and leftovers are handy for setting a mailbox post or patching a step later.
Bags or ready-mix: where the line falls
- Under about 1 yd³ (45 bags of 80 lb mix): bags are usually the practical choice.
- 1–2 yd³: either can work. Consider renting a mixer, or call local suppliers that offer small-load trailers.
- Over 2 yd³: get ready-mix quotes. Ask about minimum orders and short-load fees, and make sure the truck can reach your pour or that you’ve arranged a pump.
Post holes: concrete goes below the frost line
For fence and deck posts, a common rule of thumb is a hole about three times the post’s width. The depth should be at least one third of the post’s height above ground. Local code may require you to go below the frost line, which is deeper in cold climates. Put a few inches of gravel in the bottom of each hole for drainage, which also slightly reduces the concrete you need. Our gravel calculator works out the stone. This calculator doesn’t subtract the volume of the post, so the result includes a small margin. If you’re building a whole fence, the fence calculator counts the posts, rails and pickets and the concrete for every hole.
Thickness matters more than you’d think
Going from 4 to 5 inches thick adds 25% more concrete. Going to 6 inches adds 50%. Set the thickness for the load the slab will carry: 4 inches for foot traffic, 5–6 inches for vehicles. Check local code for anything structural.
Frequently Asked Questions
How many bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet. An 80 lb bag yields about 0.6 cubic feet, so it takes 45 bags to make one cubic yard. You need 60 bags of the 60 lb size or 90 bags of the 40 lb size. Those counts are before waste, so buy a few extra.
How much concrete do I need for a 10x10 slab?
A 10 by 10 foot slab poured 4 inches thick holds 33.3 cubic feet, or about 1.23 cubic yards. With a 10% waste allowance that is 62 bags of 80 lb concrete, or 1.5 cubic yards if you order ready-mix.
How many bags of concrete do I need per fence post?
A typical 10 inch wide, 3 foot deep post hole holds about 1.6 cubic feet, so plan on three 80 lb bags per hole. The post itself takes up a little of that space, which gives you a small built-in margin. Wider or deeper holes need more, so enter your own hole size above.
How thick should a concrete slab be?
Four inches is standard for patios, walkways and shed floors. Driveways and garage floors that carry cars are usually poured 5 to 6 inches thick. Your local building code and any engineered plans take priority over these general figures.
How much waste should I add when ordering concrete?
Add 5% to 10% for a slab on a flat, well-compacted base. Go up to 15% or 20% for hand-dug footings, uneven ground or forms that might bow. Running short mid-pour leaves a weak cold joint, so it is cheaper to have a bag or two left over.
How much area does an 80 lb bag of concrete cover?
An 80 lb bag makes about 0.6 cubic feet of concrete. Poured 4 inches thick, that covers about 1.8 square feet. At 2 inches thick it covers about 3.6 square feet.
Should I use bags or order ready-mix concrete?
Bags make sense for post holes, small pads and repairs up to about one cubic yard (45 bags of 80 lb mix). Past that, mixing by hand gets slow and the first batches can start setting before you finish. Get a ready-mix quote for anything over one or two yards, and ask about short-load fees on small orders.
Related Calculators
Sources & Assumptions
- Quikrete Concrete Mix (No. 1101) data sheet — 0.30, 0.45 and 0.60 ft³ yield per 40, 60 and 80 lb bag
- ACI 332, Residential Code Requirements for Structural Concrete — minimum slab and footing guidance
- Bag yields assume standard concrete mix. High-strength, fast-setting and lightweight mixes yield differently; check the bag.
- Volumes are rounded only for display. Bag counts and ready-mix orders always round up.
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