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Concrete calculator

Cubic yards, bags and the delivered price for your city. Free, no sign-up, step by step.

By the LocalServiceHQ editorial team
Updated Oct 28, 2026
Formulas and sources listed below
Prices checked Oct 2026
Over 1 cubic yard?A ready-mix truck is usually cheaper and faster than mixing bags.
Photo by Mitchell Luo on Unsplash

How the math works

Slab volume
Geometry
length × width × (depth in ÷ 12) = cu ft
Measure length and width in feet and depth in inches. A 4 in slab is 0.333 ft thick.
Round holes
Geometry
π × (diameter ÷ 24)² × (depth ÷ 12) × holes
Dividing the diameter in inches by 24 gives the radius in feet.
Cubic yards
Supplier standard
cu ft ÷ 27 = yd³
Ready-mix is sold by the cubic yard. Most Austin suppliers have a 1 yard minimum.
Bag yield
Manufacturer bag labels
80 lb = 0.60 · 60 lb = 0.45 · 40 lb = 0.30 cu ft
Printed yield for standard concrete mix. Add waste for spills, uneven ground and form flex.

Already worked out

Common jobs at 10% waste. Click one to load it into the calculator.

Rather hire it out?

See Austin installed prices and local permit rules.

Concrete in Austin, TX

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How to use the concrete calculator

Measure your project in feet, then measure depth in inches. For odd shapes, split the area into rectangles, run each one, and add the results.

1
Pick a shape
Slab for driveways, patios and sidewalks. Footing for wall or strip footings. Post holes for fence and deck posts.
2
Enter the size
Length and width in feet, depth in inches. Measure at a few points; use the deepest one if the ground slopes.
3
Choose waste
10% covers spills, uneven subgrade and forms that bow. Use 15% on rough, hand-dug ground.
4
Read the result
Order ready-mix by the cubic yard, or buy bags. Save the step-by-step as a PDF for your supplier or contractor.

How many bags of concrete are in a yard?

One cubic yard is 27 cubic feet. Divide that by the yield printed on the bag to get the bag count.

Bag size
Yield per bag
Bags per cubic yard
Weight per yard
80 lb
0.60 cu ft
45 bags
3,600 lb
60 lb
0.45 cu ft
60 bags
3,600 lb
40 lb
0.30 cu ft
90 bags
3,600 lb

Yields are the standard printed values for regular concrete mix. Check your bag; fast-setting and high-strength mixes can differ.

How thick should concrete be?

Thickness drives both strength and cost. Going from 4 to 5 inches adds 25% more concrete.

Project
Typical thickness
Concrete per 100 sq ft
Notes
Sidewalk, walkway
4 in
1.23 yd³
Control joints every 4–5 ft
Patio
4 in
1.23 yd³
5 in for hot tubs or heavy grills
Driveway, cars
4 in
1.23 yd³
Rebar on a compacted base
Driveway, trucks or RV
5–6 in
1.54–1.85 yd³
Thicken edges where tires run
Garage or shop slab
4–6 in
1.23–1.85 yd³
Vapor barrier underneath
Shed base
4 in
1.23 yd³
Thicken the perimeter for wall loads

Local building codes set the minimum for footings and some slabs. Confirm with your city before you pour.

Bags or a ready-mix truck?

Hand-mixing makes sense for small jobs. Past about one cubic yard, a truck is usually cheaper per yard, faster and gives a more even pour.

 
Bagged mix
Ready-mix truck
Best for
Post holes, small pads, repairs
Driveways, patios, slabs over 1 yd³
Work involved
Mix every bag by hand or mixer
Poured straight from the chute
Minimum order
None
Often 1 yd³; short-load fees below a set amount
Consistency
Varies batch to batch
One mix, set strength (PSI)
Time window
You set the pace
About 90 minutes from batching to finish

Mistakes that crack slabs

!
Skipping the baseA 4 in compacted gravel base stops settling. Pouring on loose or soft soil is the top cause of cracks.
!
No control jointsCut joints about one quarter of the slab depth, spaced no more than 2–3 times the thickness in feet (8–12 ft for a 4 in slab).
!
Adding extra waterWet concrete is easier to spread but weaker. Follow the bag or order the right slump.
!
Ordering exactly the volumeRunning short mid-pour leaves a cold joint. Always add 5–15% waste.
!
Pouring in extreme heatIn hot weather, pour early, keep it damp and shade it so it doesn't dry before it cures.
!
Forgetting reinforcementRebar or wire mesh holds cracked pieces together. Driveways should get rebar.

How long does concrete take to cure?

Stage
Typical time
What you can do
Initial set
2–6 hours
Finish and cut joints
Walk on it
24–48 hours
Light foot traffic
Drive a car on it
About 7 days
Normal passenger cars
Heavy vehicles
About 28 days
Trucks, RVs, equipment
Full design strength
28 days
Rated PSI reached

Cold weather slows curing. Keep new concrete moist for the first week for the strongest result.

Concrete calculator FAQ

How do I calculate how much concrete I need?

Multiply length (ft) × width (ft) × depth (ft), then divide by 27 for cubic yards. A 10 × 10 ft slab 4 in deep is 10 × 10 × 0.333 = 33.3 cu ft, or 1.23 yd³. Add 10% for waste.

A 10 × 10 ft slab at 4 in needs 33.3 cu ft. At 0.6 cu ft per bag that is 56 bags, or 62 bags with 10% waste.

A 10 in hole, 24 in of concrete, around a 4×4 post takes about 0.9 cu ft, so 2 bags of 80 lb mix per post.

Under about 1 cubic yard, bags usually win. Above that, ready-mix is usually cheaper per yard and saves hours of mixing.

About 4,000 lb (roughly 145–150 lb per cubic foot). Plan for that weight on the delivery path.

3,000–4,000 PSI covers most homes: patios, walks and driveways. Driveways in freeze areas often use air-entrained 4,000 PSI.

A bonded overlay can work over sound concrete. Cracked or heaving slabs should be removed; new concrete will crack in the same places.

No. This tool counts concrete only. See local installed prices on our cost guides, or get quotes from pros.