Concrete Calculators
Concrete calculators and guides for slabs, footings, posts and steps, checked by a structural engineer. Work out how much concrete you need, which mix and strength to order, how to reinforce it, and how to place and cure it so it does not crack.
How much concrete
- Concrete Slab Calculator – cubic yards and bags for any slab
- Cubic Yard Calculator – rectangles, circles and triangles to cubic yards
- Concrete Footing Calculator – continuous and pad footings
- Concrete for Post Holes Calculator – bags per post hole
- Sonotube Concrete Calculator – round columns and piers
- Concrete Steps Calculator – solid steps with a top landing
- Concrete Block Calculator – blocks, mortar and fill for CMU walls
Mix, strength and ordering
- Concrete Mix Ratio – 1:2:3 and other site mixes
- What PSI Concrete Do I Need – code minimums and when to use air-entrained concrete
- How Much Concrete Does a Truck Hold – ready-mix loads, short loads and bags vs truck
Reinforcement
- Rebar Calculator for Concrete Slabs – bars, spacing and weight for a slab grid
- Rebar Sizes and Weights – #3 to #18 size chart
- Rebar vs Wire Mesh vs Fiber – which reinforcement a slab needs
Thickness, joints and curing
- Concrete Slab Thickness – patios, driveways, garages and sheds
- Concrete Control Joint Calculator – joint spacing and saw-cut depth
- Concrete Curing Time – when you can walk, drive or build on it
Key concrete numbers
| Item | Value |
|---|---|
| 1 cubic yard | 27 cubic feet; covers 81 sq ft at 4 in thick |
| 80 lb bag of concrete mix | About 0.60 cu ft, so 45 bags per cubic yard |
| Weight of concrete | About 145 to 150 lb per cubic foot, roughly 4,000 lb per cubic yard |
| Control joint spacing | 2 to 3 times the slab thickness, in feet |
| Code minimum strength (IRC R402.2) | 2,500 psi; up to 3,500 psi air-entrained for exposed slabs in cold climates |
| Rated strength | Reached at 28 days; about 65% at 7 days |
Engineer's note: most concrete problems start before the truck arrives: soft or unevenly compacted base, too much water added to make it flow, or joints cut too late. Get those three right and the slab will outlast the house. Order 5 to 10% extra, because running short in the middle of a pour leaves a cold joint that is weaker and visible forever.
Everything in this section
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Maintained by: Engr. Abdul Latif, civil and structural engineer. Every calculator here shows its formula and the code table it follows. For planning only; confirm with your local building department.