Concrete Calculator — Slabs, Footings & Columns
Calculate cubic yards of concrete needed for slabs, footings, or round columns. Get bag equivalents and cost estimates.
Concrete Needed
2.72 cu yd
With 10% waste factor
2.47 cu yd
Exact Volume
66.7 cu ft
Cubic Feet
149 bags
60-lb Bags (alt)
$407
Estimated Cost
Order 2.72 cubic yards from a ready-mix supplier, or 149 bags of 60-lb concrete mix for smaller pours.
What is Concrete Volume?
Concrete is estimated by volume, and the arithmetic is simple: length times width times depth, converted to cubic yards. What makes concrete unforgiving is that the material sets on a schedule you do not control.
Running short during a pour is genuinely serious. Concrete placed after the previous batch has begun setting creates a cold joint — a plane of weakness through the slab that never fully bonds and frequently becomes the crack line years later. Unlike paint or tile, you cannot simply fetch more tomorrow.
That asymmetry drives the whole approach: ordering surplus costs a modest amount, whereas ordering short costs structural integrity. Every experienced concrete contractor over-orders on purpose.
The formula — how to calculate Concrete Volume
- 27
- = cubic feet in a cubic yard (3 × 3 × 3)
- Waste allowance
- = 5–10% for a formed slab on level ground; more on uneven subgrade
Depth must be converted to feet before multiplying. Using inches directly is the single most common error and overstates volume twelvefold.
Step-by-step example
- 01A patio slab 20 ft × 12 ft at 4 inches thick.
- 02Convert depth: 4 ÷ 12 = 0.333 ft.
- 03Volume: 20 × 12 × 0.333 = 79.9 cubic feet.
- 04Cubic yards: 79.9 ÷ 27 ≈ 2.96 cubic yards.
- 05Add 10% for waste and subgrade variation: 2.96 × 1.10 ≈ 3.26 cubic yards.
- 06Order 3.5 cubic yards — ready-mix suppliers typically sell in quarter-yard increments and often charge a short-load fee below about a full yard.
- 07For comparison, mixing this by hand from 80 lb bags would take roughly 133 bags. Above about half a cubic yard, ready-mix is almost always the right call.
Standard thicknesses
Typical slab depths
| Application | Usual thickness | Note |
|---|---|---|
| Footpath / walkway | 4 in | Foot traffic only |
| Patio | 4 in | Thicken to 5–6 in for a hot tub |
| Driveway, cars | 4 in | 5–6 in over soft subgrade |
| Driveway, heavy vehicles | 6 in | Consider reinforcement |
| Garage floor | 4–6 in | Local code may specify |
| Shed or outbuilding base | 4 in | Thickened edge is common |
Local building codes govern anything structural, and footings in particular are code-controlled and depend on frost depth. Check before pouring rather than after.
Why concrete cracks, and what actually helps
Concrete cracks. It is not a defect to be designed out but a property to be managed, because the material shrinks slightly as it cures and moves with temperature afterwards.
Control joints are the main tool. Cutting or tooling grooves at regular intervals creates deliberate weak lines so the inevitable crack happens where you chose rather than diagonally across the middle. A common spacing rule is roughly 24 to 30 times the slab thickness in inches — so a 4-inch slab gets joints every 8 to 10 feet.
Subgrade preparation matters more than most people expect. Concrete poured over uncompacted fill will settle unevenly and crack regardless of thickness or reinforcement. Compaction and a uniform base are worth more than an extra inch of concrete.
Reinforcement — mesh or rebar — does not prevent cracking. It holds the pieces together and limits crack width once cracking occurs, which is a genuinely different and still valuable job.
Do not add water to make it easier to place
Adding water on site to loosen a stiff mix is the classic way to weaken a slab. It raises the water-to-cement ratio, which reduces compressive strength substantially and increases shrinkage cracking. If the mix is too stiff to work, ask the supplier for a plasticiser rather than reaching for the hose.
Curing is not the same as drying
Concrete gains strength through hydration, a chemical reaction that requires moisture. It does not harden by drying out — allowing it to dry prematurely stops the reaction and permanently reduces strength.
Keeping a fresh slab damp for the first several days, typically by covering it or misting it, produces a materially stronger result than letting it sit in sun and wind. Concrete reaches roughly 70% of design strength within a week and continues gaining for months.
Weather matters at both extremes. Pouring in freezing conditions can damage the mix before it sets; pouring in high heat accelerates setting and can cause surface cracking before finishing is complete. Early morning is the usual answer in summer.
Key considerations
- Convert depth from inches to feet before calculating volume.
- Add 5–10% for waste, more on uneven or unprepared subgrade.
- Compact the base properly — it matters more than extra thickness.
- Plan control joints at roughly 24–30 times the slab thickness in inches.
- Ready-mix is usually more economical above about half a cubic yard.
- Keep the slab damp during early curing rather than letting it dry out.
- Check local codes for anything structural, especially footing depth in frost regions.
Common mistakes to avoid
- Using depth in inches without converting to feet.
- Ordering the exact calculated volume with no allowance, then running short mid-pour.
- Adding water on site, which measurably weakens the finished slab.
- Pouring over uncompacted fill and cracking regardless of thickness.
- Omitting control joints and getting random cracks instead of planned ones.
- Letting the slab dry rather than cure, which permanently reduces strength.
Frequently asked questions
Sources & references
Written and fact-checked by the CalcProLabs Editorial Team. Read our calculation methodology and editorial policy.
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