How to Calculate Concrete for a Slab, Footing, or Column
Ordering concrete is expensive and unforgiving — short one yard and you're stuck with a cold joint; order too much and you pay for what goes in the dumpster. Here's how pros calculate concrete for slabs, footings, and columns without guessing.
The core formula
Every concrete calculation — slab, footing, column, or stair — is just volume math. The formula is the same:
The divisor is 27 because one cubic yard equals 27 cubic feet (3 ft × 3 ft × 3 ft). From there, the only difference between shapes is how you measure them.
Step-by-step for a concrete slab
Measure length, width, and thickness in feet
Convert inches to feet by dividing by 12. A 4-inch slab is 0.333 ft thick. A 6-inch slab is 0.5 ft. This is the step most DIYers get wrong — plugging inches into a cubic-yard formula gives results that are 1,728× too large.
Multiply to get cubic feet
For a 20 ft × 30 ft × 4 in slab: 20 × 30 × 0.333 = 200 cubic feet.
Divide by 27 to get cubic yards
200 ÷ 27 = 7.41 cubic yards. This is the raw concrete needed, before waste.
Add waste factor (5–10%)
Spillage, over-excavation, uneven subgrade, and form bulging eat concrete. Industry standard is 5% for slabs with rigid forms and 10% for footings or rough subgrade. For the slab above: 7.41 × 1.05 = 7.78 cubic yards — order 8 yards (most plants sell in 0.25 yard increments, but you rarely want to cut it that close).
Convert to bags (only for pours under 1 yard)
If you're mixing from pre-mix bags instead of calling a ready-mix truck:
- 1 cubic yard ≈ 45 × 80 lb bags (Quikrete, Sakrete)
- 1 cubic yard ≈ 60 × 60 lb bags
- 1 cubic yard ≈ 90 × 40 lb bags
Above 1 cubic yard, bags become both more expensive and more labor-intensive than ready-mix delivery.
Footings
A footing is just a long, narrow slab. Same formula: Length × Width × Depth ÷ 27. A 30 ft continuous footing, 18 inches wide and 12 inches deep, works out to 30 × 1.5 × 1 = 45 cubic feet = 1.67 cubic yards. Add 10% waste for dug trenches: 1.84 cubic yards.
For stepped footings, calculate each step separately and sum.
Columns and piers
Round columns use the cylinder formula:
A 10-inch diameter column, 8 feet tall: radius = 0.417 ft, so π × 0.417² × 8 = 4.37 cubic feet = 0.162 cubic yards. Add 10% waste: 0.178 cubic yards. Below 0.25 cubic yards, use bags.
Stairs
Stairs are the ugliest calc in the book because every step is a wedge. The shortcut:
- Calculate the triangular side area: ½ × total rise × total run
- Multiply by stair width
- Divide by 27
This gives the minimum volume (the wedge under the steps). For cast-in-place stairs, add 20% to account for the actual tread/riser geometry that sits above the wedge.
Common mistakes
- Using inches in the formula. Cubic yards requires all dimensions in feet.
- Forgetting waste factor. Your slab will always need 5–10% more than the geometric volume. Concrete doesn't "stretch."
- Rounding down. Never round down when ordering. If the math says 7.41 yards, don't order 7 — you'll be paying an emergency short-load fee when the truck runs out before the pour finishes.
- Ignoring slab reinforcement volume. Rebar and welded wire mesh take up negligible volume, so you can ignore them. Post-tension cables similarly. But if you have large drain pockets, subtract their volume.
Frequently asked questions
How many cubic yards of concrete do I need for a 20x20 slab?
A 20 ft × 20 ft slab at 4-inch thickness needs (20 × 20 × 0.333) ÷ 27 = 4.93 cubic yards. With 5% waste, order 5.2 yards — most contractors round up to 5.5 yards to stay clear of short-load territory.
How many 80 lb bags of Quikrete make a cubic yard?
Approximately 45 bags of 80 lb pre-mix per cubic yard. At $6–$8 per bag, that's $270–$360 per yard in material alone — about 3–4× the cost of ready-mix. Use bags for pours under 0.5 cubic yards; above that, ready-mix is cheaper and faster.
What waste factor should I add to my concrete order?
Use 5% for slabs with rigid formwork on a flat, compacted subgrade. Use 10% for footings in dug trenches, stepped footings, or any pour where the subgrade is uneven. Never round your order down — ready-mix plants charge a short-load fee ($100–$250) for under-minimum deliveries, so rounding up is almost always cheaper than running short.
How do I calculate concrete for a round column or pier?
Use the cylinder formula: Volume = π × radius² × height ÷ 27. Example: a 10-inch diameter column (radius = 0.417 ft) that is 8 feet tall: π × 0.417² × 8 = 4.37 cubic feet = 0.162 cubic yards. Add 10% waste: 0.178 cubic yards. For piers under 0.25 cubic yards, use pre-mix bags — a single tube form with one 80 lb bag per foot of depth is the standard for deck footings.
How much does a cubic yard of concrete cost?
Ready-mix runs $125–$200 per cubic yard for material, depending on region and mix. Short-load fees add $100–$250 if under the minimum. Fiber reinforcement adds $15–$25 per yard. Always get a written quote with delivery fees, minimum load, and overtime (if the pour runs long) before scheduling.
Related guides
Concrete is rarely the last step. Once your slab or footing cures, you'll need to calculate materials for the next phase:
- How to Calculate Framing Lumber — studs, plates, headers, and joists for walls and floors built on top of your slab
- How to Calculate Flooring — square footage, boxes, and waste factor for the finished floor surface over your slab
- How to Calculate Paint Coverage — gallons of paint needed once walls are up and primed
Skip the math — use the calculator
BuildCalc Pro's concrete calculator handles slabs, footings, columns, and stairs automatically, including bag counts and waste factor. Free to use, no signup required.
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