How to Calculate Concrete
Volume, waste factor, and the difference between a bag job and a truck job
"How do I calculate concrete?" is really three questions stacked on top of each other. What shape are you actually pouring. What volume does that work out to. How much extra do you order so you do not run short. The math takes ten minutes. The judgment is what separates a clean pour from a ruined Saturday.
This guide covers all three, with the field shortcuts contractors use in their heads and the mistakes that send DIYers back to Google for a second look. When you want the math done for you, the concrete calculators handle every shape on this page (that's why we made them).
The Only Formula You Actually Need
Length times width times depth, all in feet, divided by 27. That is the entire equation for any rectangular pour, which covers slabs, footings, pads, and the vast majority of residential concrete work.
Cubic yards = (L ft × W ft × D ft) / 27
The reason 27 keeps showing up: a cubic yard is 3 feet by 3 feet by 3 feet, which is 27 cubic feet. Concrete suppliers in the United States price and dispatch in cubic yards. Bag manufacturers list yields in cubic feet. The 27 is the bridge between the two.
Two field shortcuts worth memorizing:
- - For depths in inches: (L ft × W ft × D in) / 324 = cubic yards. Saves the inch-to-foot conversion step when you are doing math at the form board.
- - The "magic 81" trick for 4-inch slabs: square footage divided by 81 equals cubic yards. A 162 sq ft patio at 4 inches is 2.0 yards. This is how working contractors estimate flatwork in their heads.
Tim Seay, a Tennessee concrete contractor quoted in Concrete Network, calls 81 "the magic number" for residential flatwork. Whether you are bidding a patio or sanity-checking a quote, divide square footage by 81 and you have the yardage in seconds.
Match the Formula to the Shape
Five shapes cover roughly 95 percent of residential pours. Recognize which one you have and the math becomes routine.
1. Slab or flatwork
Driveways, patios, sidewalks, garage floors, shed pads. Standard rectangular volume.
Example: 12 ft × 16 ft × 4 in = (12 × 16 × 4) / 324 = 2.37 cu yd
Use the slab calculator for any rectangular flatwork.
2. Footing
Spread footings under columns, deck posts, or structural piers, plus continuous strip footings under foundation walls. Same rectangular formula, but be ruthless about average depth on uneven ground.
Example: 24 in × 24 in × 12 in = 8 cu ft = 0.30 cu yd per footing
Spread footings: footing calculator. Continuous: strip footing calculator. For sizing rules, see the footing size chart.
3. Column or pier
Sonotubes, deck piers, porch columns. Cylinder volume: pi times radius squared times height. The diameter is what is printed on the tube. The radius is half of that. Mixing those two up is the most common error here.
Example: 12 in tube, 4 ft tall = 3.14 × (0.5)^2 × 4 = 3.14 cu ft = 0.12 cu yd
Use the column calculator, which handles round and square columns.
4. Post hole
Cylinder volume of the hole, minus the volume of the post itself. The subtraction is genuinely contested between calculator sites. Honest answer: for fast-set mix in a small hole around a 4x4, the displacement is so small that subtracting it changes the answer by less than half a bag, so most people skip it. For sonotubes or 6x6 posts in larger holes, do subtract, because you will be off by a bag or more.
The post hole calculator handles both modes. For full bag counts by post size, see bags of concrete for post holes.
5. Stairs
Break each tread plus riser into a rectangular block, sum them, then add the back triangle if the stairs are open underneath. Easier than it sounds once you see the geometry.
The stairs calculator walks you through it. For IRC code dimensions, see concrete stairs calculation.
Irregular Shapes: Break It Into Rectangles
Anything that does not look like a clean rectangle gets split into smaller pieces, calculated separately, then summed.
L-shaped slabs
Draw an imaginary line at the inside corner. You now have two rectangles. Calculate each one, add the volumes. Works for any L, T, or U shape.
Trapezoid driveways with a flared apron
For the flared section where a driveway widens at the street, use the average of the two widths and treat it as a rectangle. The contractor shorthand:
Average width = (narrow + wide) / 2
A driveway that flares from 10 ft to 18 ft over the last 8 ft averages 14 ft wide. Calculate that section as 14 × 8 × depth.
Slabs with cutouts
Drain basin, column base, tree well. Calculate the full slab volume. Calculate the cutout volume. Subtract.
Sloped slabs (the most common DIY miss)
Driveway falling from a garage to the street, patio sloped for drainage. Use the average depth, not the depth at one end:
Average depth = (deep end + shallow end) / 2
A driveway that is 6 inches at the garage and 4 inches at the street averages 5 inches deep across the whole pour. Most DIY shortfalls trace back to measuring at the form and assuming that depth holds for the entire slab.
The Waste Factor (and Why You Should Round Up)
Every other article on the internet says "add 10 percent" and stops there. The number depends on the job, and understanding what each percentage point pays for is what keeps you from running short or wildly over-ordering.
| Waste Factor | When to Use It |
|---|---|
| 5% | Clean rectangular pour, level compacted subgrade, experienced crew, indoor or covered, small job. |
| 8 to 10% | Typical residential exterior pour with 2x form lumber and normal subgrade prep. The default most ready-mix dispatchers will quote you. |
| 10 to 15% | Irregular shape, sloped or rough subgrade, first-time DIYer, footings dug by hand, long pour distance with wheelbarrows. |
What that waste factor actually pays for:
- - Over-excavation when the subgrade is not perfectly to grade
- - Form lumber bowing slightly under the pressure of fresh concrete
- - Subgrade settling under the weight of wet concrete
- - Loss of entrained air during placement and consolidation
- - Truck mixer hold-back, the 1 to 2 percent residual that does not fully discharge from the drum
- - Spillage at the chute and in the wheelbarrow
- - The 1 to 2 percent volume reduction from fresh concrete to hardened concrete
The honest punchline: running short is dramatically more expensive than over-ordering. A second short-load delivery to fix a 0.2 yard shortage typically runs $300 to $600 in fees, and worse, you end up with a cold joint where the two pours meet. A cold joint is a structural weakness that you cannot trowel away. Order the extra. The leftover goes into a wheelbarrow form for a stepping stone.
Bags vs Ready-Mix: The Break-Even
The decision is mostly volume, with a few real-world exceptions:
- Below 0.5 cubic yard: bags every time. No short-load fee, no minimum, you can mix at your pace.
- 0.5 to 1.5 cubic yards: it depends. Bags win if access is bad, the project can be split across days (fence posts, repairs), or you do not have a crew. Ready-mix wins if it is a continuous pour like a slab.
- Above 1.5 to 2 cubic yards: ready-mix almost always wins on cost, time, and finish quality.
Hand-mixing 45 bags of 80-pound mix to get one cubic yard is roughly 8 to 12 hours of physical labor and 2,160 pounds of material to move twice (off the pallet, into the mixer or wheelbarrow). Be honest with yourself about what that looks like before you commit.
For full bag math, yields by bag size, and the dry-pour question on fence posts, see how many bags of concrete do I need and the concrete bags guide. For converting between the two, the bags to yards calculator handles every common bag size.
Translating Volume Into an Order
If you are buying bags:
- - Divide cubic feet by 0.6 for 80-pound bags, 0.45 for 60-pound, or 0.3 for 40-pound
- - Always round up
- - Buy one extra bag per ten as cheap insurance, both Home Depot and Lowes will take unopened bags back
If you are calling the ready-mix plant:
- - Round up to the nearest quarter yard
- - Have the PSI, slump, aggregate size, air entrainment, delivery window, and access notes ready before you dial
- - Ask about the short-load threshold (most plants charge a fee under 5 to 9 yards)
- - Combine all your concrete work into one delivery if possible. A driveway plus walkway plus shed pad scheduled together can hit 9 yards and skip the short-load fee, saving $400 or more
For the full dispatcher conversation, including PSI and slump recommendations and how to spot a load that arrived too wet, see how to estimate concrete yards.
Five Mistakes That Blow Up the Calculation
- Forgetting to convert inches to feet. A 4-inch slab is 0.333 ft thick, not 4 ft. Treating 4 inches as 4 feet gives you a 12x over-order. Catches new DIYers every weekend.
- Measuring outside the forms instead of inside. The volume is what fits between the form boards, not what the lumber takes up.
- Calculating average depth on a sloped subgrade as if it were flat. Use (deep end + shallow end) / 2.
- Skipping the waste factor. See above. There is no good reason to do this.
- Rounding down "to be safe on cost." You are not saving money, you are buying yourself a cold joint.
For more on the cure, finish, and prep mistakes that show up after the pour, see 5 common concrete mistakes.
A Worked Example, Start to Finish
Project: a 12 x 16 ft patio at 4 inches thick, with a 12 in x 12 in x 12 in spread footing every 6 ft along one side for a future pergola (4 footings).
Slab: 12 × 16 × 4 / 324 = 2.37 cu yd
Footings: 1 × 1 × 1 = 1 cu ft each, × 4 = 4 cu ft = 0.15 cu yd
Subtotal: 2.52 cu yd
Add 10% waste: 2.52 × 1.10 = 2.77 cu yd
Round up to nearest quarter yard: 3.0 cu yd
At 3.0 cubic yards, you are well under the typical short-load threshold. Expect a short-load fee on the ticket. A common spec to call in: 3.0 yards of 3,500 PSI mix at 4-inch slump, 3/4 inch aggregate, air-entrained at 5 percent if you are in a freeze-thaw climate, morning delivery.
Total time on the calculation: ten minutes. Total time spent thinking through the shape, the slope, and the order: maybe another ten. That twenty minutes is the difference between a clean pour and a Saturday you will spend the rest of the year telling stories about.
The Bottom Line
The arithmetic on every concrete pour reduces to length times width times depth, divided by 27. The judgment is everything else. What shape you actually have. How honest you are about the subgrade and the slope. What waste factor matches the conditions. Whether the volume puts you in bag territory or truck territory. Round up.
When you want the math done for you, every shape on this page has a calculator. Jump straight to the slab, footing, column, post hole, or stairs calculator. For bag counts, the concrete bag calculator handles every common bag size.