How to Estimate Yards of Concrete
Cubic yards = length × width × depth, all in feet, divided by 27. The step almost everyone gets wrong is depth: it's naturally measured in inches, and multiplying without converting to feet first gives an answer twelve times too large. That single conversion is behind most wrong concrete estimates.
The formula
depth in inches ÷ 12 = depth in feet
| Depth | In feet |
|---|---|
| 4 inches | 0.333 ft |
| 5 inches | 0.417 ft |
| 6 inches | 0.500 ft |
| 8 inches | 0.667 ft |
| 12 inches | 1.000 ft |
A cubic yard is a cube three feet on each side: 3 × 3 × 3 = 27 cubic feet. Concrete is sold by the cubic yard but measured in feet, so every calculation ends with a division by 27. The other half of the error is depth — length and width are naturally in feet, depth is naturally in inches, and mixing the two gives an answer twelve times too large. It's by far the most common estimating mistake. If your answer looks absurd, check the depth units first.
A worked example
| Step | Calculation | Result |
|---|---|---|
| The slab | 10 ft × 10 ft at 4 in | — |
| Convert depth | 4 ÷ 12 | 0.333 ft |
| Volume in cubic feet | 10 × 10 × 0.333 | 33.3 cu ft |
| Convert to yards | 33.3 ÷ 27 | 1.23 cubic yards |
| Add 10% waste | 1.23 × 1.10 | 1.36 cubic yards |
| Order | round up | 1.5 cubic yards |
Online calculators aren't infallible, and a yardage figure that's off by a factor of two or three is a real risk if you're not checking it. The habit that protects you: compare against the coverage rule below. A cubic yard covers about 81 square feet at 4 inches. A 10×10 ft slab is 100 square feet, so it must be a little over one yard — not three, and not four. If a result and this quick check disagree by a wide margin, trust the coverage rule and recheck your inputs.
The coverage shortcut
| Thickness | Coverage per cubic yard |
|---|---|
| 3 inches | 108 sq ft |
| 4 inches | 81 sq ft |
| 5 inches | 64.8 sq ft |
| 6 inches | 54 sq ft |
| 8 inches | 40.5 sq ft |
| 12 inches | 27 sq ft |
Exact figures (27 ÷ depth in feet) — the same numbers used on the slab thickness guide.
Divide your area by the coverage figure for your thickness — that gives yardage directly, fast enough to do in your head. 100 sq ft at 4 inches: 100 ÷ 81 ≈ 1.23 yards. Compare against your long-form calculation; if they disagree, one of them is wrong. It's also the fastest way to answer the question people actually ask, which is closer to "how much concrete does my patio need" than "what's the volume of this rectangular prism."
Use the calculator below to run your own dimensions — matching the worked example above by default:
A 10×10 ft slab poured 4 in thick needs about 62 80-lb bags of concrete, or 1.36 cubic yards of ready-mix — that's roughly 5,000 lb of concrete.
Every shape you'll need
Rectangular slab
Round pier, sonotube, or post hole
radius = diameter ÷ 2
Worked: a 12-inch diameter tube 36 inches tall — π × (0.5)² × 3 = 2.36 cubic feet, before any waste allowance. A 16-inch tube of the same height — π × (0.667)² × 3 = 4.19 cubic feet.
Going from a 12-inch tube to a 16-inch tube — a modest-sounding upgrade — very nearly doubles the concrete, because volume scales with the square of the radius. That's worth knowing before deciding to upsize every footing "to be safe." This is the same relationship covered in more depth, with the actual bearing-capacity reasoning behind it, on the sonotube calculator.
The 2.36 and 4.19 cubic-foot figures above are raw volumes, before waste. The sonotube calculator's own reference table shows the same 12×36 tube at 2.59 cubic feet — that's the same number with the standard 10% waste already added in, which is what you'd actually order. Both are correct; they're just answering slightly different questions.
For setting a post rather than pouring a solid pier, a plain cylinder overstates what you need — a post displaces real volume that a solid pour doesn't have to fill. The fence post calculator subtracts the post's actual volume from the hole for a more accurate, and slightly lower, bag count than treating it as a solid cylinder.
Continuous footing or strip
For a perimeter footing, use the centerline length, not the outside length — otherwise you double-count the corners. See the footing calculator for depth and width guidance by soil type.
Steps
Treat each step as a rectangular block and sum them: the bottom step spans the full depth at the first rise, the next step is one tread shallower at twice the rise, and so on — each step shorter front-to-back than the one below it. Add the landing separately.
A flight of steps is a staircase of stacked rectangles, not a solid box and not just the visible faces. Estimating it as a solid box overestimates substantially; estimating it as only the visible faces underestimates. The block-by-block method is tedious and correct — the steps calculator does this automatically, including the option to account for a hollow-core or void-formed design.
Wall
Thickness is usually given in inches — convert to feet the same way as depth. See the poured wall calculator.
Irregular shapes
| Approach | Detail |
|---|---|
| Break it into rectangles | Divide the area into simple shapes, calculate each, and sum them — works for L-shapes, T-shapes, and most patios |
| Subtract voids | Calculate the full shape, then subtract any openings or blocked-out areas |
| Curves | Approximate with a rectangle plus a half-circle, or use the average-width method |
| Average width | For a shape that tapers, measure the width at several points, average them, and multiply by length |
When in doubt, it's always safer to slightly over-simplify in the direction that gives more concrete rather than less — see the waste section below for why.
The waste allowance isn't a flat 10%
| Condition | Allowance |
|---|---|
| Tight forms on a compacted base | 5% |
| Simple rectangular forms | 5% to 10% |
| Most DIY pours | 10% |
| Round or irregular shapes | 10% to 15% |
| Uneven ground | 15% to 20% |
| Hand-dug footings | 10% to 15% |
Waste has three sources: uneven subgrade, spillage during mixing, and over-excavation — together they typically consume 5–10% more material than the theoretical volume, and often more. The biggest of the three is the subgrade, and it's worth understanding why: the finished surface is set by your forms and screed, but the bottom is set by the ground. Every low spot in the base gets filled with concrete you didn't plan for, and unlike spillage, you can't see it happening.
This is also a real argument for grading the base carefully before you order. A well-prepared base needs 5% waste. A rough one needs 20% — and concrete is the most expensive way to fill a hole.
Ordering: round up, and here's why the asymmetry is severe
This isn't like running short on gravel, where you order more and finish tomorrow. If the first concrete starts to set before the rest arrives, the two pours don't bond — you get a cold joint, a plane of weakness through the slab that can't be repaired and will crack along its line. Industry guidance on placement is direct about the consequence: cold joints introduce shear failure planes between layers of concrete.
So the cost of ordering too little is a structurally compromised slab. The cost of ordering too much is some leftover concrete. Those aren't comparable risks — round up.
Ready-mix ordering
| Item | Detail |
|---|---|
| Increments | Typically sold in quarter-yard increments — round up to the next quarter |
| Minimum orders | Commonly cited around 3 to 5 cubic yards, though this varies by supplier — verify locally |
| Short-load fees | Applied below the minimum; delivery fees in general commonly run $75 to $200 — see the cost calculator for the fuller breakdown |
| Standard truck | Commonly 8 to 10 cubic yards |
| What to tell the plant | Strength in psi, aggregate size, and any admixtures — see the mix ratios guide |
| Timing | Concrete has a working life. Have the site, forms, crew and tools ready before the truck arrives — waiting time is billed |
Bagged concrete
Worked: a 10×10 slab at 4 inches is 33.3 cu ft. At 0.60 cu ft per 80-lb bag: 33.3 ÷ 0.60 = 56 bags before waste. With 10% waste: about 62 bags — the same figure the slab calculator gives for this exact size. See the 60 vs 80 lb bag guide for how bag size affects that count.
The crossover
| Volume | Usual answer |
|---|---|
| Under 1 cubic yard | Bagged — no minimum, no delivery, no scheduling |
| About 1 to 1.5–2 cubic yards | The decision zone — sources place the exact crossover differently in this range |
| Over 3 cubic yards | Ready-mix, essentially always |
A continuous pour is a quality advantage, not just a convenience one — mixing dozens of bags by hand almost guarantees some variation between batches. Cost figures vary by market: bagged concrete commonly runs noticeably more per cubic yard than ready-mix once you're above the crossover point, though the gap depends heavily on local pricing — this site's own cost calculator defaults to $150 per cubic yard for ready-mix as a commonly cited starting figure. Get a local quote before assuming any published number applies to you.
Mistakes worth avoiding
- Leaving depth in inches — an answer twelve times too large. The single most common error.
- Forgetting to divide by 27 — an answer in cubic feet mislabeled as yards.
- Not sanity-checking against coverage — even careful calculations benefit from a second, independent check.
- Using outside length on a perimeter footing — double-counts the corners.
- Estimating steps as a solid block — substantially overestimates.
- A flat 10% waste on rough ground — underestimates; uneven subgrade wants 15–20%.
- Ordering tight to avoid surplus — running short creates a cold joint. Not a comparable risk to ordering slightly too much.
- Not being ready when the truck arrives — waiting time is billed by the minute.
Common questions
How do you calculate cubic yards of concrete?
Multiply length by width by depth, all in feet, then divide by 27. The step people get wrong is depth — it's naturally measured in inches, and forgetting to convert to feet first gives an answer twelve times too large.
Why does my concrete estimate look way too high?
Almost always a units error: depth left in inches instead of converted to feet, or forgetting the final division by 27. Sanity-check any yardage against the coverage rule — a cubic yard covers about 81 square feet at 4 inches thick, so a 10×10 ft slab should come out just over one yard, not three or four.
How much concrete does a 12-inch sonotube need?
A 12-inch diameter tube 36 inches tall holds about 2.36 cubic feet before any waste allowance. Volume scales with the square of the radius, so a 16-inch tube of the same height holds nearly double, at about 4.19 cubic feet.
What waste percentage should I use for a concrete pour?
5% for tight forms on a well-compacted base, 10% for most DIY pours, and 15–20% for uneven ground. The subgrade is usually the biggest source of waste — every low spot in the base gets filled with concrete you didn't plan for, and you can't see it happening the way you can see spillage.
What happens if I run short of concrete mid-pour?
If the first concrete starts to set before more arrives, the two placements don't bond — that's a cold joint, a permanent plane of weakness that will crack along its line. Running short is not comparable to running out of a material like gravel, where you just order more and finish tomorrow. Always round up.