Concrete Cubic Yards Calculator
Every concrete pour fails the same way: a contractor measures in inches, divides by the wrong number, forgets the waste factor, and runs out of mud three feet before the form ends. Then it's a cold joint, a re-pour, or an emergency short-load that costs more than the slab itself. This is how to calculate cubic yards the way ready-mix plants expect you to — and the calculator that does it for you in seconds.
The core formula
Concrete volume math is the same whether you're pouring a driveway, a footing, or a deck pier. Three dimensions, one division:
The divisor is 27 because one cubic yard is a 3 ft × 3 ft × 3 ft cube — 27 cubic feet. Get the three dimensions in feet (not inches, not a mix), multiply, divide. That's the entire arithmetic. The rest of this page is about the parts most DIY guides skip: waste factor, short-load fees, columns, and when to switch from bags to a ready-mix truck.
Step-by-step calculation
Measure everything in feet
Length and width are usually already in feet. The thickness is the trap — it's almost always given in inches. Divide by 12 to convert: a 4-inch slab is 0.333 ft, a 5-inch garage slab is 0.417 ft, a 6-inch driveway is 0.5 ft. Plugging "4" directly into the formula instead of "0.333" gives a result 12× too large — and if you forget to divide by 27 on top of that, you're 1,728× off. That math error is the single most common DIY concrete failure.
Multiply length × width × thickness
Take the example we'll carry through this page: a 20 ft × 30 ft slab at 4 inches thick. In feet: 20 × 30 × 0.333 = 200 cubic feet. That's the raw volume of the void you have to fill.
Divide by 27 to get cubic yards
200 ÷ 27 = 7.41 cubic yards. This is what the geometry says you need. It is not what you order — the next step covers why.
Add the waste factor
Concrete spills, formwork bulges, the subgrade is never perfectly flat, and the finisher always leaves some in the truck. Industry rule of thumb: 5% for slabs with rigid forms, 10% for footings and rough subgrade. For our slab: 7.41 × 1.05 = 7.78 cubic yards. Most ready-mix plants sell in 0.25 yard increments, but cutting it that close on a 7-yard pour is asking for a cold joint. Order 8 yards.
Convert to bags if the pour is small
Below roughly 1 cubic yard, calling a ready-mix truck stops making sense — you'll pay the short-load fee, and the truck will leave with most of its load. For small pours, use pre-mix bags:
- 1 cubic yard ≈ 45 × 80 lb bags
- 1 cubic yard ≈ 60 × 60 lb bags
- 1 cubic yard ≈ 90 × 40 lb bags
Above 1 cubic yard, bagged concrete becomes more expensive per yard and more labor-intensive than a delivery — the labor of mixing 45 bags by hand wipes out any savings.
Footings
A footing is a long, narrow slab — same formula, different numbers. A 30 ft continuous footing, 18 inches wide and 12 inches deep: 30 × 1.5 × 1 = 45 cubic feet = 1.67 cubic yards. Add 10% waste because the trench is dug, not formed: 1.67 × 1.10 = 1.84 cubic yards. For stepped footings on sloped lots, calculate each step separately and sum the totals — trying to average the depth almost always under-orders by a quarter yard.
Per IRC R403.1, residential footings must be a minimum of 12 inches wide for one-story dwellings on average soil and extend at least 12 inches below undisturbed ground or below the frost line, whichever is deeper. Local amendments may push those minimums higher — always check with your building department before ordering.
Columns and piers
Round columns and deck piers don't use length × width — they use the cylinder formula:
For a 10-inch diameter column 8 feet tall: radius is 5 inches, or 0.417 ft. π × 0.417² × 8 = 4.37 cubic feet = 0.162 cubic yards. Add 10% waste: 0.178 cubic yards. That's well under the threshold where bags beat ready-mix — one tube form filled with pre-mix is the standard play for deck footings and small piers.
For a row of identical piers, calculate one and multiply. For mismatched piers (deck posts on a sloped lot, for example), calculate each individually — the tallest one will set whether you can still do the whole job with bags or need to call a truck.
Waste factor: 5% vs 10%
This is the number DIYers either forget or guess at. The rules:
- 5% — flat slabs with rigid formwork on a compacted, level subgrade. Garage slabs, basement floors, finished patios. Spillage and form bulge are the main losses.
- 10% — footings, stepped footings, slabs on grade where the subgrade is rough or freshly excavated. Dirt absorbs some volume during the pour, and the bottom of a dug trench is never perfectly flat.
- 15% — pump-truck pours of any kind. Anything that goes through a hose loses concrete to the line itself (typically 0.5–1 cubic yard stuck inside the boom at the end of the pour).
The reason waste factor matters more than it sounds: ready-mix plants charge a short-load fee — usually $100 to $250 — for any delivery under the truck's minimum (typically 3 to 5 cubic yards). If your geometry says 4.9 yards and you skip the waste factor, you'll be ordering an additional 0.5 yards mid-pour and paying the short-load surcharge on the second delivery. The extra $30 of concrete from a proper 10% waste factor is the cheapest insurance on the entire job.
Bags vs ready-mix: where the break-even is
The decision isn't about cubic yards alone — it's about cost per yard plus labor:
- Under 0.25 cubic yards — bags every time. A single deck pier, a small pad for an HVAC unit, repair work.
- 0.25 to 1 cubic yard — bags usually win on cost, but only if you have a mixer and a helper. Hand-mixing 30+ bags is brutal work; if you don't already own a tow-behind mixer, the rental fee tips the math toward ready-mix.
- 1 cubic yard and up — ready-mix. The per-yard cost is roughly one-third of bagged, the pour is one continuous operation (no cold joints between batches), and you finish the entire job in the same window the concrete stays workable.
One subtle catch: most ready-mix plants have a delivery minimum (often 1 yard, sometimes 2–3 in rural markets) and a short-load fee that kicks in below their economical truck load (usually 3–5 yards). Get a written quote with the minimum, the short-load threshold, and the overtime fee (if the pour runs past the truck's allotted unloading time) before you commit to a date.
Common DIY mistakes
- Plugging inches into the cubic-yard formula. Thickness must be in feet. A 4-inch slab is 0.333 ft, not 4. This single mistake is the source of more re-pours than every other math error combined.
- Forgetting the waste factor. The geometric volume is always less than what you need. Always.
- Rounding down. 7.41 yards is not 7. Round up — the alternative is an emergency short-load mid-pour, which costs more than the extra concrete by an order of magnitude.
- Ignoring the short-load minimum. If the plant's economical load is 3 yards and your pour is 2.5, you're paying for 3 either way. Adjust your order accordingly — an extra patio pad on the day of the pour is a much better use of that concrete than the dumpster.
- Skipping the trench or form check before the truck arrives. A trench dug 14 inches wide instead of 12 inches is 16% more volume — that's the difference between a 5-yard order and a 6-yard order. Measure the actual void, not the plan dimensions, on pour day.
Frequently Asked Questions
What waste factor should I use for a concrete order?
Use 5% for slabs with rigid forms on a flat, compacted subgrade. Use 10% for footings in dug trenches, stepped footings, or any pour where the subgrade is uneven. Never round down — running short forces a cold joint or a second emergency delivery, both of which cost more than the few extra dollars of waste.
Why do ready-mix plants charge a short-load fee?
Ready-mix trucks have a minimum economical load — usually 3 to 5 cubic yards. Anything below that, the plant charges a short-load fee of roughly $100 to $250 to cover the truck's time and the cost of mixing a partial batch. If your pour is borderline, it is often cheaper to order the minimum load and waste the extra than to pay the short-load surcharge.
How do I calculate concrete for a round column or pier?
Use the cylinder formula: Volume = π × radius² × height ÷ 27. For 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 for 0.178 cubic yards. For piers under 0.25 cubic yards, use a tube form with pre-mix bags rather than ordering ready-mix.
How many bags of pre-mix concrete equal one cubic yard?
Approximately 45 × 80 lb bags, 60 × 60 lb bags, or 90 × 40 lb bags equal one cubic yard. At typical retail bag pricing, that runs about 3 to 4 times the cost of ready-mix delivery — so bags are practical for pours under 0.5 cubic yards (deck piers, small pads), but above 1 cubic yard, call a ready-mix truck.
What are the most common DIY mistakes when ordering concrete?
The four most common mistakes are: plugging inches directly into the cubic-yard formula (results 1,728× too large), forgetting the waste factor entirely, rounding down instead of up, and ignoring the short-load minimum at the plant. Any one of those will leave you short on the day of the pour — and a half-finished slab cures into a cold joint that has to be ground out and re-poured.
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