Concrete Calculator
Enter your dimensions and get the volume in cubic yards and cubic metres, the number of bags, and the answer other calculators skip: whether this pour should be bags or a ready-mix truck.
Bags or a truck? This is the crossover zone. 62 bags of 80 lb means real mixing labour — rent a mixer, or price a truck and ask what the short-load fee adds. Either can win here; get the fee before you decide.
📌 Geometry says 1.23 yd³ — the figure above adds your 10% allowance. Real pours run over: an uneven subgrade, a slab that dips below its nominal thickness, spillage and mix left in the drum all take concrete the geometry never saw.
Estimate for ordering, not a structural specification · thickness, reinforcement and footing depth follow your local building code · nothing you enter leaves this browser.
Spotted a mistake or have an idea? — errors get fixed and credited publicly.
Last updated: September 13, 2026 · Formulas verified by automated tests that block every release. Editorial review dates are shown in the sources block above.
How much concrete do I need?
Multiply length × width × thickness, then divide by 27 to get cubic yards (or work in metres for cubic metres). A 10 × 10 ft slab at 4 inches is 33.3 cubic feet — 1.23 cubic yards, or about 56 bags of 80 lb mix, 62 with the 10% waste allowance professionals add. Each 80 lb bag yields 0.60 cubic feet, so a full cubic yard takes 45 of them.
Bags or a truck: the question every other calculator leaves you holding
Getting the yardage is the easy half. The decision that costs money comes next, and it turns on a number most people meet only after they phone a supplier.
Ready-mix trucks carry 9–10 cubic yards, and suppliers set a full-load minimum around 8–10. Order less and you pay a short-load fee — commonly $40–$60 per cubic yard, or a flat charge in the $50–$150 range. Suppliers rarely publish it; you get it on the phone. On a one-yard pour that surcharge can be a third of the bill, which is why small jobs so often go back to bags.
So the practical crossover sits around 1 to 1.5 cubic yards. Below half a yard, bags win almost always. Above one and a half, a truck wins on both cost and labour even after the fee — and remember what the alternative means physically: a single cubic yard is forty-five 80 lb bags, each one lifted, opened, mixed and poured before the first batch starts setting. Between those marks, make the call with the actual fee in hand.
Why the pour always eats more than the geometry
Ask any contractor and you will hear the same instruction: never order the exact number. The calculation describes a perfect box; the site does not. The subgrade dips where someone over-excavated, a slab nominally four inches thick runs four and a half across the middle, some mix stays in the drum, and some ends up on the ground. Reports of a nineteen-yard estimate taking twenty-one and a half are ordinary, not remarkable.
Five percent covers a flat, well-prepared base; ten percent is the honest default, and what the slider above starts at. The asymmetry is the whole argument: leftover concrete is an annoyance you deal with in an afternoon, while running short mid-pour leaves a cold joint — a permanent seam in the finished slab where the first batch had already started curing. You cannot sand that out.
Thickness, base and the dig depth people forget
Typical practice in the US, offered as a starting point rather than a rule: four inches for walkways, patios and shed pads; five for a car driveway; six or more for trucks, RVs, garage floors and equipment pads. Each extra inch buys a substantial gain in load capacity for a modest amount of concrete, which is why so few people regret going from four to five.
Under it goes compacted gravel — roughly four inches for light use, six under vehicles, deeper where drainage is poor. It does not change the concrete volume, but it does change your excavation: dig the slab thickness plus the base, a step that is routinely forgotten until the forms are already set. An inadequate or poorly compacted base is the most common reason a properly mixed slab cracks a couple of winters later.
What none of this can tell you is what your jurisdiction requires. Slab thickness, reinforcement, and especially footing depth are governed by local code and by the frost line where you live — the US residential code requires exterior footings below that line, and that depth ranges from nothing in the deep South to five feet or more in northern states. For anything structural, the building department is the source, not a calculator.
Cubic metres, mix grades and 50 kg cement bags
Outside North America concrete is ordered by volume in cubic metres and specified by grade, with the mix given as parts by volume. Switch the units above and the calculator works that way instead: M15 at 1:2:4 for non-structural work, M20 at 1:1.5:3 for ordinary residential slabs and beams, M25 at 1:1:2 where loads are higher.
The arithmetic behind it: wet volume is multiplied by 1.54 to get dry volume, since sand and aggregate lose bulk once mixed with water, then split along the ratio. For M20 that lands at roughly 8 bags of 50 kg cement per cubic metre, with about 0.42 m³ of sand and 0.84 m³ of aggregate. M15 needs around 6.3 bags, M25 around 11. One honest caveat: nominal ratios are a site convention for ordinary work — M25 and above are meant to be design-mixed to the governing standard rather than proportioned by a fixed recipe.
Working out the area first? The square footage calculator handles odd-shaped rooms and patios, and its result feeds straight into the length and width above.
Sources, assumptions and limitations
- Formula: volume = length × width × thickness (÷ 27 for cubic yards) · bags = volume ÷ bag yield · metric mix: dry volume = wet × 1.54, split by grade ratio
- Assumptions:
- Bag yields are the manufacturer figures: 0.60 ft³ per 80 lb bag, 0.45 per 60 lb, 0.375 per 50 lb, 0.30 per 40 lb — labelled approximate by the manufacturer itself.
- Waste allowance defaults to 10%; 5% suits a flat, well-prepared subgrade.
- Weight is estimated at about 150 lb per cubic foot (2,400 kg/m³) for planning access and delivery.
- Metric mode uses nominal mix ratios and the standard 1.54 dry-volume factor with a 50 kg cement bag at 0.0347 m³.
- Short-load fees and ready-mix prices vary by supplier and region — confirm locally before choosing bags or a truck.
- An estimate for ordering material, not a structural specification: thickness, reinforcement and footing depth follow local building code.
- Nothing you enter is stored or transmitted.
- Sources: QUIKRETE Concrete Mix No. 1101 — data sheet (bag yields, 4,000 psi at 28 days) · QUIKRETE — concrete calculator and project guidance
- Last reviewed: September 14, 2026 by the CalcNotebook team
Frequently asked questions
How many bags of concrete do I need for a 10x10 slab?
A 10 × 10 ft slab poured 4 inches thick is 33.3 cubic feet, or 1.23 cubic yards — about 56 bags of 80 lb mix, or 62 once you add the 10% waste allowance professionals use. At 5 inches thick it rises to 1.54 cubic yards. This is the size where bags stop being fun: 60-odd bags is a full day of mixing, and renting a mixer or pricing a truck is worth the phone call.
How many 80 lb bags are in a cubic yard?
Forty-five. Each 80 lb bag of standard concrete mix yields 0.60 cubic feet once mixed, and a cubic yard is 27 cubic feet. For other sizes: 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb. Those yields come from the manufacturer data sheet and are deliberately labelled approximate — they assume none is spilled and the subgrade is level.
Should I buy bags or order a ready-mix truck?
Under about half a cubic yard, bags win — a truck would bring a short-load fee for a fraction of a load. Above roughly 1.5 cubic yards, ready-mix usually wins on both money and labour even after that fee. Between the two you are in the crossover zone: get the fee from a local supplier before deciding, because it is the number that flips the answer. The calculator above tells you which zone you are in.
What is a short-load fee?
A surcharge for ordering less than a full truck. Suppliers typically run 8–10 cubic yards per load and add roughly $40–$60 per cubic yard, or a flat $50–$150, when you order below their minimum. Most do not publish it — you get it by phone. It matters more than people expect: on a one-yard order it can be a third of the bill, which is exactly why small pours often go to bags instead.
How much extra concrete should I order?
Five to ten percent more than the geometry says, and ten if the subgrade is uneven or the excavation is rough. Every experienced contractor orders over, because running short mid-pour means a cold joint in the finished slab — a permanent weak line you cannot sand out. Leftover concrete is a nuisance; a short pour is a defect.
How thick should my slab be?
Common practice: 4 inches for walkways, patios and shed pads; 5 inches for a car driveway; 6 inches or more for trucks, RVs, garage floors and equipment pads. These are typical figures rather than rules — slab thickness, reinforcement and footing depth are set by your local building code and the actual soil and load. Check with your building department before a pour that has to carry anything heavy.
How do I calculate concrete in cubic meters and cement bags?
Switch the units above to metric. Volume in cubic metres is length × width × thickness, then the nominal mix grade divides it into cement, sand and aggregate using the standard 1.54 dry-volume factor. For M20 (1:1.5:3) that works out to roughly 8 bags of 50 kg cement per cubic metre, plus about 0.42 m³ sand and 0.84 m³ aggregate. M15 needs around 6.3 bags, M25 around 11 — though M25 and above should really be design-mixed rather than built from a fixed ratio.
Do I need gravel under the slab?
Usually yes — a compacted base of 4 inches under patios and walkways, 6 inches under driveways, more where drainage is poor. It does not change how much concrete you need, but it changes how deep you dig: excavate the slab thickness plus the base. Skipping or poorly compacting the base is the most common reason a good pour cracks two winters later.
Do you store what I enter?
No. Dimensions and results exist only in this browser tab. No account, no upload, no tracking of your project.