Concrete Calculator — Yards, Bags, Cost, Weight & Volume

How to calculate concrete

Concrete is sold by volume, so every concrete question is a volume question first. For anything rectangular the whole of it is:

Volume = Length × Width × Thickness

The only difficulty is that the three measurements are rarely in the same unit. A slab is measured in feet but poured in inches, and concrete is ordered in cubic yards. So convert the thickness to feet by dividing by 12, multiply out to get cubic feet, then divide by 27 to get cubic yards:

20 ft × 20 ft × (4 in ÷ 12) = 133.333 ft³
133.333 ÷ 27 = 4.938 yd³

Both divisors are exact, not approximations. A foot is exactly twelve inches and a yard is exactly three feet, so a cubic yard is exactly 3 × 3 × 3 = 27 cubic feet.

Why estimators divide by 324

Length in feet × width in feet × thickness in INCHES ÷ 324 gives cubic yards. 324 = 12 × 27. The 12 converts the thickness from inches to feet and the 27 converts cubic feet to cubic yards; doing both at once is the shortcut US estimators use. The engine performs the two steps separately in canonical SI and never uses 324 as a constant — the figure is published so a reader can check the page's arithmetic by hand. It is the two steps above collapsed into one: 12 to get from inches to feet, 27 to get from cubic feet to cubic yards, and 12 × 27 = 324. This calculator does the two steps separately in metres, which is why mixing units on the same job can never double-apply a factor.

Concrete slab calculations

A slab is the simplest case and the most common question. Measure the length and the width of the area you are pouring, decide the thickness, and the arithmetic above gives you the order.

Two things catch people out. The first is that a slab is almost never exactly its nominal thickness: an over-excavated or rutted subgrade quietly swallows concrete, and a slab formed 4 in deep over soft ground can finish closer to 5 in in places. The second is that a slab is not always rectangular. For an L-shape, split it into two rectangles and add a second section — the project total adds them for you. For a genuinely four-sided but non-rectangular slab, use the trapezoid project type; for one that changes thickness from end to end, use the tapered slab, whose volume is the footprint times the average of the two thicknesses.

A 10 ft × 10 ft × (4 in ÷ 12) slab is 33.333 ft³ or 1.235 yd³. Doubling both plan dimensions to 20 × 20 quadruples it to 133.333 ft³ (4.938 yd³) — area scales with the square of the dimensions, which is why a slab twice as wide costs four times as much to pour, not twice.

How many bags of concrete do I need?

A bag's yield is the volume of finished concrete it makes, and the manufacturer prints it on the bag and in the product data sheet. The number of bags is that yield divided into the volume you need, rounded up — never a stored 'bags per yard' figure. Yields are PRODUCT-specific, not bag-weight-specific: QUIKRETE and Sakrete both publish 0.6 ft³ for an 80 lb bag, but for a 90 lb bag QUIKRETE publishes 0.675 ft³ and Sakrete publishes 0.66 ft³. When the number matters, read your own bag.

Bags = required volume ÷ declared bag yield, then rounded up.

Deliberately absent:

• 50 lb: Only one primary document in this dataset publishes a 50 lb concrete-mix yield (QUIKRETE No. 1101, 0.375 ft³). Publishing that as a GENERIC 50 lb figure would generalise one brand's product to the whole category, which this dataset does not do. Use the QUIKRETE product record, or read the yield off your own bag.

Concrete yards and volume conversions

Ready-mix is ordered in cubic yards in the United States and cubic metres almost everywhere else. Both appear in every result on this page, along with cubic feet and litres, and each is converted independently from the same internal figure rather than one from another — so a rounding error cannot cascade through the set.

Concrete cost

The cost of a pour is the quantity times a unit price you supply, plus whatever charges your supplier adds:

Estimated cost = required quantity × your unit price + delivery + short-load + pump + labour + tax

This page publishes no prices. Ready-mix is quoted plant by plant and moves with the region, the mix design, the load size, the season and the haul distance, and a figure printed on a web page is out of date the week after it is written. Bagged concrete is priced by the retailer. Enter the numbers you were quoted and the cost panel will total them; enter nothing and the panel stays empty rather than showing a plausible fiction.

Two charges are worth asking about specifically. A short-load fee applies when you order less than the plant's minimum — often a few cubic yards — and it can cost more than the concrete itself on a small job. Pumping is priced separately again, usually with its own minimum, and is worth it exactly when the truck cannot reach the forms.

Ready-mix or bags?

The practical dividing line is about a cubic yard. Below it, bags win: no minimum charge, no delivery window, and you can stop for lunch. Above it, the arithmetic turns: a cubic yard is 45 eighty-pound bags, roughly 4,050 lb of material to lift, open, mix and place before the first of it starts to set.

Ready-mix also buys consistency. Every bag you mix by hand is its own little experiment in water content, and water content is what governs strength; a truck delivers one mix design, batched to a specification, with a slump you can check on arrival. Against that, a truck imposes a schedule: it arrives when it arrives, the load has to be placed, and waiting time is chargeable.

Concrete weight

Weight is volume × density, and the density used here is the one ACI's own terminology standard attaches to the words 'normalweight concrete': approximately 150 lb/ft³ (2400 kg/m³). ACI's word is 'approximately', and this page keeps it. A delivered mix varies with its aggregate, its air content and its water content, and reinforcement adds weight the concrete volume does not account for. If your supplier gives you a density, use theirs.

Weight = volume × density. At 150 lb/ft³, one cubic yard weighs about 4,050 lb — 2.03 US short tons, or 1,837 kg. It matters for more than curiosity: it decides whether a trailer is legal, whether a slab-on-fill needs its bearing checked, and how many people you need to move a pour by wheelbarrow.

Source: Normalweight concrete has a density of approximately 150 lb/ft³ (2400 kg/m³). — American Concrete Institute, ACI Concrete Terminology (ACI CT-25), read 2026-08-25.

Common project types

Not every pour is a rectangle, and the shortcuts that work for a slab go wrong on a footing or a flight of stairs. This calculator carries 15 project types, each asking for the measurements that shape actually needs:

• Slabs, pads, patios and driveways — the rectangular case, plus trapezoidal and tapered variants.
• Walls — length × height × thickness. Openings are not deducted; enter them as a negative section only if you can measure them accurately.
• Footings — continuous strip footings measured as a total run, and isolated pads counted by quantity.
• Columns and piers — rectangular or round, one at a time or by the dozen.
• Post holes — with or without the post subtracted. Subtracting matters more than people expect.
• Tubes and rings — an annulus, where the inner circle is removed rather than estimated away.
• Curbs and curb-and-gutter — two rectangles sharing one run length, reported separately.
• Stairs — an explicit sum of step prisms, with an optional platform and side walls.

Waste, and the three things people call it

Three different quantities get called 'waste' and this page keeps them apart. The GEOMETRIC VOLUME is what your dimensions enclose — it is arithmetic and it has no opinion. The ESTIMATING ALLOWANCE is a percentage you choose to add for over-excavation, an uneven subgrade, spillage and form deflection; no code specifies it, this page defaults it to 0%, and it is never added on your behalf. SUPPLIER ORDERING CONSTRAINTS are a third thing again: a ready-mix plant sells in fixed increments with a minimum load and a short-load charge, and bagged concrete only comes in whole bags. Rounding up to a whole bag is not waste and neither is a plant minimum.

Keeping them apart is the whole point. The geometric volume is checkable arithmetic you can redo by hand. The allowance is a judgement you made. The supplier's minimum and the whole-bag rounding are constraints someone else imposed. A calculator that adds all three together and shows you one number has hidden two decisions inside a figure that looks like a measurement.

Thickness

The thickness buttons on this page — 2 in, 3 in, 4 in, 5 in, 6 in, 8 in — are there so you do not have to type. They are not recommendations. How thick a slab should be depends on what will stand on it, what is under it, how it is reinforced, how it is jointed and what the ground does when it freezes — and none of that is something a volume calculator can know.

What this page will tell you is a manufacturer's own scope statement: every bagged concrete mix in the dataset states it is designed for pours of 2 in or more, so a section entered thinner than that carries a note. That is the bag's limit, not a structural opinion. For anything carrying a vehicle, a structure or a code-regulated load, the thickness is a design output — ask a designer or your local building department, not a calculator.

Concrete, cement and mix materials

Cement is one ingredient; concrete is the finished material. Concrete is cement plus sand, plus coarse aggregate, plus water — and a bag of 'concrete mix' already contains all of the dry ingredients, so you add only water. This calculator sizes CONCRETE: the volume of finished material your forms will hold, and the number of bags or cubic yards of it you need to buy. It does not do mix design, and it will not tell you how many bags of portland cement, how much sand or how much gravel to combine to make that concrete yourself — those proportions depend on the strength you are designing for and are not a geometry problem.

So if you arrived searching for a "cement calculator", this page is almost certainly the one you want: it sizes the finished concrete. If you genuinely need mix proportions — so many parts cement to sand to aggregate for a target strength — that is mix design, and it depends on the aggregate you have and the strength you are designing for, not on the shape of your forms.

Worked examples

Every figure in these examples is produced by the same engine that runs the calculator above, at build time. None of them is typed in, so none of them can drift.

Formula reference

Frequently asked questions

How this calculator works

1. Normalization. Every dimension you type is converted to metres exactly once, before any geometry runs. Because each field carries its own unit selector, you can enter a slab in feet and its thickness in millimetres and the answer is the same — a conversion factor can never be applied twice.
2. Geometry. Your project type decides the formula. The formula and the substitution with your own numbers in it are both printed under "Show calculation", straight from the engine, so you can check the arithmetic by hand.
3. Conversions. Cubic feet, cubic yards, cubic metres and litres are each derived independently from the internal figure using the exact 1959 definitions, so the four agree to the last decimal and one cannot inherit another's rounding.
4. Allowance. Applied once, to the total, only if you ask for it. The exact geometric volume never disappears from the screen.
5. Bag yield. Read from the manufacturer's published data sheet, divided into your requirement, rounded up. Never a stored bags-per-yard constant.
6. Weight. Volume × density, using the density ACI's terminology standard attaches to "normalweight concrete", or your supplier's figure if you enter one.
7. Cost. Your unit price × the recommended quantity, plus the charges you entered. Nothing else — no stored prices, no regional averages, no estimates.

Nothing is rounded in the middle of that chain. Rounding happens when a figure becomes text, and at one genuinely discrete step: whole bags, which always round up and are guarded so that an order of exactly 45 bags is never quietly turned into 46.

If a value does not check out — a dimension of zero, a post wider than its hole, an inner diameter larger than the outer one — the calculator shows a structured error and no numbers at all. It never silently corrects an input or shows a partial figure.

Sources and methodology

This page separates four kinds of claim and labels every one of them:

• Exact mathematics — true by the definition of the units. Derived in code from the 1959 international yard and pound agreement, never typed as a constant.
• Manufacturer data — a named product's published figure, read from that manufacturer's own technical data sheet, with the URL and the date it was read.
• Typical estimating practice — a convention with no authority behind it. Labelled as such, and never presented as a requirement.
• Engineering and code claims — traceable to a document that actually states them. Nothing here is attributed to ACI or ASTM unless the cited document says it; a test-method standard is cited for methodology and never as the source of a number it does not publish.

The full record, with every claim, value, source, URL and retrieval date, is published as a provenance report generated from the dataset itself.

Technical review: the dataset was checked against the primary documents by Ugo Candido on August 25, 2026. That is a source-transcription check by the page's own author, not an independent third-party engineering review, and this page does not claim one. The arithmetic is held in place by an independent oracle test suite that re-derives every published figure from first principles without importing the production geometry.

Concrete engine v1.0.0 · dataset 1.0.0 · share format v1. Last reviewed August 25, 2026.

Related calculators

• Volume Calculator — the volume of any solid — cylinders, cones, tanks and pipes — when the question is geometry rather than a concrete order.
• Area Calculator — the plan area of an odd-shaped pour before you multiply by a thickness.
• Gravel Calculator — the sub-base under the slab, by tons and cubic yards.
• Unit Conversion Calculator — any other unit conversion, on the same exact factors this page uses.