Concrete Slab Calculator

From dimensions to a truck order

The volume chain is length ft × width ft × (thickness in ÷ 12) cubic feet, then ÷ 27 for cubic yards. The worked slab: 100 sq ft × 0.3333 ft = 33.33 cu ft = 1.23 cu yd (0.94 m³). Adding 10% brings it to 1.36 cu yd, and quarter-yard ordering rounds that up to 1.5 cu yd — leaving 0.14 cu yd of margin in the truck. These figures come from the engine at build time, not from copy-editing.

The units underneath that chain — why a cubic yard holds exactly 27 cubic feet, how cubic meters relate to both, and how a footprint and a thickness become a volume — are the subject of the guide Cubic feet, cubic yards, and ordering concrete, which also runs the same chain for footings and columns and ends in bag counts. Read it if the conversions are new to you. The rest of this page explains what this calculator does with the numbers you give it, stage by stage, and how to read what comes back.

What the calculator computes, stage by stage

Every estimator on this site follows the five-stage pipeline documented on the methodology page. The slab tool runs all five, and because ready-mix is sold by volume rather than by the bag, its coverage stage is the simplest on the site.

1. Geometry. Length times width gives the footprint in square feet. The thickness is divided by 12 to turn inches into feet, and the footprint times that decimal thickness is the net volume in cubic feet. From that one unrounded cubic-feet figure the engine also reports cubic yards (divided by 27) and cubic meters (multiplied by 0.028316846592, the international foot cubed). In metric mode the meters and centimeters you type are converted to feet and inches first, so the arithmetic downstream is identical in both systems.
2. Coverage. On most tools this stage divides by a published yield — how much one bag or box covers. Ready-mix has no package: the unit of purchase is the cubic yard itself, so the coverage divisor is the definition 27 cu ft per cu yd, and no manufacturer figure enters this page at all. The bag yields belong to the concrete bag calculator.
3. Waste. The engine applies net × (1 + waste ÷ 100) to the cubic-feet volume before anything is rounded. The default of 10% is a trade convention recorded in the site's materials data, not a manufacturer figure, and the form lets you replace it. The result, converted to cubic yards, is the adjusted volume: what the job is expected to consume.
4. Package rounding. The "package" here is the ordering increment. Ready-mix suppliers commonly take orders in 0.25 cu yd steps, so the engine computes ceil(adjusted ÷ 0.25) × 0.25. This is the only rounding in the chain and it always rounds up. The spare is the order minus the adjusted volume — concrete beyond the allowance that the rounding obliges you to buy. When the division lands on a whole number within floating-point noise, the engine treats it as an exact fit rather than adding a phantom quarter yard.
5. User-priced cost. If you type a price per cubic yard, the cost is that price times the ordered quantity — the rounded figure, not the adjusted one, because the rounded figure is what you buy. The site never supplies or averages a price; leave the field blank and the line does not appear.

Why each input matters

Length and width set the footprint, and the volume scales with each of them directly: doubling either doubles the order before rounding. Both go in as decimal feet. This form has no separate inches box, so a tape reading of 22 ft 6 in is entered as 22.5 — the mixed measurement converted to decimal feet, which is the site's convention for every dimension before any arithmetic runs. The feet & inches calculator handles that conversion for awkward readings. If the pour is not a single rectangle, settle the total square footage first with the square footage calculator and enter it as the length with a width of 1.

Thickness is the input to watch. It is small, it is entered in a different unit from the other two, and it comes from your plans rather than from this calculator: the tool sizes the pour for whatever thickness you give it and makes no judgment about whether that thickness suits the load, the soil, or the code. Those questions belong to a licensed engineer or your building department, as the note under the form says. Arithmetically it is a straight multiplier — the volume moves in exact proportion to it — which is why the unit slip described below is so costly.

Waste multiplies the volume, so it moves the adjusted figure proportionally and the order in quarter-yard steps. Small changes often change nothing, because the round-up absorbs them; then a further change tips the order by a quarter yard. That step behaviour is why the examples below show the adjusted volume and the order side by side.

Price per cubic yard touches only the last line. It never feeds back into the quantity, so you can add or change it after the fact without disturbing the order.

The units toggle changes what the fields expect and how the result is presented, not what is computed. In metric mode, meters are divided by 0.3048 and centimeters by 2.54 — both exact definitions — before the engine sees them, and the result panel leads with cubic meters while still reporting the US quarter-yard order.

The figures this page relies on

• Default waste, 10% — a common trade convention recorded in the site's materials data, where it is labelled a convention because no primary source publishes a universal figure; it is the common allowance for spillage and an uneven subgrade. Adjustable in the form.
• Ordering increment, 0.25 cu yd — a trade convention held in the slab engine: suppliers commonly take orders in quarter-yard steps, so it is something to confirm with your supplier rather than a specification. The tool always reports the order in this increment.
• Inches per foot, 12, and cubic feet per cubic yard, 27 — definitions, exact.
• Cubic meters per cubic foot, 0.028316846592 — the international foot of 0.3048 m, cubed; exact. Used to report the m³ line and, in metric mode, the metric order.
• Meters per foot, 0.3048, and centimeters per inch, 2.54 — exact definitions, used only to convert metric entries into the engine's feet and inches; square meters per square foot, 0.09290304, presents the footprint back in m².
• Coverage per bag — none. This page holds no product figure. A slab small enough to mix from bags is counted against the manufacturer's yields on the concrete bag calculator, not here.

Worked examples

Every figure in this table was produced at build time by the same engine that runs the form above. The inputs are chosen to show different behaviours of the pipeline; the metric row was entered as meters and centimeters and converted exactly before the engine ran, which is what the metric toggle does.

The shed pad, with and without an allowance. An 8 × 12 ft pad at 4 in is 96 sq ft and 32 cu ft — 1.19 cu yd net. With the default 10% the adjusted volume is 1.3 cu yd, ordered as 1.5 cu yd with 0.2 cu yd spare. Set the waste field to 0 and the same pad still orders 1.25 cu yd, 0.06 cu yd more than it holds, because the quarter-yard round-up does not go away when the allowance does. On a pour this size the rounding is a large share of the order, which is the scale at which the bag route is worth comparing.

The walkway. A 60 × 3 ft path at 4 in has the same thickness as the patio and a footprint of 180 sq ft. Shape does not matter to the engine, only the product of length and width: the net volume is 60 cu ft, 2.22 cu yd; the allowance takes it to 2.44 cu yd; and the order is 2.5 cu yd with 0.06 cu yd spare. If a path like this will be bought as more than one delivery, run the tool once per delivery, because the quarter-yard rounding applies to each order separately.

The driveway. 20 × 40 ft at 5 in is 800 sq ft; at 5 ÷ 12 = 0.4167 ft thick that is 333.33 cu ft, 12.35 cu yd net, or 9.44 m³. Waste lifts it to 13.58 cu yd and the order is 13.75 cu yd with only 0.17 cu yd of rounding. A large adjusted volume rarely sits far below a quarter-yard boundary, so on a big pour the spare is a small fraction and the allowance, not the rounding, is what governs the order.

The garage floor, measured in feet and inches. The tape says 22 ft 6 in by 24 ft, so the length is entered as 22.5 ft. At 6 in — half a foot — the 540 sq ft floor is 270 cu ft, exactly 10 cu yd. The 10% allowance makes that 11 cu yd, which is itself a whole number of quarter yards, so the order is 11 cu yd with 0 cu yd spare. Binary arithmetic can leave a product like this a hair above the boundary; the engine's exact-fit tolerance is what keeps it from adding a spurious quarter yard.

The priced pour. 16 × 20 ft at 4 in is 106.67 cu ft net, 4.35 cu yd after the 10% allowance, and an order of 4.5 cu yd. With $100.00 per cubic yard typed into the price field, the cost line reads $450.00 — the ordered quantity times the price, nothing else. That price is a placeholder chosen only to show the arithmetic; the site holds no concrete prices, and the cost is computed on the rounded order because the rounded order is what you buy.

The metric slab. In metric mode, 6 × 3.5 m at 12 cm reaches the engine as 19.69 × 11.48 ft at 4.72 in. The footprint is 21 m² (226.04 sq ft) and the net pour 2.52 m³ — 88.99 cu ft, 3.3 cu yd. With 10% the adjusted volume is 2.77 m³, and the quarter-yard convention rounds it to 3.75 cu yd, shown as 2.87 m³. The result panel leads in cubic meters, but the order underneath it is the same one the imperial path would produce from the converted dimensions.

How to read the result

The headline line is the purchase decision: Order N cu yd of ready-mix. Beneath it, Net pour shows the bare geometry in cubic feet, cubic yards, and cubic meters, so a typo in a dimension is visible before it is padded; With N% extra shows the waste-adjusted volume with the percentage that produced it; and Rounded to quarter-yard ordering repeats the order with the spare in brackets — the concrete the rounding added beyond the allowance. Read the net line to check your geometry, the adjusted line to check your allowance, and the order line to place the order. A cost line appears only when a price is entered. In metric mode the same lines lead with cubic meters, converted from the engine's figures with the exact factor above, and the quarter-yard line is labelled as the US ordering figure. The copy button places a short plain-text summary of the result on the clipboard.

Common misreadings

Thickness in feet. Thickness goes in as inches; length and width as feet. Enter the 4 in patio as 4 feet thick and the engine sees 48 in: the same 100 sq ft footprint becomes 400 cu ft, 14.81 cu yd net, and an order of 16.5 cu yd instead of 1.5 — 12 times the correct net volume, the most expensive typo in concrete estimating. If the cubic-yard figure looks wildly high, check the thickness field first. The guide linked above shows the slip in the other direction too.

Metric figures in the imperial fields. The metric slab above is 6 × 3.5 m at 12 cm. Type those same numbers with the toggle still on Imperial and the engine sees a 6 × 3.5 ft slab 12 in thick: 21 cu ft and an order of 1 cu yd, against the correct 3.75. The error is not a simple factor, because two of the three fields shrank and the third grew; if the order looks implausibly small for the slab, check the toggle.

Waste as a fraction. The waste field takes a percentage. The driveway at 10% orders 13.75 cu yd; typed as 0.1 it is read as 0.1%, the adjusted volume falls to 12.36 cu yd, and the order drops to 12.5 — 1.25 cu yd short of the intended allowance, with no warning, because a fraction of a percent is a valid input.

Area instead of dimensions. The two footprint fields want a length and a width, not a square footage. A footprint you already know as an area should go in as the length with a width of 1; typing the area into one field and a dimension into the other describes a different slab entirely.

Padding twice. Rounding a dimension up before entering it, or adding a margin to the order afterwards, repeats work the pipeline already does: the allowance is applied to the volume and the round-up to the allowance, exactly once each. The shed pad above shows what the rounding alone adds; the allowance is meant to be the only other margin.

Where the model stops

The engine models one rectangle of uniform thickness and assumes that shape is what gets built. It does not model thickened edges, a footprint that is anything but a rectangle, or a subgrade that is not flat — those are among the reasons real consumption differs from the geometry, and the waste allowance is the single flat percentage you are given to fold them into. It sizes the concrete and nothing else: subgrade preparation, formwork, reinforcement layout, delivery minimums, and moisture and temperature effects are outside the model, as they are for every estimator on the site. Quarter-yard rounding is a convention, not a rule; a supplier that orders in a different increment changes only the last step, and the volume underneath it does not move. And, as the note under the form says, this tool sizes the pour only — slab thickness for a load, reinforcement, and permits are decisions for a licensed engineer or your building department, and the dimensions you enter should already carry that review. The methodology page lists the simplifications every estimator shares.

Related tools and guides

Small pour, bagged mix instead of a truck? The concrete bag calculator compares every bag size for this volume. Footings under the slab edge are the concrete footing calculator's job, piers and columns the concrete column calculator's, and the steel grid your plan calls for is counted by the rebar calculator. The square footage calculator settles an irregular footprint, the feet & inches calculator turns tape readings into decimal feet, and the construction unit converter moves any figure between cubic yards and cubic meters. For the units and the reasoning behind the chain, see cubic feet, cubic yards, and ordering concrete and how material estimates are calculated, or browse all guides.

Frequently asked questions

Dimensions never leave your browser — the arithmetic runs locally. The full estimation pipeline is documented on the methodology page.