Driveway Slab Concrete Calculator

Purpose and model boundary

This calculator estimates the concrete for a slab you draw: a driveway, a patio, a shed base or a garage floor. You draw the slab's outline on a grid and block out up to five areas inside it that take no concrete, such as a column or a drain. The spreadsheet measures the net slab area from the drawing and returns the concrete to order with its waste allowance, whether ready-mix is recommended, the ready-mix trucks and the premixed bags that volume takes, and an estimate of the length of control joints at the spacing you enter. It is not a structural design or a mix design, and it does not price anything. Rebar, mesh, a base course, formwork, thickened edges, footings and delivery charges are not estimated.

Inputs and units

The drawing is the first input. Its outline is sent as an ordered list of up to 100 corner points and each block-out as an ordered list of up to 20 points, exactly as on the Area Calculator, which measures the same drawing. The US/Metric switch on the drawing chooses the unit the coordinates are written in, feet for US and metres for Metric, and the unit of every result.

The inputs beside the drawing are:

• Slab thickness: in inches (US) or centimetres (Metric), the same over the whole slab.
• Concrete waste: extra concrete for spillage, an uneven subgrade and form movement, as a percentage of the slab volume.
• Bag size: the premixed bag the bag equivalent is counted in, 60 lb or 80 lb. The same two bags are offered in Metric.
• Control joint spacing: the distance between control joints, in feet (US) or metres (Metric).

Slab thickness and control joint spacing have one input for each system, each with its own default: 4 in or 10 cm of slab, and joints every 10 ft or every 10 m. The defaults are not conversions of each other: 4 in is 10.16 cm, not 10, and the Metric spacing is worked example 11.7.6.3's 10 m, at which a slab under 10 m each way gets no joint at all, so type your own spacing. Switching between US and Metric shows the other system's inputs with their own values and never converts a number you typed; the value you typed for the system you left is kept for when you switch back, and it is not sent while it is hidden. The spreadsheet reads only the inputs of the selected system. Concrete waste (default 10%) and bag size (default 80 lb) are the same in both systems.

The defaults are the figures the requirement's worked examples use, not values from a code or a standard. Use the thickness on your plans, your supplier's waste allowance and bag, and the joint spacing your designer or local practice calls for.

Governing relationships

The net slab area is the area of the outline less the area of each block-out, each measured by the shoelace formula exactly as on the Area Calculator:

Net slab area = Gross area − (Exclusion 1 area + … + Exclusion 5 area)

With the net slab area in square feet (US) or square metres (Metric):

Slab volume before waste (US, cu yd) = net slab area × slab thickness in inches ÷ 12 ÷ 27

Slab volume before waste (Metric, m³) = net slab area × slab thickness in centimetres ÷ 100

Concrete to order = slab volume before waste × (1 + concrete waste)

Bag equivalent = concrete to order in cubic feet ÷ the bag's yield, rounded up to a whole bag, where a 60 lb bag yields 0.45 cu ft and an 80 lb bag 0.60 cu ft

Ready-mix trucks = concrete to order ÷ the truck capacity, rounded up to a whole truck, where a truck carries 10 cu yd

Ready-mix recommended = Yes when ROUND(concrete to order, 6) > ROUND(the ready-mix threshold, 6), otherwise No, where the threshold is 1 cu yd

The control joints are straight lines across the outline's bounding rectangle, the smallest rectangle that holds every point of the outline: W is its width, from the leftmost point to the rightmost, and H its depth, from the lowest point to the highest. The joints sit at every multiple of the spacing from the rectangle's lower-left corner, strictly inside it, and each one runs the rectangle's full depth or width:

Joints across the width = W ÷ control joint spacing, rounded up to a whole number, minus 1, each H long

Joints across the depth = H ÷ control joint spacing, rounded up to a whole number, minus 1, each W long

Control joint length = joints across the width × H + joints across the depth × W

A joint that would fall on the rectangle's far edge is an edge, not a joint, which is why the count is one less than the spacings rounded up.

The constants are exact: 12 inches in a foot, 27 cubic feet in a cubic yard (a yard is 3 feet), and 100 centimetres in a metre. In Metric the concrete to order is turned into cubic feet for the bag count by the exact cubic foot, 0.028316846592 m³ (0.3048 m cubed), so the same bag yields the same volume in either system. The requirement gives the ready-mix threshold and the truck capacity only in cubic yards, so in Metric they are the US 1 cu yd and 10 cu yd converted exactly: 0.764554857984 m³ (about 0.7646 m³) and 7.64554857984 m³ (about 7.6455 m³), not rounder metric figures. Waste applies once, before any rounding up, and the trucks, the bag equivalent and the ready-mix recommendation are all counted from the concrete to order with its waste included.

Calculation sequence

1. The spreadsheet measures the gross area of the outline and the area of each block-out, and subtracts the block-outs to give the net slab area.
2. It reads the slab thickness and control joint spacing of the selected system, the concrete waste and the bag size.
3. Slab volume before waste: the net slab area times the thickness, in cubic yards (US) or cubic metres (Metric).
4. Concrete to order: the slab volume times one plus the waste. It is not rounded.
5. Bag equivalent: the concrete to order in cubic feet over the chosen bag's yield, rounded to six decimals and then up to a whole bag. The six-decimal step keeps a figure that is whole on paper whole in the spreadsheet: worked example 11.7.6.4's 300 sq ft slab, 6 in thick with 10% waste, is exactly 165 cu ft, 275 bags of 80 lb, but the spreadsheet's arithmetic measures that slab as 300.0000000000005 sq ft, and without the step it would order 276.
6. Ready-mix recommended: the concrete to order and the threshold, each rounded to six decimals, are compared: Yes when the concrete is above the threshold, No at or below it. The same strict "above" holds in both systems.
7. Ready-mix trucks: the concrete to order over the truck capacity, rounded to six decimals and then up to a whole truck.
8. Control joint length: the width and depth of the outline's bounding rectangle, each over the spacing, rounded to six decimals and then up, less one, give the joints each way; each is multiplied by the rectangle's other side and the two are added.
9. The status line is chosen, as described below. When it begins NOT VALID:, every computed quantity reads zero and Ready-mix recommended reads NOT VALID.

For the drawing the page opens with, worked example 11.7.6.1, a 20 by 15 ft slab of 300 sq ft with the US defaults: 300 × 4 ÷ 12 ÷ 27 = 3.70 cu yd before waste, and 4.07 cu yd to order with 10%. That is above 1 cu yd, so ready-mix is recommended, and it takes 1 truck; as bags it is 110 cu ft, 183.3 bags of 80 lb, so 184. At 10 ft spacing the rectangle has one joint across its 20 ft width, 15 ft long, and one across its 15 ft depth, 20 ft long: 35 ft of control joints. In Metric, worked example 11.7.6.3's 25.44 m² slab, 10 cm thick with 10% waste, is 2.80 m³ to order, ready-mix recommended, 1 truck or 165 bags of 80 lb, and at its 10 m spacing no control joint. Worked example 11.7.6.5 draws the 20 by 40 ft driveway of the requirement's plan LI on its own, where that plan places it: 5 in thick, 13.58 cu yd, 2 trucks or 612 bags of 80 lb, and 100 ft of control joints.

Outputs and interpretation

Concrete to order, Ready-mix trucks and Control joint length are the three headline results. Concrete to order is in cubic yards (US) or cubic metres (Metric) to two decimals, and Slab volume before waste is the same volume without the waste allowance. Ready-mix recommended reads Yes when the concrete to order is above the threshold and No at or below it, when mixing from bags is the usual choice.

Ready-mix trucks and Bag equivalent are both always given, so the two ways of buying can be compared. Ready-mix trucks is the truckloads if you order ready-mix, whatever the recommendation: a small pad that reads No still shows 1 truck, and there the bag equivalent is the order. Bag equivalent is the number of premixed bags of the chosen size that make the concrete to order. Both are whole numbers, counted from the unrounded volume, so they can differ from a count made from the two-decimal figure on the page.

Control joint length is in feet (US) or metres (Metric), to two decimals. It is an estimate measured across the outline's bounding rectangle, so it may overstate the joints to cut: see the status logic and the limitations below. Net slab area is the area every volume is computed from, and Gross area and Exclusion 1 area to Exclusion 5 area show how the drawing arrives at it, 0 for a block-out slot you did not use; they are in square feet (US) or square metres (Metric).

A result belongs to the drawing and the inputs it was calculated for. When you change either after pressing Calculate, the result is kept but marked stale until you press Calculate again; changing the value back makes it current again. Switching between US and Metric changes what is sent, so it marks the result stale too.

Validation and status logic

The drawing tool checks every edit to the drawing before it is kept, with the Area Calculator's rules: at least three points per shape, no two consecutive points in the same place, no wall crossing another wall of its shape, every block-out inside the outline, no two block-outs overlapping, and at most five block-outs. A refused edit leaves the drawing as it was, and an imported drawing is refused as a whole if any rule fails. The drawing tool always writes a shape's points into consecutive rows, with both coordinates of every point; the spreadsheet refuses a request that leaves a blank row between them, or gives a point of the outline or of a block-out only one coordinate.

The status line is the spreadsheet's own verdict, the first of these that applies:

• NOT VALID: choose US or Metric
• NOT VALID: draw at least three outline points
• NOT VALID: every outline point needs both coordinates
• NOT VALID: every point of exclusion 1 needs both coordinates
• NOT VALID: every point of exclusion 2 needs both coordinates
• NOT VALID: every point of exclusion 3 needs both coordinates
• NOT VALID: every point of exclusion 4 needs both coordinates
• NOT VALID: every point of exclusion 5 needs both coordinates
• NOT VALID: each exclusion needs zero or at least three complete points
• NOT VALID: outline and exclusion points must fill consecutive rows, with no blank row between them
• NOT VALID: the slab outline encloses no area
• NOT VALID: the exclusions leave no slab area
• NOT VALID: slab thickness must be greater than zero
• NOT VALID: enter a concrete waste between 0% and 100%
• NOT VALID: choose a bag size of 60 lb or 80 lb
• NOT VALID: control joint spacing must be greater than zero
• CHECK: the slab is not a plain rectangle or has block-outs, so the control joint length, measured across its bounding rectangle, may be longer than the joints to cut
• OK

The checks on slab thickness and control joint spacing read the selected system's input only. The CHECK: line applies when the control joint length is more than zero and either the slab's gross area is smaller than its bounding rectangle, so the outline is not a plain rectangle, or any block-out is drawn. Worked examples 11.7.6.2, an L-shaped slab around a column, and 11.7.6.4, a slab with two angled corners, read CHECK:; the default rectangle, the Metric slab of 11.7.6.3 (which has block-outs but, at 10 m spacing, no joint), the driveway and the small pad read OK. A CHECK: verdict comes with the figures; a NOT VALID: verdict is shown in place of them.

Assumptions and limitations

• The control joint length is measured across the outline's bounding rectangle from its lower-left corner, not clipped to the slab. On an L-shaped or angled slab part of that length falls where there is no concrete, and block-outs do not shorten it, so it may overstate the joints to cut: on worked example 11.7.6.2's L-shaped slab, 8 ft of its 35 ft runs across the missing corner. The joints are not moved to re-entrant corners or columns, and isolation joints around columns and along walls are not included.
• The joint spacing is the one you enter. The calculator does not check it against the slab's thickness or size; it is not a jointing design.
• The slab is one thickness throughout. Thickened edges, footings, rebar, mesh and a base course are not modelled.
• The slab is measured as a flat plan area; a sloping driveway is measured as its footprint, so it needs slightly more concrete than the figures say.
• Walls are straight lines between points; a curved edge is drawn as short straight segments, which cut slightly inside the curve.
• The bag yields, 0.45 cu ft for a 60 lb bag and 0.60 cu ft for an 80 lb bag, are the requirement's figures. A bag that yields differently changes the count in proportion; check the yield printed on your bag. Kilogram bags are not offered.
• The ready-mix threshold of 1 cu yd and the truck capacity of 10 cu yd are the requirement's figures, converted exactly for Metric. A supplier's trucks may carry less, and short-load charges are not modelled.
• The spreadsheet trusts the drawing tool to keep block-outs inside the outline and apart from each other and does not check either itself, so this calculator is offered on its web page only, where the drawing tool enforces them.
• Drawing needs a mouse, a pen or a touch screen; a drawing prepared elsewhere can be loaded with Import.

Restrictions and non-computing states

This calculator accepts a slab thickness and a control joint spacing of zero or more, a concrete waste from 0% to 100%, and a bag size of 60 lb or 80 lb; a thickness or spacing of zero is refused by the status line. An input of the system that is not selected is not read. The drawing tool refuses any edit that would leave a shape outside its limits. Nothing is sent until you press Calculate, and pressing it again on an unchanged drawing with unchanged inputs sends nothing.

Errors and warnings

A blank required input keeps Calculate from sending, and pressing it names the input and moves to it. If the calculation service refuses an input, the status line names it and the input is marked until you change it. If the service does not answer, the page says so, keeps your drawing, your inputs and the last result, and lets you press Calculate again. A CHECK: status means the figures are returned with a reason to look again; a NOT VALID: status means they cannot be computed and none are shown.

Sources and further reading

• National Institute of Standards and Technology, Guide for the Use of the International System of Units (SI), NIST Special Publication 811, Appendix B.8. nist.gov. The exact inch (2.54 cm) and foot (0.3048 m) behind the drawing's two systems, from which 12 inches make a foot, a cubic foot is 0.3048 m cubed and a cubic yard, 27 cubic feet, is about 0.7646 m³.
• Bureau International des Poids et Mesures, The International System of Units (SI), 9th edition (2019), and its table of SI prefixes. bipm.org. The prefix behind the Metric thickness: a centimetre is a hundredth of a metre.
• Eric W. Weisstein, "Polygon Area," MathWorld, Wolfram Research. mathworld.wolfram.com/PolygonArea.html. The shoelace formula for the area of a polygon from the coordinates of its corners, with the condition it needs, a flat polygon whose sides do not cross, which the spreadsheet uses to measure the slab and each block-out, as on the Area Calculator.
• The QUIKRETE Companies, Concrete Mix, Product No. 1101, data sheet (revised October 2022). quikrete.com. One manufacturer's yields for premixed concrete: approximately 0.45 cu ft (12.7 L) from a 60 lb bag and 0.60 cu ft (17 L) from an 80 lb bag, the two yields the bag equivalent is counted at. The same sheet tells its readers to cut control joints every 6 to 8 ft (1.8 to 2.4 m) in a slab poured from that mix, closer than the 10 ft this page starts with.

The ready-mix threshold of 1 cu yd, the truck capacity of 10 cu yd, the bag yields and the default thickness, waste, bag and joint spacing are the figures of the requirement this calculator was built to, not values from these sources or from any code or standard. These sources describe the units, the measurement and one product; they do not certify a particular slab or its result. Use the thickness and joint spacing on your plans, the yield printed on your bag and your supplier's truck size, and check the figures before ordering.