Calculate how much concrete a slab, footing, column or circular pad needs — in cubic yards, and in the number of bags that works out to.
These are the calculator's default inputs, so you can press Calculate and get the same numbers back. A 12 ft by 10 ft patio slab, 4 inches thick, 10 percent allowance. Convert the depth: 4/12 = 0.333 ft. Then 12 x 10 x (4/12) = 40.00 cubic feet (keep the third as a fraction; 0.333 rounded gives 39.96), which is the base volume the tool prints in its note line. Add 10 percent and you have 44.0 cubic feet, or 44/27 = 1.63 cubic yards, or 1.25 cubic metres. For ready-mix you would order 1.75 yards, because plants sell in quarter-yard steps. At 140 to 150 lb per cubic foot that slab weighs 6,160 to 6,600 lb, which the tool shows as 3.08 to 3.30 US tons. Bagging it instead: 44.0 / 0.60 = 73.3, rounded up to 74 bags of 80 lb mix, about 5,920 lb of dry mix to carry. The allowance is applied to both figures.
Volume = length x width x depth, every dimension in the same unit. For a column or circular pad, volume = pi x radius squared x depth. Divide cubic feet by 27 for cubic yards, or by 35.3147 for cubic metres. The calculator applies your waste allowance to that volume first, then divides the result by the bag yield and rounds up, so the bag count already carries the allowance. Bag counts come from published yields; the weight row comes from a density range, which is why it prints as a range rather than a single number.
The calculator uses a density range per material, not a single number, because real aggregate varies with moisture, grading and how hard it has been compacted. Where a figure could not be traced to a published document it is labelled as an estimate.
| Material | Density used | Source |
|---|---|---|
| Ready-mixed normalweight concrete Ordinary ready-mix made with normalweight aggregate. NRMCA states this range while defining what structural lightweight concrete is not, so it is a comparison figure inside a lightweight-concrete sheet rather than a survey of plant records. If your supplier gives you a mix design, that number beats this one. |
140–150 lb/cu ft | NRMCA CIP 36, Structural Lightweight Concrete (© NRMCA 2003, 2016 — 2016 revision), section 'WHAT is Structural Lightweight Concrete': normalweight concrete 140 to 150 lb/ft3 (2240 to 2400 kg/m3). |
| Bagged concrete mix, as placed (QUIKRETE No. 1101) The unit weight QUIKRETE publishes for the same bag whose 0.60 cubic foot yield drives the bag count on this page, measured wet under ASTM C138. It sits at the bottom of the ready-mix range. The sheet says 'approximately', and Sakrete publishes no unit weight at all for comparison. |
140 lb/cu ft | QUIKRETE Concrete Mix No. 1101 data sheet, revised October 2022, Table 1 Typical Physical Properties: 'Unit Weight, ASTM C138 — Approximately 140 lb/ft3 (2242.5 kg/m3)'. Test method ASTM C138. |
These are the figures this calculator uses. Each names the document it came from, with its edition and section, so you can open it and check. Where a figure is superseded or is trade convention rather than a published value, the row says so.
| Item | Figure | Source |
|---|---|---|
| 80 lb (36.2 kg) bag yield | 0.60 cu ft (17 L) — 45 bags per cubic yard | QUIKRETE Concrete Mix No. 1101 data sheet, rev. October 2022, YIELD: 'An 80 lb (36.2 kg) bag yields approximately 0.60 ft3 (17 L)'. Sakrete High-Strength Concrete Mix TDS (©2023), Technical Data table, publishes 0.6 ft3 for the same 80 lb (36.2 kg) bag. 27 / 0.60 = 45 exactly. |
| 60 lb (27.2 kg) and 40 lb (18.1 kg) bag yields | 0.45 cu ft (12.7 L) and 0.30 cu ft (8.5 L) — 60 and 90 bags per cubic yard | Same two sheets; both state 0.45 and 0.30 ft3. The two brands agree on 40, 60 and 80 lb and disagree only at 90 lb, where QUIKRETE gives 0.675 ft3 and Sakrete 0.66 ft3, so this calculator does not offer a 90 lb bag. |
| Sakrete Canadian bag sizes | 25 kg → 0.41 cu ft; 30 kg → 0.50 cu ft | Sakrete High-Strength Concrete Mix TDS (©2023), Technical Data table, rows '25 kg Canada' and '30 kg Canada'. These are 55.1 lb and 66.1 lb bags, not the US 60 and 80 lb sizes. Yield per kilogram is the same mix (about 0.0166 ft3/kg); only the package differs. |
| Unit weight of the mixed concrete | Approximately 140 lb/cu ft (2242.5 kg/m3) | QUIKRETE Concrete Mix No. 1101 data sheet, rev. October 2022, Table 1 Typical Physical Properties, 'Unit Weight, ASTM C138'. Measured wet under ASTM C138. The Sakrete sheet lists yield, water, slump and strength but publishes no unit weight. |
| Normalweight concrete density range | 140–150 lb/cu ft (2,240–2,400 kg/m3) | NRMCA CIP 36, Structural Lightweight Concrete (© NRMCA 2003, 2016), 'WHAT is Structural Lightweight Concrete'. Stated there as the contrast against lightweight concrete, not as a survey of plant records — read the sentence it sits in before quoting it. |
| Contraction joint spacing, unreinforced slabs only | 24 to 36 × slab thickness, capped at 18 ft (5.5 m) | ACI 302.1R-04, 3.2.5.3 Contraction joints, p. 302.1R-11: 'For unreinforced, plain concrete slabs, joint spacings of 24 to 36 times the slab thickness, up to a maximum spacing of 18 ft (5.5 m), have produced acceptable results.' Superseded — ACI 302.1R-15 does not carry this sentence. |
| Contraction joint depth, conventional saw | At least 1/4 of the slab depth, or 1 in. (25 mm), whichever is greater | ACI 302.1R-04, 8.3.12 Saw-cut joints, p. 302.1R-46. (Not 8.2.8.2, which is also titled 'Saw-cutting' but sits under 8.2.8 Tools for jointing and lists saw hardware only; the -04's own cross-reference at 3.2.5.4 points to 8.3.12.) The 'whichever is greater' clause is part of the recommendation. ACI 302.1R-15, 5.2.10, refers saw-cutting detail to its own 10.3.12. Michigan Concrete Association Driveway & Sidewalk Guidelines give the same quarter-depth rule: 1 in. for a 4 in. slab. |
| Cubic feet per cubic yard | 27 — exact | Definitional: 3 ft × 3 ft × 3 ft. Not a measured or rounded figure. |
| Cubic feet per cubic metre | 35.3147 — rounded from 35.314667 | Derived from 1 ft = 0.3048 m exactly (international yard and pound agreement, 1959: 1 yd = 0.9144 m exactly). (1/0.3048)^3 = 35.31466672…, which this page rounds to 4 decimal places for display. Rounded, not exact. |
Five documents do the work: the QUIKRETE Concrete Mix No. 1101 data sheet (revised October 2022), the Sakrete High-Strength Concrete Mix technical data sheet (©2023), NRMCA CIP 36 Structural Lightweight Concrete (2016 revision), ACI 302.1R, and the Michigan Concrete Association Driveway & Sidewalk Design and Placement Guidelines, which carry the thickness minimums. The MCA document is undated; it references ACI 305R-10 and 306R-10, so it postdates 2010. Each figure names the one it came from. Two caveats. The cubic-yard conversion is exact; the cubic-metre conversion is rounded. And the ACI joint figures are quoted from the 2004 edition: ACI 302.1R-15 supersedes it and no longer publishes a spacing multiplier, so those lines are historic guidance, not current ACI recommendation. The waste allowance is trade convention, not sourced.
Thickness follows the load, not the span. The Michigan Concrete Association's Driveway & Sidewalk Design and Placement Guidelines set the citable floor: the minimum concrete thickness for a driveway is 4 inches, and when traffic will include delivery vehicles the minimum shall be 5 inches. That is the published number, and it is lower than the five to six inches often quoted. Anything above 5 inches for a house driveway is trade practice, not a requirement, and it is expensive practice: going from 4 to 6 inches raises your volume by exactly 50 percent. The material itself sets a hard floor further down. QUIKRETE's No. 1101 sheet says its mix is designed for pouring 2 inches thick or more; Sakrete says the same 2 inches for full-depth work, but asks for a minimum of 4 inches of concrete on flatwork. So 4 inches is where the manufacturer, the association and ordinary patio practice all land.
Reinforcement does not stop cracking. ACI 302.1R-15 puts it flatly: reinforcement will not prevent cracking, and if it is properly sized and located, cracks should remain tightly closed. Joints decide where the crack goes. The spacing rule everyone quotes comes from the superseded 2004 edition, section 3.2.5.3, and it applies only to unreinforced, plain concrete slabs: joint spacings of 24 to 36 times the slab thickness, up to a maximum spacing of 18 ft, have produced acceptable results. The multiplier is a pure ratio, so 4 inches gives 8 to 12 ft and 6 inches gives 12 to 18 ft, where the cap starts to bind. ACI 302.1R-15 dropped that sentence and leaves spacing to the designer. For depth, the 2004 edition asks for at least a quarter of the slab depth or 1 inch, whichever is greater. Michigan Concrete Association is tighter on spacing: not exceeding twice the slab thickness, 8 ft at 4 inches.
Two different weights appear here and they are not the same quantity. The dry mix you carry is bags times bag weight: 74 bags of 80 lb is 5,920 lb. The concrete you place is volume times density: 44 cubic feet at 140 to 150 lb per cubic foot is 6,160 to 6,600 lb. The difference is the mixing water, which QUIKRETE puts at 6 to 9 pints per 80 lb bag. Do not expect the two to reconcile exactly; the sheet calls both the yield and the unit weight approximate. Two practical notes. QUIKRETE asks for at least 1 cubic foot of mixer capacity per 80 lb bag, so 74 bags is nothing like one mixer load. And if you switch this calculator to metric, do not assume the bags on the shelf match: Sakrete's Canadian sizes are 25 kg yielding 0.41 cubic feet and 30 kg yielding 0.50, which are 55 and 66 lb bags.
Forty-five. An 80 lb (36.2 kg) bag yields 0.60 cubic feet on both the QUIKRETE No. 1101 and the Sakrete High-Strength sheets, and a cubic yard is 27 cubic feet, so 27 / 0.60 = 45 exactly. Sixty bags for 60 lb, ninety for 40 lb. The calculator will show more than this, because it adds your waste allowance before dividing.
Pi x radius squared x depth. A 12 inch hole 3 ft deep is 3.1416 x 0.5 x 0.5 x 3 = 2.36 cubic feet. Pick the column shape: the diameter field has its own inches/feet selector and defaults to inches, so type 12, not 1. With the 10 percent allowance included the tool returns 2.6 cubic feet and 5 bags of 80 lb mix.
With the selector on inches, nothing goes wrong: you get 1.05 cubic feet, 1.2 with the allowance, and 2 bags of 80 lb mix. If that field were reading feet, 8 would mean an 8 ft diameter, and because area goes with the square of the radius that is 144 times too much concrete. Check the unit beside the diameter box before you order.
No, and for most jobs you should not. A dressed 3-1/2 in. post in a 12 in. hole covers 12.25 of the hole's 113.10 square inches — roughly 11 percent, about the same as the allowance you are already adding. For something large, a column base or a void former, work out the displaced volume and subtract it yourself.
NRMCA gives normalweight concrete a density of 140 to 150 lb per cubic foot, which is 3,780 to 4,050 lb per cubic yard, or 1.89 to 2.03 US tons. The default 12 x 10 ft slab on this page comes out at 3.08 to 3.30 US tons. QUIKRETE measures its own bagged mix at approximately 140 lb per cubic foot, the bottom of that range.
Neither bag sheet publishes one, so the 10 percent default here is trade convention rather than a sourced figure. It covers spillage, an uneven subgrade and over-excavated edges, and machine-dug footings usually want more. The reason to add it is that running short mid-pour leaves a cold joint, and a cold joint in a slab is permanent.