BuildWellHow concrete is measured
Concrete is ordered by volume. Ready-mix trucks deliver it by the cubic yard, which is a cube 3 feet on each side, or 27 cubic feet. Bagged mix is sold by weight, and each bag yields a set volume once mixed, printed on the bag.
Volume is length × width × depth, with every measurement in feet. Most of the trouble comes from mixing units: slab thickness is given in inches, so a 4 inch slab is 4 ÷ 12 = 0.333 feet thick. The calculator converts for you.
Measure the forms, not the plan. Forms are rarely perfectly square, the subgrade is rarely perfectly flat, and a slab that’s 4 inches on the drawing can be 4½ inches in the low spots. That’s what the extra allowance is for.
The concrete formulas
Slab, patio or walk: length (ft) × width (ft) × thickness (in) ÷ 12 = cubic feet.
Continuous footing: length (ft) × width (in) ÷ 12 × depth (in) ÷ 12 = cubic feet.
Round columns or piers: π × (diameter (in) ÷ 24)² × height (ft) × how many = cubic feet. Dividing the diameter by 24 turns it into the radius in feet.
Then for every shape:
- Cubic yards = cubic feet ÷ 27.
- To order = cubic yards × (1 + extra).
- Bags = cubic feet to order ÷ the bag’s yield, rounded up.
Typical yields printed on bagged concrete mix are about 0.6 cubic feet for an 80 lb bag, 0.45 for a 60 lb bag and 0.3 for a 40 lb bag.
Bags per cubic yard
| Bag | Typical yield | Bags per cubic yard | Bags per 10 cu ft |
|---|---|---|---|
| 80 lb | 0.6 cu ft | 45 | 16.7 |
| 60 lb | 0.45 cu ft | 60 | 22.2 |
| 40 lb | 0.3 cu ft | 90 | 33.3 |
A yard of concrete is 45 bags of 80 lb mix. That’s 3,600 pounds of bags to unload, mix and place, which is why larger pours usually come on a truck.
Coverage by slab thickness
How far a yard goes at common thicknesses, and what 100 square feet takes.
| Thickness | Sq ft per cubic yard | Cubic yards per 100 sq ft | 80 lb bags per 100 sq ft |
|---|---|---|---|
| 3 in | 108 | 0.93 | 42 |
| 3.5 in | 92.6 | 1.08 | 49 |
| 4 in | 81 | 1.23 | 56 |
| 5 in | 64.8 | 1.54 | 70 |
| 6 in | 54 | 1.85 | 84 |
| 8 in | 40.5 | 2.47 | 112 |
Columns, piers and form tubes
Volume per foot of height for round forms, and bags for a typical 4 foot pier.
| Diameter | Cu ft per foot of height | 80 lb bags per foot | 80 lb bags for a 4 ft pier |
|---|---|---|---|
| 6 in | 0.196 | 0.33 | 2 |
| 8 in | 0.349 | 0.58 | 3 |
| 10 in | 0.545 | 0.91 | 4 |
| 12 in | 0.785 | 1.31 | 6 |
| 14 in | 1.069 | 1.78 | 8 |
| 16 in | 1.396 | 2.33 | 10 |
| 18 in | 1.767 | 2.95 | 12 |
| 24 in | 3.142 | 5.24 | 21 |
Doubling the diameter quadruples the volume. A 16 inch pier holds four times the concrete of an 8 inch one, foot for foot.
Worked examples
A 10 × 10 patio, 4 inches thick
10 × 10 × 4 ÷ 12 = 33.33 cubic feet, or 1.23 cubic yards. With 10% extra, 36.67 cubic feet, or 1.36 yards. In bags: 36.67 ÷ 0.6 = 61.1, so 62 bags of 80 lb mix. Or 82 bags of 60 lb, or 123 bags of 40 lb.
A 40 foot footing, 16 inches wide and 8 inches deep
40 × (16 ÷ 12) × (8 ÷ 12) = 35.56 cubic feet, or 1.32 cubic yards. With 10% extra, 1.45 yards, or 66 bags of 80 lb mix.
Six deck piers, 12 inches across and 4 feet deep
π × 0.5² × 4 = 3.14 cubic feet each, 18.85 for all six, or 0.70 cubic yards. With 10% extra, 20.73 cubic feet, so 35 bags of 80 lb mix.
How much extra to order
Five to ten percent is a common allowance. It covers:
- An uneven subgrade. Low spots in the base become thick spots in the slab.
- Form bulge. Forms flex a little under the weight of wet concrete.
- Spillage and what stays in the mixer, the wheelbarrow and the chute.
Running short in the middle of a pour can leave a cold joint, where fresh concrete meets concrete that has started to set. It’s a weak spot and it shows. A little left over is cheaper.
Bags or ready-mix
For a few post holes or a small pad, bags are simple. As the job grows, the bags grow fast: one yard is 45 bags of 80 lb mix, and two yards is 90. Mixing that much by hand or in a small mixer takes a crew most of a day, and the first batches can start to set before the last ones are placed.
At around a yard or more, price a ready-mix delivery against the bags. Ask the supplier about minimum loads, short-load charges and how long the truck can wait. Ready-mix also lets you specify the strength and any additives the job needs.
Common mistakes
Mixing inches and feet. 10 × 10 × 4 is 400, not 33.33. Divide the thickness by 12 first.
Forgetting the base. A slab needs a compacted gravel base, and that’s its own material and its own labor.
No control joints. Concrete shrinks as it cures and cracks where it wants to. Joints tell it where. A common rule of thumb is joints spaced two to three times the slab thickness in inches, in feet: 8 to 12 feet apart in a 4 inch slab, cut about a quarter of the slab deep.
Skipping the cure. Concrete gains strength over weeks, not hours. Keeping it moist, or using a curing compound, makes a stronger surface.
Pouring in heat and wind without a plan. A hot, windy day dries the surface fast, and that can open shallow shrinkage cracks before the slab is even finished. Dampen the base, line up enough hands to finish, and start curing as soon as the finish is done.
Ordering exactly what the math says. The math is for perfect forms on a perfect base. Add the extra.
From quantities to an estimate
The concrete itself is only part of a flatwork job. The estimate also needs demolition and haul-out if there’s an old slab, excavation and grading, gravel base, forms, reinforcement, finishing, joints, curing and cleanup.
When you estimate concrete work in BuildWell, describe the job and it plans each of those as its own line with labor hours. Materials are priced at stores near the job, checked within 30 days, and labor at your metro’s wage plus your overhead and markup. You can change any quantity, hour or price before you send it.
