Contents: units · rectangles · 324 and 81 · round shapes · stairs and curbs · waste · bags · weight · mix · rebar
1. Get every number into one unit
Most mistakes come from multiplying feet by inches. Convert first.
YardPour displays metric volume using 0.0283168 m³ per ft³ throughout. Some sites print 0.0283. The small difference adds up on big pours, so the two are never mixed in one result.
2. Rectangles: slabs, walls, square footings
Example: 10 × 10 ft slab, 4 in thick
- 4 in ÷ 12 = 0.333 ft.
- 10 × 10 × 0.333 = 33.33 ft³.
- 33.33 ÷ 27 = 1.23 yd³.
3. The 324 and ÷ 81 shortcuts
Slabs are usually measured in feet for area and inches for thickness. Combine the two conversions into one number:
The ÷ 81 shortcut, popularized by Concrete Network, is exact at 4 inches only. At 6 inches use ÷ 54. At 5 inches use ÷ 64.8. For any other thickness, use 324.
4. Round shapes: columns, tubes, post holes, round slabs
Example: 12 in tube form, 4 ft tall
- D = 12 in = 1 ft, so the radius is 0.5 ft.
- π × 0.5² × 4 = 3.14 ft³.
- 3.14 ÷ 27 = 0.116 yd³.
5. Stairs, curbs, and trapezoids
Solid stairs on grade. Each step is a box one run deep. Step k is k rises tall. Summing 1 + 2 + … + n gives n(n+1) ÷ 2. A top landing adds width × landing depth × n × rise .
Curb and gutter. Modeled as two rectangles: the curb (width × height) plus the gutter flag (width × thickness), times length. Real profiles have batters and radii. Check your agency's detail .
Trapezoid. Average the two widths: V = L × ((top + bottom) ÷ 2) × H.
6. Net volume vs order volume
Net is the geometry. Order is what you buy. A 5–10% margin is often cited for uneven subgrade, form bulge, and spillage . YardPour defaults to 0% and shows both numbers, so you choose the margin knowingly.
7. Bags
| Bag | Yield (ft³) | Bags per yd³ |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 50 lb | 0.375 | 72 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
Yields as widely cited for QUIKRETE Concrete Mix No. 1101 . Some calculators publish different bags-per-yard tables. The bag in your hand is the authority.
8. Weight
150 lb/ft³ is a common planning figure. Concrete Network cites about 4,000 lb/yd³. YardPour shows 150 as an editable assumption, not a fact about your mix .
9. Mix ratios
For 1 yd³ at 1 : 2 : 3: dry 41.58 ft³, cement 6.93 ft³ (7 bags), sand 13.86 ft³, gravel 20.79 ft³. The dry factor and bag volume are rules of thumb . A ratio is not a strength specification.
10. Rebar grid (takeoff only)
A spacing limit often attributed to ACI 318 is the lesser of 3 × slab thickness or 18 in. Concrete cover against earth is often cited around 3 in . Laps are not added unless you enter a lap length. This is a quantity takeoff, not structural design.
Thickness: typical, not code
Sidewalks and patios are often 4 in. Driveways and garages are often 4–6 in. Some pages cite an IRC slab-on-ground minimum of 3.5 in . Footing depth has no national default and depends on frost depth and your building department.
Put it to work
The concrete calculator runs every formula on this page, and the slab, yard, and bag tools focus on one job each.
Frequently asked questions
What is the formula for calculating concrete?
Volume = length × width × thickness, with every number in feet. Divide cubic feet by 27 to get cubic yards. For round shapes, use π × radius² × height.
Why do you divide by 27 for concrete?
A cubic yard is 3 ft × 3 ft × 3 ft, which is 27 cubic feet. Dividing cubic feet by 27 converts to cubic yards, the unit ready-mix is sold in.
Where does the 324 in the concrete formula come from?
When thickness is in inches, you divide by 12 to get feet and then by 27 to get yards. 12 × 27 = 324, so cubic yards = square feet × inches ÷ 324.