Concrete Calculator
Free concrete calculator. Estimate cubic yards for slabs, footings, columns, and walls, plus 60/80 lb bags and delivered cost. No signup.
A concrete calculator turns slab, footing, column, or wall dimensions into the cubic yards of ready-mix you need to order, then converts that volume into 60 lb or 80 lb bag counts and an approximate delivered cost.
How the volume is calculated
The tool works in cubic feet first, then divides by 27 because one cubic yard is 27 ft³. The shape you choose decides which formula runs:
- Slab, footing, or wall:
volume (ft³) = length × width × (thickness in inches ÷ 12). - Column or round pier:
volume = π × (diameter ÷ 2)² × height, with the diameter in feet.
Waste is applied on top of the net figure: order yards = net yards × (1 + waste ÷ 100). Bag counts then divide the ordered volume by each bag’s yield — about 0.45 ft³ for a 60 lb bag and 0.60 ft³ for an 80 lb bag — and round up, because you cannot buy part of a bag.
Worked example: a 20 × 10 ft slab at 4 inches
These are the calculator’s default values: length 20 ft, width 10 ft, thickness 4 in, waste 10%, price $150 per cubic yard.
| Step | Calculation | Result |
|---|---|---|
| Convert thickness to feet | 4 ÷ 12 | 0.333 ft |
| Volume in cubic feet | 20 × 10 × 0.333 | 66.67 ft³ |
| Net cubic yards | 66.67 ÷ 27 | 2.47 yd³ |
| With 10% waste | 2.47 × 1.10 | 2.72 yd³ |
| 80 lb bags | (2.72 × 27) ÷ 0.60 | 123 bags |
| 60 lb bags | (2.72 × 27) ÷ 0.45 | 163 bags |
| Delivered cost | 2.72 × $150 | $407.41 |
The bag figures matter only if you are mixing by hand. For a pour this size, ready-mix delivered by truck is usually cheaper and far less work.
Reference: thickness, volume, and bag yield
Net figures for a 100 ft² area, before any waste allowance.
| Slab thickness | Volume per 100 ft² | 80 lb bags | 60 lb bags |
|---|---|---|---|
| 3 in | 0.93 yd³ | 42 | 56 |
| 4 in | 1.23 yd³ | 56 | 74 |
| 5 in | 1.54 yd³ | 70 | 93 |
| 6 in | 1.85 yd³ | 84 | 111 |
Check the 4-inch row: 100 ft² × 0.333 ft = 33.33 ft³ = 1.23 yd³. Bags: 33.33 ÷ 0.60 = 55.6 (round up to 56), and 33.33 ÷ 0.45 = 74.1 (round up to 74).
Ready-mix or bags?
The break-even usually sits somewhere under a cubic yard. A full truck carries roughly 8 to 10 cubic yards, and suppliers commonly apply a short-load fee for small pours, so a single footing can be cheaper mixed by hand. Once the pour passes about a yard, or you need it placed quickly, delivered concrete wins on both cost and labour.
- Under 1 yd³: bag mixing is practical — expect around 45 eighty-pound bags per cubic yard.
- 1 to 3 yd³: compare a short-load delivery against bag cost plus your own time.
- Over 3 yd³: ready-mix is almost always the right call.
Common mistakes
- Leaving thickness in inches. A 4-inch slab is 0.333 ft, not 4. Skipping the ÷12 multiplies the order twelvefold.
- Ordering the net figure. Uneven grade, spillage, and form deflection consume 5–10% on a typical slab.
- Assuming both bag sizes cover the same area. An 80 lb bag yields roughly a third more concrete than a 60 lb bag.
- Using the column formula for a square pier. The πr² formula assumes a round cross-section; a square footing uses length × width.
How many cubic yards is a 10 × 10 slab at 4 inches?
100 ft² × 0.333 ft = 33.3 ft³ = 1.23 yd³, or about 1.36 yd³ once you add a 10% waste allowance.
Do I need waste for a footing or trench?
Yes. Trench walls are rarely straight and dry soil absorbs some mix, so 10% is a reasonable default even though the calculator lets you set waste anywhere from 0 to 50%.
Why does the column option ignore the width field?
A round pier’s volume is a cylinder: πr²h. The calculator reads the diameter field and the length field, which it treats as height, and ignores width and thickness for that shape.
How many 80 lb bags make a cubic yard?
About 45. A cubic yard is 27 ft³ and an 80 lb bag yields roughly 0.60 ft³, so 27 ÷ 0.60 = 45 bags. In 60 lb bags the same yard takes about 60.
Concrete for fence post holes
Post holes are cylinders, which is why the column shape handles them: enter the hole diameter and depth and the tool returns the volume, bags, and cost. For a typical 10-inch hole 30 inches deep around a 4×4 post, that is about 1.1 cubic feet, or roughly 2.4 sixty-pound bags per hole.
Multiply by the number of posts, then add a bag or two for spillage and imperfect holes. Tube forms use slightly more mix, because the post is not displacing concrete inside the hole. Set about a third of the post length in the ground and take the bottom below the frost line where the ground freezes.
Setting posts rather than pouring a slab? Choose the column shape above and enter the hole diameter and depth to estimate the concrete per post hole, and the bulk material calculator sizes the drainage base underneath.