How the concrete calculator works
Concrete is sold by volume, so every shape comes down to cubic feet. The calculator converts each measurement to feet, works out the volume of one pour, multiplies by how many you have, adds your waste margin and divides by the yield of one bag. A cubic yard is 27 cubic feet (3 × 3 × 3).
Slab or footing: length × width × depth Round column: π × (diameter ÷ 2)² × height Post hole: π × (hole ⌀ ÷ 2)² × depth − post area × depth Bags: total cubic feet ÷ yield per bag, rounded up
Examples
A 10 × 10 ft pad, 4 inches thick
4 ÷ 12 = 0.333 ft, so 10 × 10 × 0.333 = 33.33 cu ft (1.23 cu yd). With 10% extra that is 36.67 cu ft. At 0.60 cu ft per 80 lb bag: 36.67 ÷ 0.60 = 61.1, so 62 bags. These are the default values above.
Twelve 4×4 posts for a 6 ft fence, 10 inch holes, 3 ft deep
The 3 ft depth follows QUIKRETE’s post-setting instructions (one-third of the post’s length, which is half its 6 ft height above ground); your local requirement may differ. One hole holds π × (5 in)² × 36 in = 2,827 cubic inches, or 1.64 cu ft. The post (3½ × 3½ in) takes up 0.26 cu ft of that, leaving 1.38 cu ft of concrete per hole. For 12 holes with 10% extra that is 18.23 cu ft: 31 bags of 80 lb, or 49 bags of 50 lb. Counting each hole as a square box instead would add about 27% to the hole volume.
Post holes: to subtract the post or not?
The post fills part of the hole, so the concrete only fills the space around it. With Subtract the post’s volume switched on, the calculator removes the post’s cross-section × depth from each hole. That is the right number when the post stands in the concrete to the bottom of the hole. Switch it off to get the volume of the empty hole, for example for a footing poured before the post is set. The result always states which one you are seeing.
If you put gravel in the bottom of each hole for drainage, enter only the depth that will be filled with concrete.
Bag yields used by default
Premixed concrete bags list an approximate yield on the label. The calculator starts with the yields printed on common US bags of standard concrete mix. Other products (fast-setting, high-strength, crack-resistant) can yield different amounts, so enter the number from your bag if it differs.
| Bag size | Typical yield | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.3 cu ft | 90 |
| 50 lb | 0.375 cu ft | 72 |
| 60 lb | 0.45 cu ft | 60 |
| 80 lb | 0.6 cu ft | 45 |
Source: these figures match the yields listed on the manufacturer's data sheet forQUIKRETE® Concrete Mix (No. 1101)(revised October 2022). Fast-setting yields: QUIKRETE Fast-Setting Concrete Mix data sheet (60 lb = 0.45 cu ft) and spec sheet (50 lb ≈ 0.375 cu ft). Other brands and mixes may differ. See our methodology.
Entering measurements
- Type it the way you measured it. On a keyboard you can enter
10' 6",10 ft 6 in,5 1/2or3 m; the calculator converts it to the unit shown next to the field. On a phone, type the decimal number and pick the unit. - Measure inside the forms, not the outside. Form boards add width that never fills with concrete.
- Use the deepest point if the ground is uneven, or average several depth readings. A half-inch dip over a large slab adds up quickly.
- Switching units converts your number, so 10 ft becomes 3.048 m rather than 10 m.
Common mistakes
- Entering inches as feet. A 4 in slab entered as 4 ft is 12 times too much concrete.
- Treating round holes as squares. A box around a round hole holds about 27% more than the hole itself.
- Ordering the exact amount. Running short mid-pour can leave a cold joint. The ready-mix industry group NRMCA suggests ordering 4–10% extra (CIP 8).
- Going by bag weight. Always use the yield in cubic feet printed on the bag.
Limitations
The result is a material estimate, not an engineering specification. It does not tell you what size, depth, strength or reinforcement your project needs. Those depend on loads, soil, frost depth and local building code. Ask your local building department or a qualified professional for anything structural.
Frequently asked questions
How many 80 lb bags of concrete are in a yard?
At the typical yield of 0.60 cu ft per bag, one cubic yard (27 cu ft) takes 27 ÷ 0.60 = 45 bags.
How many bags of concrete do I need per fence post?
It depends on the hole. A 10 in hole 3 ft deep around a 4×4 post holds about 1.38 cu ft: roughly 2.3 bags of 80 lb mix or 3.7 bags of 50 lb mix. Choose Post holes above and enter your hole size for an exact number.
Should I use bags or order ready-mix?
Bags suit small jobs like post holes, steps and small pads. For larger pours, compare with a ready-mix quote in the Concrete Slab Calculator. Suppliers price by the cubic yard and may charge extra for small loads.
Full walkthrough: How Much Concrete Do You Need?
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