Concrete Calculator
Enter your pour size and get cubic yards, cubic feet, and exactly how many bags to buy — updated live as you type.
Bags to buy
Bag vs Ready-Mix: Which Costs Less?
The question every DIY pour comes down to. Prices vary by store and region, so edit the numbers to match your local quotes — the verdict updates with your calculator results above.
Short-load note: this pour is under 1 cubic yard. Most ready-mix suppliers bill a 1 yd³ minimum (already applied above), and many add a short-load fee on top — which makes bags even more attractive at this size.
Rule of thumb: bags usually win under about 1 cubic yard. Above that, the truck wins on price — and always wins on labor. Mixing 60 bags by hand is a full day of heavy work; a truck places the same pour in under an hour.
Quick Reference: Bags for Common Pours
Bag counts are rounded up and shown before waste — add about 10% so you don't run short. Based on standard yields: an 80 lb bag makes 0.60 ft³, a 60 lb bag makes 0.45 ft³.
| Pour | Volume | 80 lb bags | 60 lb bags |
|---|---|---|---|
| Slab 10 × 10 ft, 4 in thick | 1.23 yd³ | 56 | 75 |
| Slab 10 × 10 ft, 6 in thick | 1.85 yd³ | 84 | 112 |
| Slab 12 × 12 ft, 4 in thick | 1.78 yd³ | 80 | 107 |
| Slab 12 × 12 ft, 6 in thick | 2.67 yd³ | 120 | 160 |
| Post hole 12 in wide × 36 in deep (each) | 2.4 ft³ | 4 | 6 |
| Post hole 10 in wide × 36 in deep (each) | 1.6 ft³ | 3 | 4 |
| Sonotube 12 in × 48 in (each) | 3.1 ft³ | 6 | 7 |
How to Calculate Concrete (The Formula)
For a slab or footing, concrete volume is just length × width × thickness — with one catch: your measurements are in mixed units, so convert the thickness to feet first.
- Measure the length and width in feet.
- Convert thickness: divide inches by 12. (4 in = 0.333 ft, 6 in = 0.5 ft.)
- Multiply: length × width × thickness in feet = cubic feet.
- Cubic yards: divide cubic feet by 27.
- Bags: divide cubic feet by the yield printed on the bag (80 lb = 0.60 ft³, 60 lb = 0.45 ft³, 50 lb = 0.375 ft³, 40 lb = 0.30 ft³) and round up.
- Add waste: multiply by 1.10 for a 10% cushion.
Volume (ft³) = L × W × (T ÷ 12) · Yards = ft³ ÷ 27
Worked example — a 10 × 10 ft patio at 4 in thick: 10 × 10 × (4 ÷ 12) = 33.3 ft³. That's 33.3 ÷ 27 = 1.23 yd³, or 33.3 ÷ 0.60 = 55.6, so 56 bags of 80 lb mix before waste.
For round holes and tubes, use the cylinder formula: π × radius² × depth, converting inches to feet first (radius in feet = diameter in inches ÷ 24). A 12 in × 36 in post hole: π × 0.5² × 3 = about 2.4 ft³ per hole.
How Thick Should Your Concrete Be?
| Project | Recommended thickness | Notes |
|---|---|---|
| Patios, walkways, sidewalks | 4 in | The standard residential slab. |
| Shed floors and pads | 4 in | Go 6 in if the shed will hold heavy equipment. |
| Driveways (cars) | 4–6 in | Use 6 in for trucks, RVs, and delivery vehicles. |
| Garage floors | 6 in | Handles vehicle weight and jack stands. |
| Footings | 8–12 in deep, 16–24 in wide | The bottom must sit below the frost line — check your local code. |
| Post holes | Depth = ⅓ of the post's above-ground height | Diameter about 3× the post width; deeper in frost country. |
Thickness is the cheapest strength you'll ever buy: going from 4 in to 6 in adds 50% more concrete but far more than 50% more load capacity. Never pour thinner than 4 in for a slab — thin pours crack and spall.
Concrete Calculator FAQ
How many bags of concrete do I need for a 10x10 slab?
At the standard 4 in thick, a 10 × 10 ft slab needs 33.3 ft³ (1.23 yd³) of concrete: 56 bags of 80 lb mix, or 75 bags of 60 lb. Add 10% for waste and buy 62 × 80 lb bags. At 6 in thick, the same slab needs 84 × 80 lb bags before waste.
How many 80 lb bags of concrete are in a cubic yard?
45 bags. Each 80 lb bag yields 0.60 ft³ and a cubic yard is 27 ft³: 27 ÷ 0.60 = 45. For other sizes: 60 bags of 60 lb, 72 bags of 50 lb, or 90 bags of 40 lb per cubic yard.
How much concrete do I need for a post hole?
It depends on the hole. A typical fence post hole — 12 in wide and 36 in deep — holds about 2.4 ft³, which is 4 bags of 80 lb mix per hole. A 10 in × 36 in hole takes about 3 bags. Dig the hole roughly 3× the width of the post, and deep enough that the bottom is below the frost line.
Should I buy bags or order ready-mix?
Under about 1 cubic yard, bags usually win: no delivery fee, no minimum order, no short-load charge, and you can mix at your own pace. Above 1 yd³, ready-mix usually costs less per yard and saves enormous labor — 45 bags per yard is a lot of lifting, mixing, and wheelbarrow trips. Use the cost comparison above with your local prices to find your exact crossover.
How deep should a concrete slab be?
4 inches for patios, walkways, and shed pads. 6 inches for driveways, garage floors, and anywhere heavy vehicles will park. Thickness matters more than almost anything else for crack resistance, and the extra concrete is cheap insurance compared to tearing out a failed slab.
Estimates Only
Results are planning estimates. Actual concrete needs vary with ground conditions, form accuracy, over-digging, and product yield. Bag yields follow manufacturer figures for standard mixes; your product may differ slightly. Always order a little extra — running short mid-pour creates a weak cold joint. For structural work (foundations, load-bearing footings, retaining walls), consult a licensed professional and follow your local building code.