Concrete Calculator
Calculate how much concrete you need in cubic yards, plus the number of premixed bags required.
About the Concrete Calculator
Ordering too little concrete stalls a pour halfway through, and ordering too much wastes money — this calculator converts a slab's length, width, and thickness into the exact volume of concrete needed, plus how many premixed bags that translates to if you're not ordering from a truck.
How It Works
The calculator multiplies length by width by thickness (converted from inches to feet) to get cubic feet, then divides by 27 to convert to cubic yards, the standard unit ready-mix concrete is sold in. It also divides the cubic feet figure by the yield of standard 60-pound and 80-pound premixed bags to estimate bag counts.
Cubic Feet = Length × Width × (Thickness ÷ 12); Cubic Yards = Cubic Feet / 27; 60lb bags ≈ Cubic Feet / 0.45; 80lb bags ≈ Cubic Feet / 0.6.
Worked example
Scenario: A 10 ft × 10 ft slab poured 4 inches thick.
- Thickness in feet: 4 ÷ 12 = 0.333 ft.
- Volume: 10 × 10 × 0.333 = 33.33 cubic feet.
- Cubic yards: 33.33 ÷ 27 = 1.23 cu yd.
- Bags: 33.33 ÷ 0.45 = 74.1 → 75 sixty-pound bags, or 33.33 ÷ 0.6 = 55.6 → 56 eighty-pound bags.
Result: 1.23 cubic yards, or 75 × 60 lb / 56 × 80 lb bags.
How much concrete for common slab sizes
The table below runs the calculator for the slabs people ask about most, with a 10% allowance for spillage and uneven ground. Bag yields are the typical figures printed on premixed bags: 0.30 cubic feet for a 40 lb bag, 0.45 for 60 lb and 0.60 for 80 lb. Bag counts are rounded up, because you cannot buy part of a bag.
| Slab | Thickness | Concrete (cu yd) | Order with 10% waste | 40 lb bags | 60 lb bags | 80 lb bags |
|---|---|---|---|---|---|---|
| 10 × 10 ft | 4 in | 1.23 | 1.36 | 123 | 82 | 62 |
| 12 × 12 ft | 4 in | 1.78 | 1.96 | 176 | 118 | 88 |
| 10 × 20 ft | 4 in | 2.47 | 2.72 | 245 | 163 | 123 |
| 20 × 20 ft | 4 in | 4.94 | 5.43 | 489 | 326 | 245 |
| 24 × 24 ft | 4 in | 7.11 | 7.82 | 704 | 470 | 352 |
| 10 × 10 ft | 6 in | 1.85 | 2.04 | 184 | 123 | 92 |
| 20 × 20 ft | 6 in | 7.41 | 8.15 | 734 | 489 | 367 |
Answers to "how many bags for a 10×10 slab" range from 56 to 65 depending on whether waste is included: 56 bags of 80 lb mix is the exact volume with nothing spare, and 62 bags includes 10% waste. Above roughly one cubic yard — about 45 bags of 80 lb mix — mixing by hand becomes a long day's work, and many people price a ready-mix delivery instead. Always check the yield printed on the bag you buy, since mixes differ.
Tips and common mistakes
- Order 5–10% extra to cover spillage and an uneven base.
- Ready-mix trucks usually have a minimum order around 1 cubic yard; above about 1 yd³, delivery is often easier than bags.
- Standard slabs are 4 in thick; driveways are usually 5–6 in.
Assumptions & Limitations
- Assumes a uniformly flat slab with consistent thickness across the entire area, with no allowance for waste built in.
- Bag yield figures are standard manufacturer approximations for 60lb and 80lb premixed concrete bags.
- Most contractors add 5-10% extra to the calculated volume to account for spillage, uneven subgrade, and form leakage — order accordingly.
- Does not account for rebar, mesh, or other reinforcement that displaces a small amount of concrete volume.
Planning a concrete order
Treat the calculated volume as the geometric minimum, not automatically the amount to purchase. Real pours can need extra material because the subgrade is uneven, forms bow or leak, and some concrete remains in tools or the delivery system. For a large or structural pour, confirm slab thickness, reinforcement, local requirements and the supplier’s minimum-load or short-load charges before ordering.