Concrete Calculator
Calculate concrete volume for slabs, footings, or circular pads.
Plan your concrete project
Choose a shape and enter measurements. Every result updates instantly.
1Choose the concrete form
2Enter dimensions
3Ordering and costs
5–10% is common for normal projects.
Adjust any measurement to update the entire estimate.
How to calculate concrete
This calculator estimates how many cubic yards of concrete you need for a slab or circular pour — the number ready-mix suppliers actually sell by, so getting it right avoids both a costly extra delivery and paying for concrete you didn't need.
How the calculation works
Rectangular slab: Volume (yd³) = Length (ft) × Width (ft) × (Thickness in inches ÷ 12) ÷ 27. Circular slab: Volume (yd³) = π × (Diameter ÷ 2)² × (Thickness in inches ÷ 12) ÷ 27. Both formulas convert your dimensions into cubic feet, then divide by 27 (the number of cubic feet in a cubic yard) since that's the standard unit concrete is ordered in.
Example
A rectangular patio slab 12 ft × 10 ft at 4 inches thick: Volume = 12 × 10 × (4÷12) ÷ 27 = 120 × 0.333 ÷ 27 ≈ 1.48 cubic yards. Most suppliers deliver in whole or half-yard increments and recommend ordering 5–10% extra to cover uneven subgrade and minor spillage, which would put a real order for this slab at roughly 1.6–1.65 yards.
Frequently asked questions
How much extra concrete should I order?
A common rule of thumb is 5–10% over your calculated volume to account for subgrade unevenness, minor form leaks, and waste. For irregular or uneven sites, some contractors go as high as 10–15%.
What thickness should I use for a driveway versus a patio?
Patios and walkways are commonly poured at 4 inches, while driveways supporting vehicle weight are typically 5–6 inches, and areas expecting heavier trucks may need more — check local codes and your specific use case.
Concrete Calculator
Rectangular slab: Volume (yd³) = Length (ft) × Width (ft) × (Thickness in inches ÷ 12) ÷ 27. Circular slab: Volume (yd³) = π × (Diameter ÷ 2)² × (Thickness in inches ÷ 12) ÷ 27. Both formulas convert your dimensions into cubic feet, then divide by 27 (the number of cubic feet in a cubic yard) since that's the standard unit concrete is ordered in.
Let's understand your concrete result.
Calculate a result above and this guide will help you interpret it using this calculator's own formula and explanation.
Pro Tips for Concrete
- Order 5–10% more than the calculated volume — uneven ground, form leakage, and waste during placement are normal and this buffer avoids running short mid-pour.
- Double-check your thickness measurement in inches, not feet — a 4-inch slab is standard for patios and walkways, while driveways typically need 5–6 inches for vehicle loads.
- Ready-mix trucks usually have minimum order quantities (often around 1 yard); for very small pours, factor in whether a minimum charge applies even if your calculated volume is smaller.
Common Concrete Mistakes to Avoid
- Forgetting to convert thickness from inches to feet before multiplying — this calculator handles that conversion internally, but if you're doing it by hand, mixing units here is the single most common concrete math error.
- Not accounting for slab thickening at edges or footings, which can require more concrete than a flat-thickness estimate alone would suggest for structural pours.
When to Use This Calculator
This calculator estimates how many cubic yards of concrete you need for a slab or circular pour — the number ready-mix suppliers actually sell by, so getting it right avoids both a costly extra delivery and paying for concrete you didn't need.