Concrete Slab Calculator
Use this tool to estimate how much concrete you need for a slab and get quick bag/count and cost estimates.
How this concrete slab calculator helps
When you are planning a patio, driveway, shed pad, or garage floor, the first question is always the same: how much concrete do I need? A concrete slab calculator removes guesswork by converting your slab dimensions into volume, then giving that volume in both cubic feet and cubic yards.
This is useful whether you are:
- Ordering ready-mix concrete by the yard
- Buying 60 lb or 80 lb concrete bags
- Building a budget for materials and labor
- Adding a waste allowance to avoid coming up short
Concrete slab volume formula
For a rectangular slab, the calculation is straightforward:
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (ft)
Thickness (ft) = Thickness (inches) ÷ 12
Volume (cubic yards) = Cubic feet ÷ 27
After volume is calculated, a waste factor is applied. Most jobs use 5% to 15% extra depending on complexity, grade variation, and formwork quality.
Step-by-step: measuring your slab correctly
1) Measure length and width
Use the longest and widest dimensions in feet. If your slab shape is irregular, split it into rectangles and calculate each section separately.
2) Confirm slab thickness
Typical residential slab thickness:
- 4 inches for walkways/patios
- 5 to 6 inches for driveways (vehicle load)
- 6+ inches for heavier loads or special cases
3) Add waste factor
Always include extra concrete for uneven subgrade, spillage, and finishing variation. A common default is 10%.
4) Estimate cost
If you know your local ready-mix rate, enter cost per cubic yard to get a quick total material estimate.
Example calculation
Suppose your slab is 20 ft × 12 ft × 4 in with a 10% waste factor:
- Thickness in feet = 4 ÷ 12 = 0.333 ft
- Base volume = 20 × 12 × 0.333 = 79.92 cu ft
- With 10% waste = 87.91 cu ft
- In cubic yards = 87.91 ÷ 27 = 3.26 yd³
That means you should plan for about 3.3 cubic yards of concrete.
Bag concrete vs ready-mix: quick planning guide
The calculator also estimates bag counts for small projects:
- 60 lb bags (approx. 0.45 cu ft yield each)
- 80 lb bags (approx. 0.60 cu ft yield each)
As a rule of thumb, once your project is above about 1 cubic yard, ready-mix delivery often saves major time and labor.
Common mistakes to avoid
- Forgetting to convert thickness from inches to feet
- Skipping waste factor and running short during pour
- Not accounting for thicker slab edges or footings
- Ignoring reinforcement and base preparation requirements
- Assuming one fixed concrete price across all suppliers
FAQ: concrete slab calculator
How many cubic feet are in a cubic yard?
There are exactly 27 cubic feet in 1 cubic yard.
Is 4 inches thick enough for a slab?
For many patios and walkways, yes. For driveways and heavy loads, you may need 5 to 6 inches or more, depending on local code and soil conditions.
How much extra concrete should I order?
Most contractors add 5% to 15%. Use higher values when site conditions are uneven or access is difficult.
Can I use this for non-rectangular slabs?
Yes. Break the area into rectangles, calculate each one, and add totals before ordering concrete.
Final takeaway
A reliable concrete slab calculator gives you a faster, cleaner estimate for volume, bags, and cost. Measure carefully, include a realistic waste allowance, and confirm local structural requirements before you pour.