Concrete Slab Calculator: Yards, Bags & Cost
This concrete slab calculator turns your length, width and thickness into the cubic yards of concrete you need, plus the number of 40, 60 or 80 lb bags and an estimated delivery cost. Getting the volume right up front prevents costly short-pours or wasted material on patios, walkways and floating slabs.
Estimated concrete
Estimates only. Add ~5–10% for waste. Bag yields: 40 lb ≈ 0.30, 60 lb ≈ 0.45, 80 lb ≈ 0.60 cu ft. Concrete ≈ 150 lb per cu ft.
How the Concrete Slab Calculator works
- 1
Enter your dimensions
Type the length, width and thickness of your slab, footing or pad and choose your units, feet, inches or meters.
- 2
Get instant volume
See the result in cubic yards, cubic feet and cubic meters, plus weight, the moment you type, no page reloads.
- 3
Buy the right amount
We convert volume into the exact number of 40, 60 and 80 lb bags, truck loads and cost so you order without waste or shortfalls.
The formula
Cubic yards = [Length_ft x Width_ft x (Thickness_in / 12) x Qty] / 27
- ·Length_ft = slab length (feet)
- ·Width_ft = slab width (feet)
- ·Thickness_in = slab thickness (inches)
- ·Qty = number of identical slabs
- ·27 = cubic feet per cubic yard
The calculator finds cubic feet first, divides by 27 for yards, and adds a default 10% for waste and spillage. Concrete weighs about 150 lb per cubic foot if you also need the load weight.
Worked examples
Example 1: 10 x 10 ft slab at 4 inches thick
- Area = 10 x 10 = 100 ft2
- Thickness = 4 / 12 = 0.333 ft
- Volume = 100 x 0.333 = 33.33 ft3
- Yards = 33.33 / 27 = 1.23 yd3, +10% waste = 1.36 yd3
Result: About 1.23 cubic yards, or 1.36 yd3 ordered with 10% waste
Example 2: 12 x 12 ft patio at 5 inches thick
- Area = 12 x 12 = 144 ft2
- Thickness = 5 / 12 = 0.417 ft
- Volume = 144 x 0.417 = 60 ft3
- Yards = 60 / 27 = 2.22 yd3, +10% waste = 2.44 yd3
Result: About 2.22 cubic yards, or 2.44 yd3 with waste
Concrete calculators by project type
Concrete bag yield & coverage
How much mixed concrete a standard bag yields, and how far a cubic yard goes at common slab depths.
| Bag size | Yield (cu ft) | Bags per cubic yard |
|---|---|---|
| 40 lb | 0.30 | 90 |
| 60 lb | 0.45 | 60 |
| 80 lb | 0.60 | 45 |
- 40 lb
- 60 lb
- 80 lb
Related guides
Concrete slab calculator FAQs
How many yards of concrete do I need for a 10x10 slab at 4 inches?
A 10 ft × 10 ft slab poured 4 inches thick is 100 ft² × 0.333 ft = 33.3 ft³, or about 1.23 cubic yards. Add roughly 10% for spillage and subgrade variation, so order about 1.35 yards.
How thick should a concrete slab be?
Most patios, walkways and floating slabs are poured 4 inches thick. Go to 5–6 inches where vehicles or heavy loads will sit on the slab, and add rebar or wire mesh for extra strength.
How many 80 lb bags of concrete are in a cubic yard?
One 80 lb bag yields about 0.60 ft³, and a cubic yard is 27 ft³, so you need roughly 45 bags of 80 lb mix per yard. That's about 60 bags at 60 lb or 90 bags at 40 lb.
Should I use bagged concrete or order ready-mix for a slab?
Bagged concrete is practical for slabs under about 1 cubic yard, but the labor of mixing dozens of bags adds up fast. For anything larger, ready-mix at roughly $125–$165 per yard delivered is usually cheaper and gives a more consistent pour.
How much does a concrete slab cost per yard?
Ready-mix concrete typically runs about $125–$165 per cubic yard delivered, before forms, rebar, finishing and labor. Small loads and short-load fees can raise the effective price on tiny slabs.
How much extra concrete should I order for a slab?
Order about 5–10% more than the calculated volume to cover uneven subgrade, form deflection and spillage. Running short mid-pour causes a cold joint, so it's safer to have a little extra.
What is the formula for concrete slab volume?
Multiply length in feet by width in feet, then by thickness in feet (inches divided by 12). That gives cubic feet, and dividing by 27 converts it to cubic yards. For a 10 x 12 slab at 4 inches: 10 x 12 x 0.333 = 40 ft3, or 1.48 yd3.
How do I convert cubic feet of concrete to cubic yards?
Divide the cubic feet by 27, because one cubic yard equals 27 cubic feet. So 54 cubic feet is 2 cubic yards. Ready-mix is almost always ordered in cubic yards, while bagged concrete is rated in cubic feet of yield.
How many 60 lb bags do I need for a small slab?
Each 60 lb bag yields about 0.45 cubic feet, so divide your total cubic feet by 0.45 and round up. A 4 x 4 ft pad at 4 inches is 16 x 0.333 = 5.33 ft3, which is about 12 bags. Bagged concrete gets expensive fast above roughly half a cubic yard.
Can I pour a 4-inch slab without rebar?
A 4-inch slab for a patio or shed floor is often reinforced with welded wire mesh or #3/#4 rebar on a grid rather than left plain. Reinforcement controls cracking and holds the slab together if the ground shifts, so it is strongly recommended for anything load-bearing. Fiber-mesh added to the mix is a common supplement, not a full replacement.
Does making the slab thicker change how much concrete I need?
Yes, volume scales directly with thickness. Going from 4 to 6 inches on a 10 x 10 slab raises the volume from 33.33 to 50 cubic feet, a 50% increase. Always confirm the thickness your project actually requires before ordering.
How do I calculate concrete for an irregular or L-shaped slab?
Break the shape into rectangles, calculate the volume of each separately, then add them together. For circles or curves, use the round or area calculator for those sections. Summing simple shapes is more accurate than trying to average one big rectangle.
How soon can I walk on a freshly poured slab?
Most slabs are firm enough for foot traffic after 24 to 48 hours, but concrete only reaches roughly 70% of strength at 7 days and full design strength at 28 days. Keep vehicles and heavy loads off until at least the 7-day mark. Curing conditions like temperature and moisture affect the timeline.