Home / All calculators / Concrete slab
CONCRETE & MASONRY / MATERIAL ESTIMATOR
Concrete Slab Calculator.
Calculate the concrete needed for a patio, driveway, shed pad, or slab. Enter the dimensions, adjust the waste allowance, and estimate cubic yards, bag counts, and optional ready-mix cost.
Your concrete estimate
Visualize the slab dimensions before calculating volume
Enter your measurements below to calculate material quantities.
How to use the calculator
- Measure length and width. Enter the slab dimensions in feet.
- Enter thickness. Use inches, based on the design requirements of your project.
- Set your waste allowance. Adjust the percentage to account for uneven ground, spillage, and measurement uncertainty.
- Review quantity and cost. Compare cubic yards with bag counts, and enter local ready-mix pricing when available.
How concrete volume is calculated
Cubic feet = length (ft) × width (ft) × thickness (in) ÷ 12
Divide cubic feet by 27 to convert to cubic yards. Apply the waste allowance to the calculated volume before planning an order.
Typical bag yields vary by product. As a rough reference, a 60 lb bag yields about 0.45 cubic feet, while an 80 lb bag yields about 0.60 cubic feet. Check the manufacturer’s stated yield.
10 × 10 ft slab, 4 inches thick
100 sq ft × (4 ÷ 12) ft = 33.33 cubic feet. This is 1.23 cubic yards before waste, or approximately 1.36 cubic yards with 10% extra.
At a nominal yield of 0.60 cubic feet per 80 lb bag, that is approximately 62 bags with 10% waste.
How much concrete do I need for a slab?
The amount of concrete depends on the slab’s footprint and its thickness. Square footage measures the surface; concrete volume measures the space to fill. Two slabs with identical floor areas can require different amounts of concrete when their thicknesses differ.
Enter your slab dimensions in the calculator above for an instant estimate, then use this guide to understand how quantities, bag yields, waste allowances, and delivery choices affect the result.
Concrete slab volume formula
Volume in cubic yards = cubic feet ÷ 27. To allow for waste, multiply the volume by 1 + (waste percentage ÷ 100).
Thickness is divided by 12 because it is entered in inches while the other dimensions are in feet. One cubic yard contains 27 cubic feet. Ready-mix suppliers commonly quote concrete by cubic yard.
Concrete slab size chart
Examples below assume a uniform 4-inch thickness. The final column includes 10% additional material; quantities are rounded.
| Slab size | Area | Base yd³ | With 10% extra |
|---|---|---|---|
| 8 × 8 ft | 64 sq ft | 0.79 | 0.87 yd³ |
| 10 × 10 ft | 100 sq ft | 1.23 | 1.36 yd³ |
| 12 × 12 ft | 144 sq ft | 1.78 | 1.96 yd³ |
| 12 × 20 ft | 240 sq ft | 2.96 | 3.26 yd³ |
| 20 × 20 ft | 400 sq ft | 4.94 | 5.43 yd³ |
How thickness affects concrete quantity
For a given footprint, volume increases directly with thickness. A 6-inch slab uses 50% more concrete than a 4-inch slab with the same area. Choose slab thickness based on loads, soil conditions, reinforcement, applicable codes, and an appropriate design—not simply on a generic rule of thumb.
| Thickness | 10 × 10 ft, base | With 10% extra |
|---|---|---|
| 3 in | 0.93 yd³ | 1.02 yd³ |
| 4 in | 1.23 yd³ | 1.36 yd³ |
| 5 in | 1.54 yd³ | 1.70 yd³ |
| 6 in | 1.85 yd³ | 2.04 yd³ |
How many bags of concrete do I need?
Bag counts depend on the manufacturer’s stated yield, not just bag weight. As planning approximations, a 60 lb bag may yield about 0.45 cubic feet and an 80 lb bag about 0.60 cubic feet. Divide the adjusted concrete volume in cubic feet by the yield per bag and round up.
Worked example: A 10 × 10 ft slab at 4 inches thick requires about 36.67 cubic feet after adding 10% waste. That works out to approximately 82 bags of 60 lb mix or 62 bags of 80 lb mix at those nominal yields. Confirm actual yield on the bag.
Bagged concrete vs. ready-mix
Bagged concrete
Bagged concrete is convenient for smaller pads, repairs, and places where truck access is difficult. For large quantities, consider the time needed to transport, mix, and place dozens of bags. Include mixing equipment and labor when comparing costs.
Ready-mix delivery
Ready-mix can be practical for larger continuous pours. Ask suppliers about the price per cubic yard, minimum loads, short-load fees, delivery charges, truck access, and waiting time. Total costs depend on your location and project, so neither option is automatically cheaper in every case.
Choosing a concrete waste allowance
A 10% allowance is a useful planning starting point, but it is not universal. Uneven subgrade, form tolerances, spillage, and complex layouts can change the amount needed. Carefully measured and prepared sites may require less. Do not use a waste allowance to hide significant missing volumes such as thickened edges or footings.
Estimating irregular or L-shaped slabs
Split an irregular footprint into non-overlapping rectangles. Calculate each part’s volume separately, then add the totals. If thickness varies, calculate each thickness zone individually. For thickened perimeter edges, footings, or steps, add their additional volume rather than treating the entire slab as one uniform thickness.
Common concrete estimating mistakes
- Entering inches as feet when specifying thickness.
- Ordering concrete using square footage without considering slab depth.
- Forgetting to convert cubic feet to cubic yards.
- Assuming every bag has an identical yield.
- Overlooking uneven excavation, thickened edges, or separate footings.
- Confusing a quantity estimate with structural engineering advice.
Before ordering concrete
Recheck inside form dimensions, confirm slab thickness and any extra sections, choose a realistic allowance, and discuss mix specifications and delivery logistics with your supplier. For load-bearing slabs, foundations, or uncertain ground conditions, consult a qualified professional and follow local requirements.
Related planning tools: concrete slab cost calculator, gravel calculator, and cubic yard calculator.
Turn your estimate into an ordering plan
A volume result is only the starting point. Confirm the details below before purchasing concrete or scheduling a delivery.
01. Check the footprint
Measure inside the forms. Split L-shaped slabs into rectangles and add any thickened edges separately.
02. Confirm the depth
Use the specified structural thickness. Low spots and uneven subgrade can increase actual volume.
03. Choose the supply method
Compare bag quantities with ready-mix delivery, including handling, labor, access, and minimum charges.
04. Verify the order
Discuss mix specifications, expected yield, delivery timing, and contingency with your supplier.
“I have a rectangular slab measuring 10 by 10 feet at a specified 4-inch thickness. My estimated volume is 1.23 cubic yards before allowance, or about 1.36 cubic yards with 10% extra. What mix, minimum order, and delivery charges apply?”
Concrete slab FAQs
Understand quantities and ordering decisions before starting your pour.
How many bags of concrete do I need for a 10 × 10 slab?
At 4 inches thick, a 10 × 10 ft slab requires approximately 1.36 cubic yards including 10% waste, equivalent to about 62 bags of 80 lb mix at a nominal 0.60 cu ft yield per bag.
Should I order extra concrete?
Allowing for waste can help account for spills, variations in slab depth, and uneven subgrade. Ten percent is a useful planning starting point, but the right allowance depends on the job.
Should I use bagged concrete or ready-mix?
Bagged concrete can be practical for smaller pours. Larger pours may be easier with ready-mix delivery. Compare total delivered prices, minimum order charges, equipment, and labor in your area.
Does the calculator determine structural slab thickness?
No. It calculates material quantities from the dimensions you provide. Structural thickness, reinforcement, base preparation, and local code requirements should be confirmed with a qualified professional where needed.