Concrete
Concrete Slab Calculator
Estimate concrete volume in cubic feet and cubic yards, the number of bags from your chosen bag yield, and a rough slab cost using your own price per bag plus national-average cost ranges for context.
Calculator inputs
Concrete Slab Calculator Results
Results update in your browser as you edit inputs. They are planning estimates, not complete shopping lists.
Enter project dimensions to calculate material quantity.
Assumptions
- The slab is treated as a simple rectangle with even thickness.
- Base material, forms, reinforcement, vapor barrier, and finishing materials are not included.
- Bag yield must come from the bag label or your supplier.
- Bag count is a planning estimate for concrete quantity, not a ready-mix quote, delivery estimate, or labor estimate.
- Check local building codes, permit requirements, product labels, and qualified professionals for structural or safety-critical work.
- Cost outputs use only the unit prices you enter.
Formula
- Volume in cubic feet = length x width x thickness in feet.
- Cubic yards = cubic feet divided by 27.
- Waste-adjusted volume = volume x (1 + waste factor).
- Bags = waste-adjusted cubic feet divided by selected bag yield.
How to Use the Concrete Slab Calculator
- Enter the slab length and width in feet.
- Enter thickness in inches, using the design or product requirement for your project.
- Choose the concrete bag yield from the product label or use the cubic-yard output for planning.
- Add waste for uneven excavation, form variation, overdig, or small losses.
- Use the result for material quantity only, then plan forms, reinforcement, base material, permits, and finishing supplies separately.
Concrete Slab Calculator Examples
10 ft x 10 ft slab at 4 in thick
A 10 ft by 10 ft slab at 4 in thick is about 33.33 cu ft, or 1.23 cu yd, before waste. With 5% waste, it is about 35.00 cu ft, or about 59 bags at 0.60 cu ft per bag.
- Area: 100 sq ft
- Thickness: 4 in
- With 5% waste: about 35.00 cu ft
- Bag count: about 59 at 0.60 cu ft/bag
12 ft x 12 ft shed slab at 4 in thick
A 12 ft by 12 ft slab at 4 in thick is about 48.00 cu ft before waste. With 5% waste, the estimate is about 50.40 cu ft, or 1.87 cu yd.
- Area: 144 sq ft
- Thickness: 4 in
- With 5% waste: about 50.40 cu ft
- Cubic yards: about 1.87
Units and Parameters Quick Reference
Concrete slab inputs
Concrete volume is very sensitive to thickness. A one-inch thickness change can add several bags on small slabs.
| Input | Unit | Planning note |
|---|---|---|
| Slab length and width | ft | Use finished form dimensions. |
| Slab thickness | in | Confirm the required thickness before estimating bags. |
| Bag yield | cu ft/bag | Use the yield printed on the bag. |
| Waste factor | % | Adds extra concrete after base volume is calculated. |
| Cubic yards | cu yd | Useful when comparing larger projects, but not a quote. |
This calculator does not include forms, rebar, mesh, gravel base, finishing tools, delivery, or labor.
Planning reference
Common concrete bag yields
Use the yield printed on your actual product label when possible. These are common planning approximations.
| Bag size | Approximate yield | Approximate bags per cubic yard |
|---|---|---|
| 40 lb bag | 0.30 cu ft | 90 bags |
| 60 lb bag | 0.45 cu ft | 60 bags |
| 80 lb bag | 0.60 cu ft | 45 bags |
Bag sizes and yields vary by product. The calculator lets you select a preset or enter a custom yield.
10 x 10 slab thickness reference
This quick reference uses a 10 ft by 10 ft slab, 5% waste, and an 80 lb bag yield of 0.60 cubic feet.
| Thickness | Concrete with waste | 80 lb bags |
|---|---|---|
| 3 in | 26.25 cu ft / 0.97 cu yd | 44 bags |
| 4 in | 35.00 cu ft / 1.30 cu yd | 59 bags |
| 5 in | 43.75 cu ft / 1.62 cu yd | 73 bags |
| 6 in | 52.50 cu ft / 1.94 cu yd | 88 bags |
Do not use this table for structural design. Confirm slab thickness, base preparation, reinforcement, and permits separately.
How much does a concrete slab cost?
This concrete slab calculator focuses on material quantity, but most projects also need a cost estimate. As a national-average planning range, a finished poured concrete slab in the United States often runs about $4 to $8 per square foot installed, covering concrete, basic site prep, and labor. A bare 4-inch slab can sit near the low end, while thicker slabs, decorative finishes, heavy reinforcement, or difficult access push toward the high end and beyond.
These ranges are national averages for context only, not a local quote. The calculator itself never uses live or local prices: if you want a material-only number, enter your own price per bag and it will multiply that by the estimated bag count. For a full installed price, always confirm with local ready-mix suppliers and concrete contractors, because regional labor rates, delivery minimums, permits, and site conditions vary widely.
Concrete slab cost ranges (national average)
Approximate U.S. national-average planning figures, not a local quote. Costs vary by region, thickness, finish, reinforcement, and access.
| Project / approach | Typical cost basis | Approx. cost range |
|---|---|---|
| 4-inch slab, bagged DIY (materials only) | Concrete bags only, your own labor | $2 to $4 per sq ft |
| 4-inch slab, ready-mix installed | Concrete, prep, and labor | $4 to $7 per sq ft |
| 6-inch slab, ready-mix installed | More concrete, prep, and labor | $5 to $9 per sq ft |
| 10x10 slab (100 sq ft) installed | Typical patio or shed slab | $400 to $800 total |
| 20x20 slab (400 sq ft) installed | Larger patio or garage pad | $1,600 to $3,600 total |
National-average ranges for planning only, not a local quote. Bagged DIY saves on materials but adds labor and time. Confirm installed pricing with local suppliers and contractors.
Concrete slab examples
Use the worked examples when you want to see the same formula with fixed slab dimensions before adjusting the calculator.
Example calculation
Example: 10 x 10 slab
A 10 ft by 10 ft slab at 4 in thick is about 33.33 cubic feet, or 1.23 cubic yards, before waste.
Common mistakes
- Entering slab thickness in feet instead of inches.
- Ignoring uneven excavation or overdig.
- Using bag count for a project better suited to ready-mix delivery.
FAQ
Does this calculate ready-mix truck pricing?
No. It estimates volume only. Any cost estimate uses your own entered bag price.
What bag yield should I enter?
Enter the cubic-foot yield printed on the concrete bag you plan to use.
Can this work as a concrete slab cost calculator?
Only as a user-entered material estimate. If you enter your own bag price, the calculator can multiply that price by the estimated bag count. It does not include ready-mix delivery, labor, contractor quotes, local prices, permits, forms, reinforcement, or finishing work.
How much does a concrete slab cost?
As a planning range, a poured concrete slab often runs about $4 to $8 per square foot installed in the United States, so a 10x10 slab is roughly $400 to $800 and a 20x20 slab is roughly $1,600 to $3,200. These are national-average figures, not a local quote: thickness, reinforcement, site prep, finish, and your region can push the price above or below this range. Bagged DIY concrete usually costs less in materials but more in labor and time. Always confirm with local suppliers and contractors.
How do I calculate concrete for an L-shaped or irregular slab?
Split the slab into simple rectangles, run each rectangle through the calculator at the same thickness, then add the cubic feet, cubic yards, and bag counts together. For a curved or rounded edge, use the smallest rectangle that fully contains the shape and rely on the waste factor, or estimate the area separately and treat it as one more rectangle of equal area.
How much does a 4-inch slab cost compared to a 6-inch slab?
A 6-inch slab uses 50% more concrete than a 4-inch slab of the same length and width, because volume scales directly with thickness. On a 10x10 slab that is roughly 18 cubic feet more, or about 30 extra 80 lb bags. Material cost rises by about the same 50% for the concrete itself, and a thicker slab may also need more base, more reinforcement, and more labor. The figures here are national-average planning numbers, not a local quote.
How many 80 lb bags of concrete are in a cubic yard?
A cubic yard is 27 cubic feet, and an 80 lb bag yields about 0.60 cubic feet, so a cubic yard takes about 45 bags of 80 lb concrete (27 / 0.60). A 60 lb bag yields about 0.45 cubic feet, so it takes about 60 bags per cubic yard, and a 40 lb bag takes about 90. Confirm the exact yield on your product label.
Should I use ready-mix delivery or bagged concrete?
Bagged concrete is convenient for small slabs, repairs, and posts, but mixing many bags is slow and the cost per cubic yard is usually higher. As a rough rule, once a slab needs more than about 1 to 2 cubic yards (roughly 50 to 90 bags), ready-mix delivery is often easier and more economical. Get a delivery quote from a local supplier, since this calculator does not provide ready-mix pricing.
Does a concrete slab need rebar?
It depends on the slab use, soil, and local code. Many patios and shed slabs use welded wire mesh or rebar to control cracking, while driveways and structural slabs usually require reinforcement sized by a professional. This calculator estimates concrete volume only and does not include rebar, mesh, or a reinforcement design. Confirm requirements with your local building department.
How many 80 lb bags do I need for a 10x10 slab?
At 4 inches thick and 0.60 cu ft per bag, a 10x10 slab needs about 56 bags before waste and about 59 bags with 5% waste.
Should I estimate concrete in bags or cubic yards?
Use bag count for small bagged projects and cubic yards when you need to discuss volume with a supplier. This site does not provide ready-mix quotes.
Does slab thickness change the estimate a lot?
Yes. Volume equals length x width x thickness, so increasing thickness from 4 in to 6 in increases volume by 50%.
Related Planning Tools
Use these related calculators when the same project needs another material estimate. Each link opens a browser-based tool with its own assumptions and formulas.
Related Material Guides
Read these planning guides when you want more context for the assumptions, depth, thickness, waste factor, or bag-yield inputs used by this calculator.
Compare concrete volume and bag counts for a 10 ft by 10 ft slab at common planning thicknesses.
Concrete guide Recommended Concrete Slab Thickness for Patios, Sheds, and DrivewaysLearn how slab thickness affects concrete volume, bag count, and planning assumptions before estimating material quantity.