Concrete
Concrete Slab Calculator
Estimate concrete volume in cubic feet and cubic yards, bag count from your chosen bag yield, and optional material cost from your own price per bag.
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
35 ft x 8 ft patio slab demolition weight
A 35 ft by 8 ft slab at 4 in thick is about 93.33 cu ft before waste. At roughly 150 lb per cu ft, that is about 14,000 lb, or about 7 tons of concrete before debris, soil, or reinforcement.
- Area: 280 sq ft
- Thickness: 4 in
- Volume: about 93.33 cu ft
- Approximate concrete weight: about 7 tons
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.
When to use this concrete slab calculator
Use this page when you know slab length, width, and thickness and need concrete quantity in cubic feet, cubic yards, or bags before checking product labels.
The same volume output can also support rough weight planning for demolition or handling. Normal-weight concrete is often estimated around 150 lb per cubic foot, but actual weight varies with mix, moisture, aggregate, and debris.
Common slab volume and weight checks
Weight uses about 150 lb per cubic foot of normal-weight concrete. Use this as a rough material-planning check only.
| Slab scenario | Volume before waste | Approx. weight |
|---|---|---|
| 10 ft x 10 ft x 4 in | 33.33 cu ft | about 5,000 lb / 2.5 tons |
| 12 ft x 12 ft x 4 in | 48.00 cu ft | about 7,200 lb / 3.6 tons |
| 20 ft x 10 ft x 4 in | 66.67 cu ft | about 10,000 lb / 5.0 tons |
| 35 ft x 8 ft x 4 in | 93.33 cu ft | about 14,000 lb / 7.0 tons |
| 35 ft x 8 ft x 5 in | 116.67 cu ft | about 17,500 lb / 8.75 tons |
Do not treat rough weight as a dumpster, hauling, or disposal-cost recommendation. Check local rules and service limits separately.
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.
- Using volume output as a structural thickness, reinforcement, or disposal plan.
FAQ
Does this calculate ready-mix truck pricing?
No. It estimates volume and bag count only. Any material cost estimate uses only the bag price you enter.
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 concrete do I need for a 10x10 slab?
At 4 inches thick, a 10x10 slab needs about 33.33 cu ft before waste. With 5% waste, that is about 35.00 cu ft, 1.30 cu yd, or 59 bags at 0.60 cu ft per bag.
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 more concrete is a 6-inch slab than a 4-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 about 18 extra cubic feet before waste.
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.
Can I estimate concrete slab weight from this calculator?
You can use the cubic-foot result for a rough weight check by multiplying by about 150 lb per cubic foot for normal-weight concrete. This is only a planning approximation for material handling or demolition load, not a disposal quote.
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%.
How much does a concrete slab weigh?
For rough planning, normal-weight concrete is often estimated at about 150 lb per cubic foot. Multiply the calculator's cubic feet by 150 to approximate pounds, then divide by 2,000 for tons.
Can I use the calculator for demolition disposal planning?
Use it only for rough volume and weight. It does not include rebar, base gravel, soil, moisture, dumpster limits, local disposal rules, or hauling costs.
Should I include waste when estimating slab weight?
For a new pour, use the waste-adjusted volume for ordering. For demolition weight, use the actual slab dimensions and thickness first, then add separate allowances for debris, base material, or uncertainty.
Sources and review notes
Reviewed . These references support the unit conversions and planning assumptions on this page. Confirm the exact product label and local project requirements before buying material.
- Methodology and calculation boundaries HomeMaterialCalc. Site-wide formula, rounding, local-calculation, and user-entered price policies.
- NIST Guide to the SI, Appendix B: Conversion Factors National Institute of Standards and Technology. Length, area, and volume unit conversions used in calculator formulas.
- QUIKRETE Concrete Mix No. 1101 data sheet QUIKRETE. Planning yields for 40 lb, 60 lb, and 80 lb concrete bags.
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.