Concrete Slab Calculator
Use our free concrete slab calculator to determine exactly how much concrete you need for any slab project. Designed for contractors, estimators, and DIY homeowners, this tool calculates concrete volume (cubic yards or meters), the number of bagged concrete needed (60 lb and 80 lb bags), and estimated cost from your project dimensions. Enter your length, width, thickness, and waste percentage to get instant, accurate results.
How to use this calculator.
- Select your unit system. Choose imperial (feet, inches) or metric (meters, centimeters) using the toggle above.
- Enter your slab dimensions. Input the length, width, and thickness of your slab. For irregular shapes, break the area into rectangles and calculate each separately.
- Set the waste factor. A 10% waste factor accounts for form leakage, subgrade variation, and spillage. Increase to 15% for irregular shapes or difficult access.
- Review your results. The calculator displays total concrete volume, the number of 60 lb and 80 lb bags needed, and an estimated cost at $150 per cubic yard.
- Print or save. Use your browser's print function for a jobsite-ready summary.
How do you calculate concrete volume, bags, and cost?
To calculate concrete volume, multiply the area (length × width) by the thickness, convert to cubic yards, then apply the waste factor.
Volume (cu yd) = Length (ft) × Width (ft) × Thickness (in) × (1 + Waste%) ÷ 324
60 lb Bags = Volume (cu yd) × 27 ÷ 0.45
80 lb Bags = Volume (cu yd) × 27 ÷ 0.60
Where:
- 324 comes from 12 (inches to feet) × 27 (cubic feet to cubic yards).
- 0.45 ft³ is the yield of one 60 lb bag of concrete.
- 0.60 ft³ is the yield of one 80 lb bag of concrete.
- Waste factor is expressed as a percentage (10% = 0.10).
- Cost is estimated at $150 per cubic yard (ready-mix, US average).
Example calculation.
Let's calculate the concrete needed for a patio slab.
Scenario: You're pouring a patio that is 20 ft long, 15 ft wide, and 4 inches thick, with a 10% waste factor.
- Calculate the base volume: 20 ft × 15 ft × 4 in ÷ 324 = 3.70 cu yd
- Apply waste factor: 3.70 × 1.10 = 4.07 cu yd → order 4.5 cu yd
- 60 lb bags needed: 4.07 × 27 ÷ 0.45 ≈ 244 bags
- 80 lb bags needed: 4.07 × 27 ÷ 0.60 ≈ 183 bags
- Estimated cost: 4.07 × $150 = $610.50
Result: You need approximately 4.5 cubic yardsof concrete for this patio slab. That's about 183 eighty-pound bags if you're mixing by hand, or one short-load ready-mix truck.
Tips from the field.
- Always round up to the nearest 0.5 yard. It is far cheaper to have a little extra concrete than to call a short-load truck for a second delivery. A cold joint ruins a slab.
- Bagged concrete is for small jobs only. If you need more than 30-40 bags, call a ready-mix truck. Bags are 2-3× more expensive per cubic yard than ready-mix.
- Check your subgrade. A well-compacted, level gravel base prevents cracking and ensures uniform slab thickness. Low spots consume extra concrete.
- Minimum thickness for foot traffic is 4 inches. For vehicle traffic (driveways), use at least 5-6 inches with reinforcement.
- Order your concrete early in the day. If you run short, the batch plant is still open for a top-up load. Afternoon pours that run short mean an overnight cold joint.
Sources: Standard concrete density (150 lb/ft³) and slab thickness guidance reflect ACI 318 (Building Code Requirements for Structural Concrete) and ACI 302 (Guide for Concrete Floor and Slab Construction). Consult a licensed structural engineer for load-bearing designs.
Disclaimer: This calculator provides estimates for planning purposes only. Actual material requirements vary based on site conditions, subgrade levelness, form accuracy, and spillage. For structural slabs or load-bearing applications, consult a licensed structural engineer. PavementCalculator.com assumes no liability for material overages or shortages.
Cost, price, and density figures on this page reflect approximate U.S. averages as of August 2026 and vary by region, supplier, and material availability.
Frequently asked questions.
How much concrete do I need for a 20x20 slab?
A 20 ft by 20 ft slab at 4 inches thick requires approximately 4.94 cubic yards of concrete (20 × 20 × 4 ÷ 324 = 4.94). Add 10% for waste and uneven subgrade: order 5.5 cubic yards.
How many 80lb bags of concrete per yard?
It takes 45 bags of 80 lb concrete mix to make one cubic yard. An 80 lb bag yields approximately 0.6 cubic feet of concrete. One cubic yard equals 27 cubic feet, so 27 ÷ 0.6 = 45 bags.
How thick should a concrete slab be?
Residential concrete slabs for patios and walkways should be 4 inches thick. Driveways require 4-5 inches. Garage floors need 4-6 inches depending on vehicle weight. Always use wire mesh or rebar reinforcement.
How much does a concrete slab cost?
A concrete slab costs $4 to $8 per square foot installed on average, including materials and labor. A 20x20 patio slab (400 sq ft) typically costs $1,600-$3,200. The concrete material alone costs about $150 per cubic yard delivered.
How do I calculate concrete slab weight?
Concrete weighs approximately 150 lb per cubic foot (4,050 lb per cubic yard). To calculate slab weight: multiply length × width × thickness (in feet) × 150 lb. For a 20×20×4-inch slab: 20 × 20 × (4÷12) × 150 = 20,000 lb (about 10 tons). Use our calculator's volume output × 4,050 for weight in pounds.
What PSI concrete do I need for a slab?
For residential patios and walkways, 3,000-3,500 PSI concrete is standard. Driveways and garage floors need 3,500-4,000 PSI. Commercial and industrial floors require 4,000-5,000 PSI. Higher PSI means stronger, more durable concrete.
How many yards of concrete do I need calculator?
Use our calculator above: enter your slab's length and width in feet and thickness in inches. The formula is (length × width × thickness in inches) ÷ 324 = cubic yards. Always order 10% extra to account for uneven subgrade and spillage.
How many bags of concrete do I need for a 10×10 slab?
A 10 ft × 10 ft slab at 4 inches thick requires approximately 1.36 cubic yards of concrete with a 10% waste factor. That's about 61 eighty-pound bags or 82 sixty-pound bags. Use our calculator above for your exact dimensions.
How much does a yard of concrete cover?
One cubic yard of concrete covers 81 square feet at 4 inches thick, 65 square feet at 5 inches, or 54 square feet at 6 inches. The formula is: Coverage (sq ft) = 324 ÷ Thickness (inches).
Bagged concrete vs. ready-mix: which is cheaper?
Ready-mix concrete typically costs $130-$180 per cubic yard delivered. Bagged concrete costs $4-$7 per 80 lb bag, which works out to $180-$315 per cubic yard. For any job over 1 cubic yard, ready-mix is almost always cheaper and produces a better result.