Concrete Calculator
Calculate concrete for slabs, footings, columns, and stairs in cubic yards or bags
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Typical: 4" for patios, 4-6" for driveways
Quick Answer
To calculate concrete: Length × Width × Depth (all in feet) ÷ 27 = Cubic Yards. A 10×10 slab at 4" thick needs 1.23 cubic yards or about 45 80-lb bags.
Whether you're pouring a patio slab, setting deck footings, casting columns, or building steps, this concrete calculator helps you determine exactly how much concrete you need. Get results in cubic yards for ready-mix delivery or bag counts for DIY projects.
Dr. Snezana Lawrence
Mathematical Historian
15+ years experience
PhD from Yale University. Published mathematical historian ensuring precision in all calculations.
Education
PhD in Mathematical History - Yale University
Concrete Coverage Chart
| Thickness | Sq Ft/Cu Yd | 80 lb Bags/Sq Ft |
|---|---|---|
| 3 inches | 108 | 0.4 |
| 4 inches | 81 | 0.5 |
| 5 inches | 65 | 0.6 |
| 6 inches | 54 | 0.8 |
| 8 inches | 41 | 1.0 |
Bag Size Comparison
| Bag Size | Cu Ft/Bag | Bags/Cu Yd |
|---|---|---|
| 40 lb bag | 0.30 | 90 |
| 60 lb bag | 0.45 | 60 |
| 80 lb bag | 0.60 | 45 |
Recommended Thicknesses
| Project Type | Thickness | Notes |
|---|---|---|
| Walkways/Patios | 4" | Light foot traffic only |
| Driveways (cars) | 4-5" | Standard passenger vehicles |
| Driveways (trucks) | 5-6" | Heavy vehicles, RVs |
| Garage Floors | 4-6" | Depends on vehicle weight |
| Shed Foundations | 4" | Light structures |
| Post Footings | 12-24" | Below frost line |
Concrete Pro Tips
- • Order 10% extra to account for spillage and uneven subgrade
- • For slabs over 1" thickness, pour in one continuous session
- • Compact and level subgrade before pouring
- • Use 4000 PSI mix for driveways and high-traffic areas
- • Add fiber mesh or rebar for slabs over 100 sq ft
- • Pour when temperature is 50-85°F for proper curing
- • Keep concrete moist for 7 days for maximum strength
- • Don't add extra water - weakens the final product
Frequently Asked Questions
How many bags of concrete do I need?
For a 10×10 slab at 4" thick: 10×10×0.33÷27 = 1.23 cubic yards. At 0.015 cu yd per 80 lb bag, you need about 82 bags.
How much does a cubic yard of concrete cover?
At 4" thick: 81 sq ft. At 6" thick: 54 sq ft. At 3" thick: 108 sq ft per cubic yard.
Should I use bagged or ready-mix concrete?
Under 1 cubic yard: bags are practical. 1-3 yards: either works. Over 3 yards: ready-mix is more economical and faster.
How thick should a concrete slab be?
Patios/walkways: 4". Driveways: 4-6". Garage floors: 4-6". Heavy equipment: 6-8". Building footings: per code.
How much extra concrete should I order?
Order 10% extra for waste, spillage, and uneven subgrade. For complex forms or slopes, add 15%.
How many 80 lb bags in a cubic yard?
About 45 80-lb bags equal 1 cubic yard. 60-lb bags: 60 per yard. 40-lb bags: 90 per yard.
What is the concrete mix ratio?
Standard mix: 1:2:3 (cement:sand:gravel) by volume. High strength: 1:1.5:3. Pre-mixed bags have correct ratios.
How long does concrete take to cure?
Initial set: 24-48 hours (can walk on). Full strength: 28 days. Keep moist for first 7 days for proper curing.
Dr. Snezana Lawrence
Mathematical Historian | PhD from Yale
Dr. Lawrence is a published mathematical historian with a PhD from Yale University. She ensures mathematical precision and accuracy in all our calculations, conversions, and academic score calculators. Her expertise spans computational mathematics and educational assessment.
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