Concrete Calculator
Calculate concrete volume, bag counts, and costs for slabs, circles, columns, footings & walls.
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What is a Concrete Calculator?
A Concrete Calculator is an essential tool for anyone planning construction, renovation, or DIY projects involving concrete. It helps you accurately estimate the amount of concrete needed for your project, preventing costly over-ordering or the frustration of running short during a pour.
Our advanced calculator goes beyond simple volume calculations. It supports 5 different shapes (rectangular slabs, circular pads, columns, footings, and walls), works with both Imperial and Metric units, and includes a wastage factor to account for real-world conditions like spillage and uneven ground.
Whether you're building a patio, driveway, foundation, or fence post, this calculator gives you exact bag counts for 80lb, 60lb, and 50kg bags, plus site-mix quantities (cement, sand, gravel) if you prefer mixing on-site. You can even enter the price per bag to get an instant cost estimate for budgeting.
5 Shape Types
Calculate for slabs, circles, columns, footings, and walls. Each shape uses precise mathematical formulas for accurate results.
Imperial & Metric
Switch between feet/inches and meters/cm instantly. All calculations update in real-time without losing your data.
Cost Estimation
Enter price per bag to get total material cost instantly. Budget your project accurately before buying materials.
Download & Print
Save calculations as text file or print them. Take your shopping list directly to the hardware store.
How to Use the Concrete Calculator
Select Your Shape
Choose from 5 types: Slab/Patio for flat rectangles, Circle/Round for fire pits, Column for pillars, Footing for foundations, or Wall for vertical slabs.
Choose Unit System
Select Imperial (feet and inches) for US measurements or Metric (meters and cm). You can switch anytime.
Enter Measurements
Input dimensions (length, width, thickness). Results calculate instantly as you type - no button needed!
Adjust & Download
Set wastage factor (5-25%), optionally add price per bag for cost estimate. Download or Print your results.
Understanding Concrete Volumes
Concrete is measured in cubic yards (or cubic meters). One cubic yard is 27 cubic feet - enough to pour a 9×9 foot slab at 4 inches thick.
Our calculator handles the complex math automatically:
- Rectangular Slabs: Length × Width × Thickness
- Circular Pads: π × (Diameter/2)² × Thickness
- Columns: π × (Diameter/2)² × Height
Bags vs Ready-Mix
Bagged concrete is ideal for small projects (under 2 cubic yards). It's convenient, requires no minimum order, and can be stored.
Ready-mix trucks deliver up to 10 cubic yards and are more economical for larger pours. Most companies require 1-2 yard minimums.
- Under 1 cubic yard: Use bags (about 45-60 bags)
- 1-3 cubic yards: Either option works
- Over 3 cubic yards: Ready-mix is usually better
Recommended Concrete Thickness
Choosing the right thickness is critical. Too thin and concrete cracks under load; too thick wastes money. Here's a quick reference:
| Application | Thickness | Notes |
|---|---|---|
| Sidewalks & Paths | 4 inches | Pedestrian traffic only |
| Patios | 4 inches | Standard residential |
| Driveways (Cars) | 4-5 inches | Regular vehicles |
| Driveways (Trucks/RVs) | 5-6 inches | Heavy vehicles |
| Garage Floors | 4-6 inches | Depends on use |
| Footings | 8-12 inches | Check local codes |
Pro Tips for Your Concrete Project
Always order 10% extra - Running short during a pour is worse than having leftover.
Prepare subgrade - Compact soil and add 4-inch gravel base for drainage.
Don't add too much water - Soupy concrete is weaker. Follow package instructions.
Cure properly - Keep concrete moist for 5-7 days by covering or spraying.
Add control joints - For slabs over 10 feet to prevent random cracking.
Safety first - Wet concrete is caustic. Wear gloves and eye protection.
Common Use Cases
Driveways & Parking
Calculate concrete for car and truck driveways. Use 4-6 inch thickness. Plan expansion joints every 10-12 feet.
Patios & Outdoor
Perfect for backyard patios, BBQ areas, and outdoor kitchens. Standard 4-inch thickness works for most uses.
Foundations & Posts
Calculate footings for decks, sheds, fence posts, and mailbox bases. Use Column shape for post holes.
Frequently Asked Questions
How do I calculate concrete for a slab?
To calculate concrete for a rectangular slab, multiply Length × Width × Thickness. Enter dimensions in our calculator: length and width in feet, thickness in inches. The calculator converts everything to cubic yards and shows how many bags you need. For a 10×10 ft patio with 4-inch thickness, you'd need about 1.23 cubic yards or roughly 56 bags of 80lb concrete.
How many bags of concrete do I need?
The number of bags depends on your project size. An 80lb bag yields 0.6 cubic feet, a 60lb bag yields 0.45 cubic feet. For example, a 100 sq ft slab at 4 inches thick needs about 33 cubic feet, which equals approximately 55 bags of 80lb concrete. Our calculator does this math instantly and adds wastage for accuracy.
What's the difference between 60lb and 80lb concrete bags?
The main difference is coverage and handling. An 80lb bag covers 0.6 cubic feet and is more economical per cubic foot but heavier to carry. A 60lb bag covers 0.45 cubic feet, is easier to handle, but you'll need more bags. For large projects, 80lb bags save money; for small DIY projects or if you have back problems, 60lb bags are more manageable.
How thick should my concrete be?
Thickness depends on the application: 4 inches is standard for patios, sidewalks, and light-duty driveways. 5-6 inches is recommended for driveways with heavy vehicles or RVs. 6-8 inches for heavy equipment areas. Footings typically need 8-12 inches. Thicker concrete is stronger but uses significantly more material.
How do I calculate concrete for a round/circular slab?
For circular concrete like round patios or fire pit bases, use the formula: π × radius² × thickness. Enter the diameter in our Circle/Round calculator - we automatically calculate the area using pi (3.14159). For a 10-foot diameter circle at 4 inches thick, you need about 0.97 cubic yards or 44 bags of 80lb concrete.
What is the concrete mix ratio for site mixing?
The standard mix ratio is 1:2:4 (1 part cement, 2 parts sand, 4 parts gravel/aggregate) by volume. For M20 grade concrete, use 1:1.5:3 ratio. Our calculator shows site-mix quantities: approximately 6-7 bags of cement, 0.4 cubic feet of sand, and 0.8 cubic feet of gravel per cubic foot of concrete.
How much concrete does a ready-mix truck hold?
A standard concrete mixer truck holds 10 cubic yards of ready-mix concrete. Mini mixers hold 2-5 cubic yards. For projects over 3 cubic yards, ready-mix delivery is often more economical than bags. Most companies have minimum orders of 1-2 cubic yards.
Should I add extra concrete for waste?
Yes! Always add 5-10% extra for spillage, uneven ground, and over-excavation. Our calculator has a wastage slider (0-25%) that automatically adjusts quantities. For uneven terrain, use 15-20% extra. It's better to have leftover concrete than to run short mid-pour.
How do I convert cubic feet to cubic yards?
Divide cubic feet by 27 to get cubic yards. For example, 54 cubic feet ÷ 27 = 2 cubic yards. Our calculator shows both units automatically. To convert from cubic meters, multiply by 1.308 to get cubic yards or 35.3147 for cubic feet.
What affects concrete curing time?
Concrete reaches 70% strength in 7 days and full strength in 28 days. Factors affecting curing: Temperature (faster in warm weather, slower in cold), moisture (keep it damp for first week), mix water ratio (too much water weakens concrete), and thickness. Don't walk on fresh concrete for 24-48 hours, and don't drive on it for at least 7 days.