Enter your project dimensions to instantly calculate the concrete needed.
How the Concrete Calculator Works
Choose Slab or Round Post/Column
Enter Dimensions
Calculate Total Volume Needed
Convert to Cubic Yards or Bags
What is a Concrete Calculator?
A Concrete Calculator estimates how much concrete you need for a slab, footing, sidewalk, or post/column, converting your project's dimensions into either cubic yards (for ready-mix truck delivery) or the number of pre-mixed bags (for smaller DIY projects). Concrete is unforgiving once poured, so accurately estimating the quantity beforehand helps you avoid a costly mid-pour shortage or an expensive over-order.
The Concrete Calculation Formulas
Slab Volume (ft³) = Length × Width × (Thickness in inches ÷ 12)
Round Post Volume (ft³) = π × (Diameter/2)² × Height (in feet, diameter converted to feet)
Cubic Yards = Total Volume (ft³) ÷ 27
For bagged concrete, the calculator divides your total volume by the yield of your selected bag size — a standard 60lb bag yields about 0.45 cubic feet of mixed concrete, while an 80lb bag yields about 0.6 cubic feet.
Recommended Slab Thickness by Application
| Application | Recommended Thickness |
|---|---|
| Sidewalk/walkway | 4 inches |
| Standard patio | 4 inches |
| Residential driveway | 4 – 6 inches |
| Garage floor | 4 – 6 inches |
| Heavy-duty/RV parking | 6 – 8 inches |
Ready-Mix Delivery vs Bagged Concrete
- Ready-mix truck delivery: Most cost-effective and practical for projects requiring more than about 1 cubic yard, delivered fresh and ready to pour.
- Bagged concrete (mix yourself): Better suited for small projects like fence post footings or small patches, where a full truck delivery wouldn't be practical or economical.
Most ready-mix suppliers have delivery minimums, so if your project falls right around 1 cubic yard, compare the cost of a minimum truck delivery against the cost of the equivalent number of bags before deciding.
Tips for a Successful Concrete Pour
- Order 5-10% extra: Uneven sub-grade and minor spillage typically require a small buffer above the exact calculation.
- Prepare the sub-base properly: A compacted gravel base beneath your slab improves drainage and reduces cracking risk.
- Plan for control joints: Cutting joints at regular intervals helps control where cracking naturally occurs as concrete cures.
- Pour in appropriate weather: Avoid pouring in extreme heat, cold, or rain, which can all affect proper curing.
Frequently Asked Questions
How many bags of concrete do I need for fence posts?
Switch to the "Round Column/Post" tab, enter your post hole diameter and depth, and the number of posts, to get an accurate bag count for all your post footings at once.
Should I order ready-mix or use bags for my project?
For projects requiring more than roughly 1 cubic yard of concrete, ready-mix truck delivery is typically more cost-effective and less labor-intensive than mixing dozens of bags by hand.
How long does concrete take to cure?
Concrete typically reaches enough strength for foot traffic within 24-48 hours, but full curing strength takes approximately 28 days.
What if my slab has an irregular shape?
Break the irregular area into simple rectangular sections, calculate each separately using this tool, and add the volumes together for your total concrete needed.