Concrete Project Calculator & Estimator
Planning a pour shouldn't require a separate notepad for unit conversions. This calculator handles the math for you, allowing you to input Feet and Inches directly without converting decimals first. Whether you are filling sonotubes for a deck or pouring a garage slab, use this tool to generate an instant shopping list for 80lb, 60lb, or 50lb bags, or determine the exact Cubic Yardage for a ready-mix truck order.
Recommended: 5-10% to account for spillage and uneven subgrades
Enter dimensions and click Calculate
Note: These calculations include your specified waste factor to account for uneven subgrades or spillage. For projects requiring more than 2 cubic yard of concrete, it is often more cost-effective to order a truck delivery from a ready-mix supplier rather than mixing bags by hand.
Steps to a Perfect Shopping List
This tool is designed to move you from "Estimation" to "Checkout" as fast as possible. Here is how to use the specific features:
- The "Feet & Inches" Advantage: Most calculators force you to convert "4 inches" into "0.33 feet" before typing. We built this tool with split inputs. If your form is 10 feet and 6 inches long, just type 10 in the
ftbox and 6 in theinbox. We handle the conversion in the background. - Switching for Footings (The "Post" Tab): Don't use the Slab tab for fence posts! Click the "Post / Cylinder" tab to calculate round volumes. This uses the Pi formula automatically, which is critical for accurate sonotube or auger hole calculations.
- Using the PDF Export: Once you have your results, click "Download Results as PDF" at the bottom. This saves a clean, formatted document to your phone that includes your specific dimensions and bag counts—perfect for showing the associate at the lumber yard loading dock.
Interpreting Your Results
- Bag Counts: We display counts for 80lb, 60lb, and 50lb bags simultaneously. You do not need to choose one before calculating; simply look at the row that matches the stock at your local store.
- The "Truck" Threshold: If your "Total Cubic Yards" result is > 2.0, the tool calculates that you would need nearly 90 bags of concrete. At this volume, we recommend calling a supplier for a "short load" delivery to save labor.
Project Planning Examples
Scenario A: The "Sonotube" Deck Footing
- The Job: You are digging 12 holes for a new composite deck.
- The Tool Setting: Switch to Post / Cylinder.
- The Input: Diameter: 12 inches. Depth: 4 feet (48 inches). Quantity: 12.
- The Output: ~1.47 Cubic Yards.
- The Decision: This equals roughly 66 bags (80lb). Since you can pour these one by one at your own pace, this is a perfect DIY bag project.
Scenario B: The Driveway Extension
- The Job: Widening a driveway by 3 feet.
- The Tool Setting: Stay on Slab / Rectangular.
- The Input: Length: 20 ft. Width: 3 ft. Thickness: 5 inches.
- The Output: ~0.97 Cubic Yards.
- The Decision: This is about 44 bags (80lb). It is borderline, but manageable with a rented mixer.
Common Questions & Expert Tips
Q: Does this calculator include the weight of the water?
A: No, the bag counts are based on the cured volume yield (how much space the concrete takes up after mixing). This ensures you buy enough dry bags to fill the hole, regardless of water weight added later.
Q: Why do I see a "Waste Factor" default of 5%?
A: We pre-fill this field because "perfect" pours are impossible. The 5% accounts for the concrete that stays stuck inside your mixer or spills over the edge of the form. You can set this to 0% if you want the exact mathematical volume, but we don't recommend it for purchasing.
Q: My thickness is in fractions (e.g., 3.5 inches). Can I type that?
A: Yes! The inputs support decimals. If you are pouring a standard 2x4 form (which is actually 3.5 inches high), simply type 3.5 into the thickness box for precision accuracy.
Q: How long before I can walk or drive on my new slab?
A: While concrete hardens noticeably within hours, it takes time to reach full strength. Generally, you can walk on it after 24 to 48 hours, but you should wait at least 7 days before driving a vehicle on a driveway, and 28 days for it to reach its maximum "fully cured" strength.
Q: Should I include my gravel base in the "Thickness" input?
A: No. Only enter the thickness of the actual concrete. For example, if you dig 8 inches deep to install 4 inches of gravel and 4 inches of concrete, you should only enter 4 inches into the calculator. Including the gravel depth will double your estimated cost unnecessarily.
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This calculator/tool is provided for estimation purposes only. Results vary based on material quality, site conditions, waste factors, and more. Definitive Calc is not responsible for material shortages or structural integrity. Always verify measurements and always consult a professional contractor and/or engineer.