Formula Used
Rectangular pour: Volume = Length × Width × Thickness ÷ 12.
Round post hole: Volume = π × Radius² × Depth × Quantity.
Total volume: Concrete Volume + Waste Volume.
Bags needed: Ceiling of Total Volume ÷ Bag Yield.
Total cost: Bags Needed × Price Per Bag.
Example Data Table
| Project | Size | Bag Size | Waste | Approx Bags |
|---|---|---|---|---|
| Small pad | 4 ft × 4 ft × 4 in | 80 lb | 10% | 10 |
| Walkway | 12 ft × 3 ft × 4 in | 60 lb | 10% | 30 |
| Post holes | 4 holes, 12 in × 36 in | 80 lb | 10% | 7 |
How to Use This Calculator
Select the project type first. Use slab or footing for rectangular areas. Use post hole for round holes. Enter length and width in feet. Enter thickness in inches. For post holes, enter diameter and depth in inches. Add the quantity if you have repeated areas or holes. Pick a bag size. Use custom yield when your product label shows a different yield. Add waste for spills, uneven subgrade, and overdigging. Enter the price per bag. Press calculate. Review the volume, rounded bag count, cost, and chart.
Sakrete Bag Planning Guide
Concrete work needs careful measuring. A small error can change the number of bags. This calculator helps you plan before mixing starts. It converts project dimensions into cubic feet. Then it compares that volume with the yield of each bag.
Most small projects are simple rectangles. These include pads, shed bases, steps, and narrow footings. Their volume comes from length, width, and thickness. Thickness is entered in inches because many pours are planned that way. The calculator converts it into feet automatically.
Post holes use a different shape. They are cylinders. The calculator uses diameter and depth to estimate each hole. It then multiplies the result by the number of holes. This is useful for fence posts, mailbox posts, deck supports, and small sign bases.
Waste is important. Soil may be uneven. Forms can bow outward. Holes may be wider than planned. Some material may stay in the mixer or wheelbarrow. A waste allowance gives a safer buying number. Ten percent is a common starting point for many small jobs.
Bag yield also matters. Different bag weights cover different volumes. Always check the product label before buying. Moisture, compaction, and job conditions can slightly change real use. The custom yield field lets you match the calculator to the exact bag you plan to buy.
The final bag count is rounded up. You cannot buy part of a bag. Rounding up also helps prevent delays. Running short during a pour can create cold joints and weak spots. Extra bags are often easier to return than missing bags are to find during work.
Use the cost estimate for budgeting. It only covers bag material. It does not include gravel base, reinforcement, tools, delivery, tax, or labor. For structural work, local code and professional advice should guide the final design.
FAQs
1. What does this calculator estimate?
It estimates concrete volume, waste volume, total bags, and material cost for slabs, footings, and round post holes.
2. Does it work for post holes?
Yes. Choose the post hole option. Then enter diameter, depth, and quantity. The calculator uses a cylinder formula.
3. Why are bags rounded up?
Concrete bags are sold as whole bags. Rounding up helps avoid shortages during mixing and placement.
4. What waste percentage should I use?
Ten percent is a common starting point. Use more for uneven ground, loose soil, deep holes, or complex forms.
5. Can I use a custom bag yield?
Yes. Select Custom and enter the cubic feet shown on your product label for a more accurate estimate.
6. Is the cost estimate exact?
No. It estimates bag cost only. Taxes, delivery, tools, gravel, reinforcement, and labor are not included.
7. Which units should I enter?
Enter rectangular length and width in feet. Enter slab thickness, post diameter, and post depth in inches.
8. Should I buy extra bags?
Usually yes. Extra material helps cover waste, uneven forms, spillage, and measurement differences.