Enter Project Details
Example Data Table
| Project | Size | Net Volume | Waste | Approximate 80 lb Bags |
|---|---|---|---|---|
| Small slab | 8 ft × 6 ft × 4 in | 16.00 ft³ | 10% | 30 bags |
| Post holes | 8 holes, 12 in diameter, 30 in deep | 15.71 ft³ | 10% | 29 bags |
| Footing strip | 20 ft × 1.5 ft × 8 in | 20.00 ft³ | 10% | 37 bags |
Formula Used
Rectangular projects: Volume = length × width × depth × quantity.
Round holes: Volume = π × radius² × depth × quantity.
Solid stairs: Volume = width × tread depth × riser height × n(n + 1) / 2 × quantity.
Waste volume: Waste = net volume × waste percentage ÷ 100.
Final bags: Bags = final cubic feet ÷ yield per 80 lb bag. The calculator rounds up because partial bags cannot be purchased.
How to Use This Calculator
- Select the project shape that best matches your pour.
- Enter dimensions using the unit selectors beside each field.
- Add quantity when you have repeated slabs, holes, or stair sets.
- Enter a waste percentage for spills and uneven ground.
- Keep the default 0.6 ft³ yield, or edit it from your bag label.
- Add a bag price when you want a cost estimate.
- Press the calculate button and review the result above the form.
- Use the CSV or PDF buttons to save your estimate.
Why This Calculator Helps
Concrete work needs careful material planning. A small shortage can stop the pour. Extra bags can waste money and storage space. This calculator turns project dimensions into cubic volume, bag counts, weight, and cost. It focuses on 80 pound bags, yet it also lets you change yield. That helps when your mix brand lists a different yield.
Measure Before You Mix
Start with accurate length, width, depth, and quantity values. Use inches for slab thickness when possible. Use feet for long measurements. For round post holes, enter diameter and depth. The tool converts every unit into feet. It then finds volume in cubic feet and cubic yards. Waste percentage is added after the net volume is known.
Planning Slabs and Footings
For slabs, multiply length by width by thickness. Footings use the same idea. Many jobs need several identical sections. The quantity field handles that need. You can enter ten pads, four footings, or many small forms. The result shows exact bags and rounded bags. Rounded bags are the bags you should buy.
Planning Round Holes
Post holes use a cylinder formula. The radius equals half the diameter. The tool multiplies radius squared by pi and depth. Then it multiplies by quantity. This is useful for fence posts, deck posts, mailboxes, and sign bases. Always measure the hole after digging. Loose soil and uneven sides can increase concrete need.
Using Waste and Cost
Waste allowance protects the job. Spills, uneven ground, deeper corners, and leftover material all matter. Five to ten percent is common for simple work. Complex pours may need more. Cost helps build a quick shopping budget. Enter your local bag price. The calculator multiplies that price by the rounded bag count.
Practical Pour Tips
Buy all bags before starting. Stage water, mixer, tools, and forms nearby. Mix only what you can place quickly. Check slope, thickness, and level during the pour. Keep notes from the result table. They help compare estimates, receipts, and future projects. Good planning makes concrete work cleaner, faster, and safer.
Save a printed copy near the mixer. It reduces mistakes when several people share measuring duties during busy work.
FAQs
How much concrete does one 80 lb bag make?
Many 80 lb concrete bags yield about 0.6 cubic feet. Always check your bag label. Different mixes can vary by brand, aggregate, moisture, and product type.
Why does the calculator round bags upward?
You cannot usually buy part of a concrete bag. Rounding upward helps prevent shortages during placement. It also allows room for small measuring errors.
What waste percentage should I use?
Use five to ten percent for simple pours. Use more for rough ground, irregular forms, deep corners, long handling time, or beginner work.
Can I calculate post holes?
Yes. Choose round post holes. Enter the hole diameter, depth, and quantity. The tool applies the cylinder formula and then adds waste.
Can this estimate slabs and footings?
Yes. Choose the rectangular option. Enter length, width, and thickness. Use quantity for repeated pads, footing sections, or similar forms.
What unit should I use for thickness?
Inches are easiest for slab thickness. Feet work well for long dimensions. The calculator converts selected units into feet before calculating volume.
Does price affect the bag count?
No. Price only affects the estimated cost. Bag count depends on volume, waste percentage, and the cubic foot yield per 80 lb bag.
Should I buy extra bags?
Usually yes. A small extra amount helps cover spills, uneven excavation, and form variation. Unopened bags may be returnable at some stores.