Calculate how much concrete you need for a slab, column, or any rectangular volume. Results include a 10% waste allowance and the number of pre-mixed bags required.
Concrete Calculator
How It Works
The calculator finds the volume of your shape in cubic feet, adds a 10% waste allowance, converts to cubic yards (the standard ordering unit for ready-mix concrete), and estimates bags needed. 1 cubic yard = 27 cubic feet. A 60 lb bag covers ~0.45 ft³; an 80 lb bag ~0.60 ft³.
Example Projects
| Project | Dimensions | Cubic Yards | 80 lb Bags |
|---|---|---|---|
| 10×10 ft patio | 10×10 ft, 4 in deep | 1.37 | 62 |
| 20×30 ft driveway | 20×30 ft, 4 in deep | 8.23 | 370 |
| Fence post (6 in, 3 ft) | 6 in dia × 3 ft | 0.04 | 2 |
Frequently Asked Questions
Why is there a 10% waste factor?
Uneven ground, spillage, form bulging, and pour depth variations can all use more concrete than calculated. A 10% buffer is standard to avoid running short mid-pour.
When should I use bags vs. ready-mix?
For projects under about 1 cubic yard, pre-mixed bags are practical and economical. For larger projects, ordering ready-mix by the truck is faster, more consistent, and usually cheaper per yard.
How thick should a concrete slab be?
4 inches is standard for residential patios, walkways, and driveways. 6 inches is recommended for heavy vehicles. Footings are typically 8–12 inches depending on load and local codes.
Can I mix different bag sizes?
Yes — the bag count is a guide. Mix 60 lb and 80 lb bags as available. The key is reaching the total volume needed; bag size only affects how many you carry.
Does temperature affect concrete?
Yes. Avoid pouring below 40°F (4°C) or above 90°F (32°C) without special precautions. Cold slows curing and risks freezing; heat accelerates curing and can cause cracking.