How to Use This Concrete Calculator
This calculator tells you exactly how much concrete you need. It works for patios, fence posts, and foundations. Get volume in cubic yards (for ready-mix) and bag counts (for DIY).
Step 1: Select Your Project Type
Different projects have different needs. Pick yours:
- Slab/Patio/Driveway - Flat surfaces with uniform thickness
- Footing/Foundation - Below-grade support, typically deeper
- Wall/Retaining Wall - Vertical structures needing forms
- Column/Post - Cylindrical shapes (diameter × height)
- Steps/Stairs - Stepped structures with multiple levels
- Curb/Border - Linear edge structures
Step 2: Enter Your Dimensions
For rectangles, enter length, width, and depth. For columns, enter diameter and height. Use feet for length/width. Use inches for depth/thickness.
Common slab thicknesses:
- 4 inches - Patios, sidewalks, foot traffic only
- 5-6 inches - Driveways, garage floors, residential use
- 8+ inches - Heavy equipment, commercial use
Step 3: Choose Concrete Strength
Strength is measured in PSI. This is the compressive strength after 28 days:
- 2,500 PSI - Interior floors, non-structural work
- 3,000 PSI - Standard residential, most common choice
- 3,500 PSI - Driveways, sidewalks, exterior slabs
- 4,000 PSI - Commercial projects, heavy vehicle traffic
- 4,500+ PSI - High-strength and industrial use
Understanding Concrete Volume
Concrete sells two ways:
Ready-Mix Concrete comes by truck. It sells by the cubic yard. One cubic yard = 27 cubic feet. That covers about 80 sq ft at 4" thick. Use ready-mix for projects over 1 cubic yard. It costs less and has consistent quality.
Bagged Concrete comes in 40, 60, and 80-lb bags. An 80-lb bag makes about 0.6 cubic feet. For small projects under 1 cubic yard, bags work well. But mixing by hand is hard work.
Concrete Coverage & Bag Calculator Table
Use this table to quickly estimate bags needed for common slab sizes at 4" thickness:
| Slab Size | Cubic Feet | Cubic Yards | 80-lb Bags | 60-lb Bags |
|---|---|---|---|---|
| 4' × 4' | 5.3 | 0.20 | 9 | 12 |
| 6' × 6' | 12.0 | 0.44 | 20 | 27 |
| 8' × 8' | 21.3 | 0.79 | 36 | 48 |
| 10' × 10' | 33.3 | 1.23 | 56 | 74 |
| 12' × 12' | 48.0 | 1.78 | 80 | 107 |
| 16' × 16' | 85.3 | 3.16 | 142 | 190 |
| 20' × 20' | 133.3 | 4.94 | 222 | 296 |
*Includes 10% waste factor. Based on 4" slab thickness. 80-lb bag = 0.6 cu ft, 60-lb bag = 0.45 cu ft.
Concrete Strength (PSI) Selection Guide
| PSI Rating | Best For | Typical Cost/Yard | Cure Time |
|---|---|---|---|
| 2,500 PSI | Interior floors, patios, light residential | $120-140 | 28 days |
| 3,000 PSI | Standard residential, foundations, walls | $130-150 | 28 days |
| 3,500 PSI | Driveways, sidewalks, pool decks | $140-160 | 28 days |
| 4,000 PSI | Commercial floors, heavy traffic areas | $150-175 | 28 days |
| 4,500+ PSI | Industrial, high-rise, precast | $165-200+ | 28 days |
Why Add a Waste Factor?
Always order 10% extra. This covers:
- Low spots in the subgrade needing more material
- Spillage during pouring and finishing
- Form irregularities and expansion
- Running short mid-pour (cold joints)
Running out mid-pour is a disaster. It creates weak "cold joints" that crack and fail. Always order more than you think you need.