Concrete Bag Calculator
Calculate how many bags of concrete (40lb, 60lb, or 80lb) you need to pour a slab, walkway, patio, or fill post holes.
Results are estimates. Check physical dimensions before purchasing.
walkways are typically 4\" deep
80 lb is the most common standard size
How to calculate concrete bag quantities
Calculating how many bags of pre-mixed concrete you need is a matter of determining the volume of the pour in cubic feet, and then dividing it by the yield of your chosen bag weight size.
The Concrete Formulas
Volume (Cubic Feet) = Length (ft) × Width (ft) × (Depth (inches) / 12) Volume (Cubic Yards) = Volume (Cubic Feet) / 27 Bags Needed = Ceiling(Volume (Cubic Feet) / Bag Yield)
Concrete yields by bag size
Pre-mixed concrete bags do not yield as much volume as their heavy weight implies:
- 80 lb bag: Yields approximately 0.60 cubic feet. (It takes 45 bags to make 1 cubic yard).
- 60 lb bag: Yields approximately 0.45 cubic feet. (It takes 60 bags to make 1 cubic yard).
- 40 lb bag: Yields approximately 0.30 cubic feet. (It takes 90 bags to make 1 cubic yard).
When to order ready-mix delivery
Hand-mixing concrete in a wheelbarrow or small mixer is heavy, exhausting work. If your pour volume exceeds 1.0 cubic yard (which equals 45 bags of 80 lb weight), buying, transporting, and mixing individual bags becomes highly impractical. We recommend contacting a ready-mix concrete supplier or renting a towable concrete trailer for any pour size larger than 1 cubic yard.
Frequently Asked Questions
A 4ft × 4ft patio slab poured to a standard depth of 4 inches requires about 5.3 cubic feet of concrete. Using standard 80-pound bags (0.60 cu ft yield each), you will need 9 bags. If using 60-pound bags (0.45 cu ft yield), you will need 12 bags.
If your project volume exceeds 1 cubic yard (27 cubic feet), you would need forty-five 80-pound bags. Mixing and pouring that much by hand is extremely labor-intensive and slow. For pours larger than 1 cubic yard, ready-mix delivery or a towable mixing trailer is highly recommended.
For pedestrian pathways and standard patio slabs, 4 inches is the residential standard. For driveways or slabs supporting heavy equipment, a depth of 6 inches is required to prevent cracking.