Concrete Bag Calculator
Estimate how many ready-mix bags of concrete (80lb, 60lb, 50lb, 40lb) you need for slabs, post holes, footings, or volume with waste and cost estimation.
Calculation Results
Total cubic feet of mixed wet concrete required including waste margin.
Equivalent volume measured in cubic yards.
Number of standard 80 lb bags needed.
Number of 60 lb bags needed.
Number of 50 lb bags (often fast-setting mix) needed.
Number of lightweight 40 lb bags needed.
Estimated material cost based on chosen per-bag price.
How This Calculator Works
Follow these quick steps to calculate your bagged concrete requirements:
- Select Your Project Type: Choose between a rectangular slab (patio, walkway), cylindrical post holes (fencing, deck footings), or enter a known volume directly.
- Enter Project Dimensions: Input measurements in feet or inches. The calculator converts all dimensions accurately.
- Include Waste: A 10% waste buffer accounts for irregular hole digging, spillage during mixing, and subgrade dips.
- Select Bag Size: View exact counts across all 4 standard bag weights (80 lb, 60 lb, 50 lb, 40 lb) and estimated total material cost.
Formula & Calculations
Concrete mix bags are packaged by dry weight and provide standard volumetric yields of consolidated wet concrete after mixing with water (ASTM C387):
1. Volumetric Yields per Bag
- 80 lb bag: Yields
0.60 cubic feet(~0.0222 cubic yards). Exactly 45 bags yield 1 cubic yard. - 60 lb bag: Yields
0.45 cubic feet(~0.0167 cubic yards). Exactly 60 bags yield 1 cubic yard. - 50 lb bag: Yields
0.375 cubic feet(~0.0139 cubic yards). Exactly 72 bags yield 1 cubic yard. - 40 lb bag: Yields
0.30 cubic feet(~0.0111 cubic yards). Exactly 90 bags yield 1 cubic yard.
2. Post Hole Geometry (Hole Minus Post Displacement)
For fence posts, the post displaces volume within the drilled cylinder hole:
Concrete Volume = [π × (Hole Diameter / 2)² × Depth - Post Displacement] × Number of Posts
- 4x4 Post (3.5" x 3.5"): Displaces
0.085 cu ftper foot of buried depth. - 6x6 Post (5.5" x 5.5"): Displaces
0.210 cu ftper foot of buried depth. - 2-3/8" Metal Post: Displaces
0.031 cu ftper foot of buried depth.
Step-by-Step Calculation Examples
Four Fence Post Holes (10in Hole x 30in Deep, 4x4 Posts)
Calculating concrete bags for setting four 4x4 fence posts with a 10% waste buffer.
- project_type: post
- hole_diameter: 10
- hole_depth: 30
- post_size: 4x4
- num_posts: 4
- waste: 10
- cost_per_bag: 6.5
- net_volume_cuft: 5.06 cu ft
- bags_80lb: 9
- bags_50lb: 14
- total_cost_80lb: $58.50
Small Walkway Slab (10ft x 6ft x 4in)
Pouring a small front walkway slab requiring 22 cubic feet of concrete.
- project_type: slab
- slab_length: 10
- slab_width: 6
- slab_thickness: 4
- waste: 10
- cost_per_bag: 6.5
- net_volume_cuft: 22.00 cu ft
- volume_cuyd: 0.81 cu yd
- bags_80lb: 37
- bags_60lb: 49
Frequently Asked Questions
How many bags of concrete do I need for a 4x4 post hole?
For a standard 4x4 fence post in an 8-inch diameter hole dug 24 inches deep, you will typically need about 2 bags of 50 lb fast-setting concrete or 1.5 bags of 80 lb concrete. If the hole is 10 inches wide and 30 inches deep, budget 3 to 4 bags of 50 lb mix per hole.
How many 80 lb bags equal 1 cubic yard of concrete?
Exactly 45 bags of 80 lb concrete mix make one cubic yard. Since 1 cubic yard equals 27 cubic feet and each 80 lb bag yields 0.60 cubic feet: 27 / 0.60 = 45 bags.
How many 60 lb bags equal 1 cubic yard?
Exactly 60 bags of 60 lb concrete mix yield 1 cubic yard (27 / 0.45 = 60 bags).
What is the difference between Fast-Setting and Standard concrete mix?
Standard concrete (e.g., Quikrete 5000 or yellow-bag mix) contains coarse gravel aggregate and takes 24 to 48 hours to set, reaching high structural strength. Fast-setting concrete (red-bag) contains special accelerating additives and smaller aggregate that sets in 20 to 40 minutes, making it ideal for setting fence posts without bracing, but it is not intended for heavy structural load slabs.
How much water should I mix with an 80 lb or 60 lb bag?
An 80 lb bag of concrete mix typically needs about 3 quarts (2.8 L) of clean water, while a 60 lb bag requires about 2.25 quarts (2.1 L). Adding excess water drastically reduces cured compressive PSI and causes surface flaking.
How many bags of concrete can fit in a standard pickup truck?
A half-ton pickup truck (such as an F-150 or Chevy 1500) has a typical payload capacity of 1,500 to 2,000 lbs. That equals roughly 18 to 25 eighty-pound bags. Always verify your specific truck payload rating on the driver door sticker to avoid damaging springs or axles.