Concrete Bag Calculator: How Many Bags Do You Need?
Estimate how many 40 lb, 60 lb, and 80 lb concrete bags you need for slabs, post holes, footings, patios, walkways, and small pads.
Written by Calzivo Editorial Team
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Quick answer
In short
- Formula: Bags needed = project cubic feet / bag yield, rounded up.
- Common yields: Typical planning yields are about 0.30 cu ft for 40 lb, 0.45 for 60 lb, and 0.60 for 80 lb bags; check the label.
- Use the tool: Compare bag sizes and optional cost in the Concrete Bags Calculator.Open calculator
Calculate bags: Use the Concrete Bags Calculator, compare larger pours with the Concrete Calculator, check volume in the Cubic Yard Calculator, or convert dimensions with the Unit Converter.
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Planning estimate only: Concrete needs vary by measurements, thickness, forms, uneven base, waste, bag yield, water mix, reinforcement, local requirements, and supplier specs. Always check the bag label and project requirements.
Quick answer
Calculate project volume in cubic feet, divide by bag yield, round up, and add a waste allowance. Bagged concrete works best for smaller pours where mixing by hand or small mixer is realistic.
Concrete bag formula
Cubic feet = length x width x depth in feet Bags = cubic feet / cubic feet per bag
Common bag yields
| Bag size | Planning yield | Best use |
|---|---|---|
| 40 lb | about 0.30 cu ft | Lighter lifting, more bags. |
| 60 lb | about 0.45 cu ft | Common DIY compromise. |
| 80 lb | about 0.60 cu ft | Fewer bags, heavier handling. |
10x10 slab example
A 10 ft by 10 ft slab at 4 inches thick needs about 33.3 cubic feet, or about 1.23 cubic yards.
80 lb bags: 33.3 / 0.60 = 55.5, round to 56 bags 60 lb bags: 33.3 / 0.45 = 74 bags 40 lb bags: 33.3 / 0.30 = 111 bags
Post hole example
For a 10-inch diameter hole 30 inches deep, use the cylinder formula. Radius is 5 inches, or 0.417 ft; depth is 2.5 ft. Volume is about 1.37 cubic feet, or roughly 3 bags at 0.60 cu ft per 80 lb bag before extra allowance.
Bags vs cubic yards
Bags are convenient for posts, small pads, repairs, and short walkways. If the calculator shows dozens or hundreds of bags, compare ready-mix or cubic-yard planning with Concrete Bags vs Cubic Yards Explained.
Common mistakes
- Forgetting slab thickness.
- Using inches directly without converting depth to feet.
- Rounding down bag counts.
- Mixing bag sizes without separate yields.
- Skipping waste allowance for uneven ground or forms.
FAQs
How many 80 lb bags of concrete do I need?
Divide cubic feet by the bag yield, often about 0.60 cu ft, then round up and check the label.
How many 60 lb bags of concrete do I need?
Divide cubic feet by the bag yield, often about 0.45 cu ft, then round up.
How much concrete do I need for a 10x10 slab?
At 4 inches thick, about 33.3 cubic feet or 1.23 cubic yards before waste.
Is it better to use bags or ready-mix?
Bags make sense for small jobs. Ready-mix may make more sense when bag counts get high.
Concrete bag estimating starts with volume, then bag yield. Round up, add waste allowance, and compare ready-mix when the bag count becomes large.
Use the tool instead
Use the matching calculator when you want to plug in your own numbers and get a result faster.
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