Concrete Bag Calculator
Estimate bags of concrete needed for slabs, footings, post holes, and pads.
Bags Needed: —
How it works
Volume = length × width × (thickness ÷ 12). Divide by bag yield and round up. Always buy 5–10% extra for waste.
| Bag size | Approx yield |
|---|---|
| 80 lb | 0.60 cu ft |
| 60 lb | 0.45 cu ft |
| 50 lb | 0.375 cu ft |
Related tools
Concrete Bag FAQ
How many 80 lb bags per yard?
About 45 bags of 80 lb concrete equal one cubic yard.
Should I buy extra?
Yes — add 5–10% for waste, spills, and form irregularities.
What bag size should I choose?
80 lb bags are standard for slabs. 60 lb bags are easier to handle for one person.
Results are estimates for planning purposes. Verify with product labels and local requirements before purchasing materials.
How to calculate how many bags of concrete you need
Estimating concrete bags starts with calculating the total volume of your project in cubic feet. Multiply the length by the width to get the area, then multiply by the thickness in feet — divide inches by 12 to convert. Divide the resulting cubic footage by the yield of your chosen bag size and round up to the nearest whole bag.
The formula is: Bags needed = (Length ft × Width ft × Thickness in ÷ 12) ÷ Bag yield
Always buy at least 5–10% extra. Running short mid-pour forces you to stop, which creates cold joints — weak seams where fresh concrete meets partially set concrete.
Concrete bag yield by size
| Bag size | Approx. yield | Bags per cu yd | Best for |
|---|---|---|---|
| 80 lb | 0.60 cu ft | ~45 bags | Slabs, footings, large pads |
| 60 lb | 0.45 cu ft | ~60 bags | General use, easier to lift |
| 50 lb | 0.375 cu ft | ~72 bags | Small repairs, post holes |
| 40 lb | 0.30 cu ft | ~90 bags | Small patches, anchoring |
Common project estimates
| Project type | Dimensions | Thickness | 80 lb bags needed |
|---|---|---|---|
| Small shed pad | 8×10 ft | 4 in | 45 |
| Standard patio | 10×12 ft | 4 in | 67 |
| Driveway apron | 12×16 ft | 4 in | 107 |
| Sidewalk section | 4×20 ft | 4 in | 45 |
| Fence post hole | 10 in dia | 36 in deep | 1–2 per post |
| Deck footing | 12 in dia | 36 in deep | 2–3 per footing |
Step-by-step: calculating concrete for a slab
Here is a worked example for a 12×16 ft shed pad at 4 inches thick:
Step 1: Convert thickness to feet: 4 ÷ 12 = 0.333 ft
Step 2: Calculate volume: 12 × 16 × 0.333 = 63.9 cubic feet
Step 3: Divide by bag yield: 63.9 ÷ 0.60 = 106.5 bags
Step 4: Round up and add 10% waste: 107 × 1.10 = 118 bags
Purchase 118 bags of 80 lb concrete for this project. If you switch to 60 lb bags, you would need approximately 156 bags.
Tips for a better concrete pour
- Dampen the subgrade before pouring — dry soil pulls moisture from the mix and weakens the slab.
- Mix one bag at a time and add water slowly — excess water is the number one cause of weak concrete.
- Use a screed board to level the surface immediately after placing. Work quickly — concrete begins setting within 45 minutes in warm weather.
- Cover the finished slab with plastic sheeting for 24–48 hours to slow the curing process and increase final strength.
- Do not walk on the slab for 24 hours. Avoid heavy loads for at least 7 days. Full curing takes 28 days.
Frequently asked questions about concrete bag calculations
How many bags of concrete do I need for a 10x10 slab?
A 10×10 ft slab at 4 inches thick requires about 33.3 cubic feet of concrete. Using 80 lb bags that yield 0.60 cubic feet each, you need about 56 bags. Add 10% for waste, so purchase 62 bags.
How many 80 lb bags of concrete are in a cubic yard?
One cubic yard equals 27 cubic feet. At 0.60 cubic feet per 80 lb bag, you need approximately 45 bags to fill one cubic yard of space.
How many bags of concrete do I need for fence post holes?
A standard 10-inch diameter post hole at 36 inches deep needs roughly 1 to 2 bags of 80 lb concrete per post. Use the quick-set variety for post holes — it sets in about 20-40 minutes without mixing.
How do I calculate how much concrete I need for a slab?
Multiply length × width × thickness (in feet) to get cubic feet. Divide by 0.60 for 80 lb bags, 0.45 for 60 lb bags. Round up and add 10% for waste.
When should I use bagged concrete vs. ready-mix?
Use bagged concrete for projects under 1 cubic yard (about 45 bags of 80 lb). For larger projects like full driveways or foundations, ready-mix concrete is more economical and far less labor-intensive.
How long does bagged concrete take to set?
Standard bagged concrete reaches initial set in 24-48 hours. You can walk on it after 24 hours and drive on it after 7 days. Full strength (4000 PSI) is reached after 28 days of curing.
How much does a bag of concrete cost?
An 80 lb bag of standard concrete mix costs approximately $6-$8 at most hardware stores. Specialty mixes like fast-setting or high-strength run $8-$12 per bag.
Do I need to wet the ground before pouring concrete?
Yes. Dampen the subgrade before pouring to prevent dry soil from pulling moisture out of the fresh concrete, which weakens the final product. Do not make it muddy — just moist.
Related tools
- Square Footage Calculator — calculate your slab or pad area first
- Concrete Volume Calculator — get cubic yards for ready-mix orders
- Fence Calculator — estimate posts and sections for your fence project
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