Concrete Bag Calculator
Enter your dimensions — get bag counts for all sizes at once.
80 lb bags
—
0.60 cu ft each
60 lb bags
—
0.45 cu ft each
50 lb bags
—
0.375 cu ft each
Cubic feet
—
Cubic yards
—
for ready-mix orders
Results include your selected waste factor. Running short mid-pour creates cold joints — always buy extra.
Common project presets — click to calculate
Bag size guide — which should you choose?
| Bag size | Yield | Bags per yd³ | Best for |
|---|---|---|---|
| 80 lb | 0.60 cu ft | ~45 bags | Slabs, footings, large pads — standard choice |
| 60 lb | 0.45 cu ft | ~60 bags | Easier to lift solo; good for general use |
| 50 lb | 0.375 cu ft | ~72 bags | Small repairs, post holes, touch-ups |
| 40 lb | 0.30 cu ft | ~90 bags | Small patches, anchoring hardware |
For projects over 1 cubic yard (~45 bags of 80 lb), consider ordering ready-mix concrete — it's faster, cheaper per yard, and far less labor.
Step-by-step example — 12×16 ft shed pad at 4 inches
| Step | Calculation | Result |
|---|---|---|
| Convert thickness to feet | 4 ÷ 12 | 0.333 ft |
| Calculate volume | 12 × 16 × 0.333 | 63.9 cu ft |
| Divide by bag yield (80 lb) | 63.9 ÷ 0.60 | 106.5 → 107 bags |
| Add 10% waste | 107 × 1.10 | 118 bags |
Tips for a better pour
- Dampen the subgrade before pouring — dry soil pulls moisture from the mix and weakens the slab.
- Add water slowly — excess water is the #1 cause of weak concrete.
- Use a screed board immediately after placing. Work quickly — concrete begins setting within 45 minutes in warm weather.
- Cover the slab with plastic sheeting for 24–48 hours to slow curing and increase final strength.
- Don't walk on the slab for 24 hours. No heavy loads for 7 days. Full strength takes 28 days.
- Never stop mid-pour on a slab — cold joints are permanent weak points. Have all your bags on site before you start.
Frequently asked questions
How many bags of concrete for a 10×10 slab?
At 4 inches thick: 62 bags of 80 lb (including 10% waste). At 6 inches thick: about 93 bags. Click the 10×10 preset above to calculate instantly with any waste factor.
How many bags of concrete for a 12×12 slab?
At 4 inches thick: 89 bags of 80 lb (with 10% waste). At 6 inches: about 133 bags. Use 60 lb bags if you're working alone — the 12×12 preset above shows all three sizes at once.
How many 80 lb bags per cubic yard?
One cubic yard = 27 cubic feet. At 0.60 cu ft per 80 lb bag, you need approximately 45 bags per cubic yard. At 60 lb (0.45 cu ft/bag), it's about 60 bags per yard.
How many bags for fence post holes?
A 10-inch diameter hole at 36 inches deep needs 1–2 bags of 80 lb fast-setting concrete per post. Use fast-set (Quikrete or similar) — pour dry into the hole, add water, done in 20–40 minutes, no mixing required.
How many bags for a 4×4 post hole?
A 4×4 post typically uses a 10-inch hole at 36–42 inches deep — about 1 to 2 bags of 80 lb per post. Larger posts (6×6) in a 12-inch hole need 2–3 bags.
How do I calculate bags of concrete?
Length (ft) × Width (ft) × Thickness (in ÷ 12) = cubic feet. Divide by bag yield: 0.60 for 80 lb, 0.45 for 60 lb, 0.375 for 50 lb. Round up, then add 10% for waste. The calculator above does all of this instantly.
Bagged concrete vs. ready-mix — which should I use?
Use bags for anything under 1 cubic yard (about 45 bags of 80 lb). Above that, ready-mix is cheaper per yard, arrives mixed, and saves enormous labor. A full driveway almost always warrants a ready-mix truck.
How long does concrete take to set?
Initial set in 24–48 hours (walk on it after 24 hrs). Drive on it after 7 days. Full 4000 PSI strength at 28 days. Fast-setting concrete (like Quikrete Fast-Setting) hardens in 20–40 minutes — ideal for posts but not slabs.
How much does a bag of concrete cost?
An 80 lb bag runs about $6–$8 at Home Depot or Lowe's. Fast-setting and high-strength mixes are $8–$12. Buying in bulk pallets can save 10–15% per bag.
Related calculators
- Concrete Volume Calculator — cubic yards for ready-mix orders
- Square Footage Calculator — calculate your slab area first
- Gravel Calculator — estimate base material for your slab
- Fence Calculator — posts, sections, and concrete for fencing
- PSI to Bar Converter — pressure conversions for equipment
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