Find out exactly how many bags of concrete you need for any pour — slabs, footings, columns, or tube forms. Supports 40, 60, and 80 lb bags with adjustable waste factor. Used by contractors, builders, and DIY crews for accurate material estimates before ordering.
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Shop Express Tools →Concrete bag calculations follow a simple volume formula. The key is to use consistent units — convert all dimensions to feet before calculating cubic feet, then account for bag yield and waste.
To convert to cubic yards (for ready-mix comparison): divide cubic feet by 27. A 10×10 ft slab at 4 inches thick is 33 cu ft ÷ 27 = 1.22 cubic yards. At a typical ready-mix price of $150–175 per yard plus delivery, this approaches the cost breakeven point versus bags at a hardware store.
| Bag Size | Yield (cu ft) | Bags per CY | Notes |
|---|---|---|---|
| 80 lb bag | 0.60 cu ft | 45 bags | Most economical — standard contractor choice |
| 60 lb bag | 0.45 cu ft | 60 bags | Good for smaller crews and residential work |
| 40 lb bag | 0.30 cu ft | 90 bags | Small patches, difficult access, overhead work |
| Ready-mix (1 CY) | 27 cu ft | — | Best for pours > 1 CY — call for local pricing |
| Application | Typical Size | 80 lb Bags (approx) | Recommendation |
|---|---|---|---|
| 10×10 ft slab, 4" thick | 33 cu ft = 1.2 CY | ~61 bags | Consider ready-mix at this size |
| 8×8 ft slab, 4" thick | 21 cu ft = 0.8 CY | ~39 bags | Use bags or small-load ready-mix |
| 4×4 ft landing, 4" thick | 5.3 cu ft | ~10 bags | Bags straightforward |
| Fence post, 10" dia × 2 ft deep | 1.1 cu ft | ~2 bags | Use tube form; 1 bag per post typical |
| Deck footing, 12" dia × 3 ft deep | 2.4 cu ft | ~4–5 bags | One 80 lb bag per 0.6 cu ft depth |
| Sidewalk 3 ft wide × 40 ft × 4" | 40 cu ft = 1.5 CY | ~73 bags | Order ready-mix at this size |
Bag counts include 10% waste factor. Your exact count may vary with pour dimensions.
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Start free trialA 10×10 ft slab at 4" thick = 33.3 cubic feet (10 × 10 × 0.333). An 80 lb bag yields approximately 0.60 cu ft of mixed concrete. With a 10% waste factor, you need: 33.3 ÷ 0.60 × 1.10 = approximately 61 bags. At 6" thick, the slab is 50 cu ft, requiring about 92 bags. Use this calculator to adjust for your exact dimensions and slab thickness.
An 80 lb bag of concrete (Quikrete 80 lb, Sakrete 80 lb) yields approximately 0.60 cubic feet of mixed concrete. A 60 lb bag yields 0.45 cu ft, and a 40 lb bag yields 0.30 cu ft. These yields are from the bag manufacturer — actual yield varies slightly with water content and mixing thoroughness.
If your pour exceeds 1 cubic yard (27 cubic feet), ready-mix concrete is usually more economical and practical. A standard concrete truck delivers 7–10 cubic yards with a minimum order of around 1 yard at most plants. For small pours under 0.5 CY (14 cu ft, about 23 bags of 80 lb), bags are typically faster and cheaper. Between 0.5 and 1 CY, compare bag cost vs. minimum load ready-mix charge.
80 lb bags are more economical per cubic foot but significantly heavier to handle — most professional concrete contractors specify 80 lb bags unless site access or crew limitations require smaller bags. 60 lb bags are a good balance for smaller crews and residential work. 40 lb bags are best for small patches, difficult-access areas, or where overhead mixing is needed. All standard premix bags use approximately the same concrete strength (3000–4000 psi) — the only difference is bag size.
Concrete forms are never perfectly square, ground is never perfectly level, and there's always some waste in the mixing and pouring process. A 10% waste factor is standard for flatwork (slabs, sidewalks). Use 15% for rough forms, uneven ground, or footing excavations with irregular bottoms. For tube forms, a 5–8% waste factor is usually sufficient since the tube controls volume precisely.