Concrete Bag Calculator
Determine how many bags of pre-mix concrete you need for slabs, footings, posts, or sidewalks. Compares 40lb, 60lb, 80lb, and 90lb bags.
Reviewed & updated for 2026 · How we calculate
Bag yields and quick math for typical projects
A bag of premix concrete yields less concrete than the bag weighs in cubic feet because dry mix is heavy: about 145 lb per cubic foot of finished concrete. The conversions: 40 lb bag = 0.30 cu ft, 60 lb = 0.45 cu ft, 80 lb = 0.60 cu ft, 90 lb = 0.68 cu ft. A cubic yard (27 cu ft) takes 45 of the 80-lb bags or 60 of the 60-lb bags.
Common project quick estimates: a single 4-inch fence post hole 10 inches across needs about 0.18 cu ft, one 60 lb bag covers it. A 6-foot square 4-inch slab pad (12 cu ft) needs 20 of the 80 lb bags plus 10% waste = 22 bags. A 60-foot sidewalk 3 feet wide by 4 inches thick is 60 cu ft, 100 of the 80 lb bags, but at this scale you should price ready-mix instead.
Bag cost (2026 pricing) runs $5-$8 for 60 lb and $6-$10 for 80 lb at major home centers. Ready-mix concrete from a truck runs $150-$200 per cubic yard delivered, with a typical $80-$150 short-load fee for orders under 4-5 cubic yards. The crossover where ready-mix becomes cheaper is roughly 60-80 bags (about 1-1.5 cu yards), and you should factor in saved labor mixing 60+ bags by hand.
Mixing pre-mix concrete the right way
Manufacturers specify water in pints or quarts per bag, typically 3 quarts (96 oz) per 80 lb bag. Too much water is the single most common DIY mistake. A wet, runny mix is easier to work with but loses 20-30% of its compressive strength. The slump test approximation: scoop wet mix into a cone shape; it should hold a 3-4 inch slump, not collapse flat.
For small batches: pour 80% of the water into a wheelbarrow first, dump in the dry mix, hoe back and forth in a chopping motion until uniform color, then add remaining water in small splashes. The hoe should leave a clear mark that holds its shape, not pool with water around it.
Working time after mixing: 30-90 minutes depending on temperature. Above 90°F it sets fast, work in shade and have a sprayer ready. Below 40°F, hydration slows dramatically; some mixes won't fully cure in cold weather without insulated blankets. Initial set in 2-4 hours; foot traffic OK in 24 hours; vehicle traffic in 7 days; full design strength at 28 days (75% by day 7).
Common bag-calculation mistakes
- Forgetting to convert depth from inches to feet: A 10×10×4 inch slab is 33 cu ft, not 400. Always divide inch dimensions by 12 before multiplying.
- No waste factor: Spills, overpour, irregular subgrade, and that one bag that gets wet in the rain all add up. Always add 10% for slabs, 15% for irregular shapes, 20% for footings with rebar.
- Rounding down on bags: Running short by one bag at hour 2 of pouring is a disaster, the joint between yesterday's pour and today's is a cold joint and a weakness. Always round up.
- Wrong depth for use case: 4" is enough for foot traffic, sidewalks, and patio. Driveways need 5-6". Areas with vehicle turning or heavy loads need 6-8" plus rebar or wire mesh.
- Ignoring the gravel base: Most slabs need 4 inches of compacted gravel underneath for drainage and frost protection. Don't subtract this from your concrete depth, it's separate.
FAQs
How many bags of concrete per cubic foot?
Approximate yields: 40lb bag = 0.30 cu ft. 60lb bag = 0.45 cu ft. 80lb bag = 0.60 cu ft. 90lb bag = 0.68 cu ft. So for 10 cu ft you need: 22 of 40lb, 16 of 60lb, 13 of 80lb, or 12 of 90lb bags.
What's the difference between 60lb and 80lb bags?
60lb bag: ~0.45 cu ft yield, easier to lift and carry. Good for small jobs and DIYers. 80lb bag: ~0.60 cu ft yield, fewer bags needed, slightly cheaper per cu ft. Common pro choice. 90lb bag: ~0.68 cu ft, even fewer bags but heavy to handle alone.
How much does a bag of concrete cost?
2026 prices vary by location and brand. 60lb bag: $4-$8. 80lb bag: $5-$10. 90lb bag: $6-$12. Quikrete, Sakrete, and Ash Grove are common brands. Bulk delivery is much cheaper per cubic foot for larger jobs (10+ cu yards).
How many bags for a concrete slab?
For a 10' × 10' × 4" slab = 33.3 cu ft. At 80lb yield of 0.6 cu ft = 56 bags. Add 10% waste = 62 bags. Total: $370-$620 in materials. For slabs over 10 cu yards (~700 bags), order ready-mix instead, cheaper per cu ft.
When should I use bags vs ready-mix?
Bags: under 1 cu yard (27 cu ft), slabs under ~10x10x4", post holes, small repairs. Ready-mix truck: over 1 cu yard, driveways, foundations, large slabs. Truck delivery costs $150-$300 minimum plus per cu yard. Crossover point: usually around 50-100 bags.