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 by Rakesh Choudhary, PhDMethodology & sources · How we calculate
Bag yields and quick math for typical projects
Every bag prints how much mixed concrete it makes, and that yield, not the weight, is what the calculator divides by. Typical label yields: 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. These are common manufacturer figures rather than an official standard, so the label on your product governs. At those yields 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 post hole 10 inches across and 2 feet deep holds about 1.09 cu ft before the post goes in, so 2 of the 80 lb bags. A 6-foot square pad 4 inches thick is 12 cu ft: 20 of the 80 lb bags, or 22 with 10% waste. A 60-foot sidewalk 3 feet wide by 4 inches thick is 60 cu ft, 100 of the 80 lb bags, and at that scale a ready-mix quote is worth getting too.
Bag prices vary by store and region, so the calculator leaves the price to you: enter what your store charges for an 80 lb bag. For a larger pour, ask a ready-mix supplier for a delivered price per cubic yard, including any small-load charge, and weigh it against the bag total plus the work of mixing by hand.
Mixing pre-mix concrete the right way
Manufacturers print the water per bag on the bag, and that figure is the one to follow. Too much water is the most common DIY mistake: a wet, runny mix is easier to place but cures weaker. A rough check: scoop some mix into a cone shape; it should sag a little and hold, 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 depends on the product and the temperature. In hot weather it sets fast, so work in shade and have a sprayer ready. In cold weather curing slows, and some mixes need insulated blankets to cure properly. The bag lists its working time, when the surface can take foot and vehicle traffic, and any temperature limits. Concrete strength is conventionally specified at 28 days, so it keeps gaining strength well after it looks hard.
Common bag-calculation mistakes
- Forgetting to convert depth from inches to feet: A 10 × 10 ft slab 4 inches thick is 33.3 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. A common convention is about 10% for a plain slab and more for irregular shapes and trenched footings; it is a rule of thumb, not a standard.
- 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: Walks and patios are commonly poured thinner than driveways, and slabs that carry vehicles or heavy loads are thicker and often reinforced. The right thickness depends on the load and the ground; check your local code or ask a builder.
- Ignoring the gravel base: Many slabs sit on a compacted gravel base for drainage. Don't subtract it from your concrete depth; it's separate.
FAQs
How many bags of concrete per cubic foot?
Typical label yields run 0.30 cu ft for a 40 lb bag, 0.45 for 60 lb, 0.60 for 80 lb and 0.68 for 90 lb, but check the figure printed on your product. Ten cubic feet then takes 34 of the 40 lb size, 23 of the 60 lb, 17 of the 80 lb or 15 of the 90 lb, each rounded up to a whole bag.
What's the difference between 60lb and 80lb bags?
Mostly handling. The heavier bag makes more concrete (about 0.60 cu ft against 0.45 at typical yields), so you buy fewer, but it is harder to lift and tip into a mixer on your own. The 90 lb size goes further again at 0.68 cu ft. To compare value, divide each shelf price by the yield on that bag.
How much does a bag of concrete cost?
It depends on the store, the region and the product, so this page does not quote a price. Type what your store charges for an 80 lb bag into the calculator and it multiplies that by the bag count. Comparing sizes? Divide each price by its stated yield to get a cost per cubic foot.
How many bags for a concrete slab?
Find the volume first. A 10 by 10 foot slab poured 4 inches thick holds 33.3 cu ft, which at a typical 0.60 cu ft per 80 lb bag means 56 bags, or 62 once you allow 10% for spills and an uneven base. Ten cubic yards would need 450 of those bags, well into ready-mix territory.
When should I use bags vs ready-mix?
Bags suit post holes, repairs and small pads you can mix and place in one session. A common rule of thumb is to price ready-mix once a pour passes about a cubic yard (27 cu ft, or 45 of the 80 lb bags), because mixing that many batches quickly enough to avoid cold joints is hard work. Ask suppliers about any minimum or small-load charge before you decide.