𧱠Concrete Bag Calculator
Bags, water, and weight for slabs, footings, and post holes.
Bags Needed (10% waste included)
Estimates for planning only. Follow the mix manufacturer\'s instructions and local building code for structural work.
How to Use This Calculator
Enter the length, width, and depth of your pour, then pick 60 lb or 80 lb bags. The calculator computes the volume, adds a 10 percent waste factor for spillage and uneven subgrade, and rounds up to whole bags. You also get the water estimate and the total weight of dry mix so you can plan hauling and mixing.
Measure the pour in feet for length and width. For footings or post holes, enter the same number for length and width to approximate a square area, or convert round holes using the circle area formula.
Enter the depth in inches. Standard slabs are 4 inches; driveways and garage floors are 5 to 6 inches. Measure your actual form depth, since a half inch of error changes the bag count noticeably.
Pick your bag size. 80 lb bags cover 0.6 cubic feet and are more economical per pound; 60 lb bags cover 0.45 cubic feet and are easier to lift and mix in a wheelbarrow.
Check the water estimate and total weight. If the bag count exceeds about 45 bags (1 cubic yard), price a ready-mix truck before you commit to mixing bags.
The Concrete Volume Formula
The 10 percent waste factor covers spillage, over-excavation, and the reality that subgrade is never perfectly flat. If your base is rough or your forms flex, consider 15 percent. Bag yield figures come from the standard industry numbers used by the major bagged-mix brands: an 80 lb bag reliably yields 0.6 cubic feet when mixed correctly.
Water is the variable most beginners get wrong. The rule is to use as little water as possible while keeping the mix workable. Excess water does not evaporate harmlessly: it leaves voids in the hardened concrete and can cut compressive strength by 20 to 30 percent. Mix in a wheelbarrow for small pours (under 10 bags) and rent an electric mixer for anything larger.
Worked Example
A 12x8 patio slab at 4 inches deep using 80 lb bags. Volume = 12 x 8 x 4 / 12 = 32 cubic feet. With 10 percent waste: 35.2 cubic feet. Bags = ceil(35.2 / 0.6) = 59 bags. Total dry weight = 59 x 80 = 4,720 lb. Water = 59 x 0.75 = 44.25 gallons. That is just over 1.3 cubic yards, right at the boundary where a ready-mix truck (often with a short-load fee) starts to look attractive.
Small example: one 12-inch diameter post hole 36 inches deep. Area = 3.1416 x 0.5 x 0.5 = 0.785 sq ft, depth 3 ft, volume = 2.36 cu ft. With waste: 2.6 cu ft. That is 5 bags of 80 lb mix (5 x 0.6 = 3.0 cu ft) or 6 bags of 60 lb mix. In practice most people use 2 to 3 bags per post hole because the hole is rarely that clean.
Tips for a Good Pour
Prep the base before you mix anything. Compact the soil, add 2 to 4 inches of compacted gravel for slabs, and wet the base lightly so it does not suck water out of your mix. Set forms dead level and brace them well: wet concrete is heavy (about 150 lb per cubic foot) and will blow out weak forms.
Pour in one continuous session if you can. If you must stop, plan a construction joint at a logical place like a control joint line. Strike off with a straight 2x4, float the surface, and cut control joints every 8 to 10 feet for 4-inch slabs so cracks form where you want them. Then keep the surface moist for several days: curing is a chemical reaction that needs water, and a slab that dries too fast ends up weak and dusty.
Frequently Asked Questions
How many bags of concrete do I need for a 10x10 slab at 4 inches?
A 10x10 slab at 4 inches deep is 33.33 cubic feet. With 10 percent waste that is 36.67 cubic feet, which needs 62 bags of 80 lb mix (0.6 cu ft per bag) or 82 bags of 60 lb mix (0.45 cu ft per bag). That is roughly 5,000 pounds of dry mix, so plan for delivery or several truck trips.
How thick should a concrete slab be?
4 inches is standard for walkways, patios, and shed floors. Driveways and garage floors need 5 to 6 inches to handle vehicle weight. Footings depend on soil and load, but 8 to 12 inches deep is common for residential work. Thicker slabs resist cracking and frost heave better, and the extra concrete is cheap insurance.
How much water goes in each bag?
An 80 lb bag needs about 3 quarts (0.75 gallons) of water, and a 60 lb bag needs about 2.25 quarts. Add water gradually and mix to a stiff, workable consistency, not soup. Too much water weakens concrete dramatically: every extra gallon per bag can cut final strength by hundreds of PSI.
When should I order ready-mix instead of bags?
Once your project passes about 1 cubic yard (27 cubic feet), a ready-mix truck is usually cheaper and far less labor. One cubic yard equals 45 bags of 80 lb mix. For anything over 2 cubic yards, bag mixing is genuinely punishing: you will spend all day mixing and risk cold joints between loads.
How long does concrete take to cure?
Concrete sets enough to walk on in 24 to 48 hours and reaches about 70 percent of its strength in 7 days. Full design strength takes 28 days. Keep it moist for the first few days (mist it or cover with plastic) and keep vehicles off new driveways for at least 7 days.