Work out how many bags of cement you need for a slab, footing or patio. Enter your dimensions, pick a mix ratio, and get bags, sand, aggregate and cost.
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This cement calculator turns slab or footing dimensions into a bag count you can take to the yard. Enter length, width, and thickness, choose a mix ratio like 1:2:4, then set your bag size and waste factor. You’ll see cement weight, sand and aggregate volumes, water needed, and total cost. Handy for patios, footings, and small pours where guessing gets expensive.
Pouring a slab by the bag is one of those jobs where ten percent short means a second trip to the store, and ten percent over means sacks hardening in the garage. This cement calculator takes the dimensions of your pour (length, width, depth), the mix ratio you want, and your bag size, then hands back the bag count plus the sand, aggregate, and water that go with it.
It’s built for anyone pouring a patio slab, a shed footing, fence post bases, or a small driveway apron. The default 1:2:4 mix covers most residential work at roughly 3500 PSI. Change the ratio, bag size, or waste factor and the numbers update as you type. Nothing leaves your browser. The math runs client-side, so you can use it on a job site with no signal.
Pick your currency from the top dropdown. All cost figures follow that choice.
Enter the length of the pour, then set the unit beside it: ft, in, m, cm, mm, or yd.
Do the same for width.
Enter the depth or thickness. For slabs this is usually in inches.
Choose a concrete mix ratio. 1:2:4 is preselected.
Adjust the waste factor. Ten percent is a good default.
Confirm the concrete density. 145 lb/ft³ covers normal-weight mixes.
Set your bag size, then add a price per bag and any extra cost.
The Summary tab shows the bag count first. Detailed Breakdown lists every intermediate number, from wet volume to water in liters.
At its core, the calculator finds the wet volume of your pour, inflates it to a dry volume, and splits that dry volume across the mix parts. Cement is a fraction of the total, so it only gets its share.
Formula: Cement Bags = ⌈ ((Cement Part ÷ Total Parts) × L × W × D × 1.54 × (1 + Waste)) × 94 ÷ Bag Size in lb ⌉
Every dimension gets converted to feet before multiplying. A depth of 4 inches becomes 0.333 ft. Wet volume is length × width × depth in cubic feet. That’s the space your concrete fills, not the volume of dry material you buy.
The 1.54 factor converts wet volume into dry volume, covering the air voids and shrinkage that happen as cement hydrates and water evaporates. Cement itself has a density of 94 lb/ft³ in the formula, which is the standard figure used across the industry.
Ratio parts decide how the dry volume splits. In a 1:2:4 mix, the total is 7 parts. Cement gets 1/7, sand gets 2/7, aggregate gets 4/7. The bag count is rounded up, since you can’t buy a fraction of a sack.
If any dimension is zero or blank, the results clear. That’s it for validation. There’s no upper limit, so a very large pour will return a very large number.
Say you’re pouring a patio that’s 12 ft long, 9 ft wide, and 5 inches thick, using a 1:2:4 mix with a 10% waste factor and 80 lb bags.
Depth in feet: 5 ÷ 12 = 0.4167 ft
Wet volume: 12 × 9 × 0.4167 = 45.0 ft³
Dry volume: 45.0 × 1.54 = 69.3 ft³
With 10% waste: 69.3 × 1.10 = 76.23 ft³
Cement volume: (1 ÷ 7) × 76.23 = 10.89 ft³
Cement weight: 10.89 × 94 = 1,024 lbs
Bags needed: 1,024 ÷ 80 = 12.8, rounded up to 13 bags
Sand comes to 21.78 ft³ (about 0.81 yd³). Aggregate lands at 43.56 ft³ (about 1.61 yd³). Water works out near 61 gallons using a 0.5 water-to-cement ratio.
Thirteen bags covers the pour with a small buffer. If you drop the waste factor to 5%, you’d get 13 bags still, so the buffer absorbs it. Drop it to zero and you’d be at 12. On a pour this size, that one extra bag is cheap insurance.
A 10 ft × 10 ft slab at 4 inches thick with a 1:2:4 mix needs about 9 bags of 80 lb cement. That assumes a 10% waste factor. Bump the thickness to 6 inches and the count rises to roughly 14 bags.
That’s a ready-mix question, not a cement one. A cubic yard of concrete weighs about 4,000 lbs and takes roughly 45 to 50 bags of 80 lb concrete mix. This calculator returns cement bags only, plus the sand and aggregate you’d combine with them.
1:2:4 is the standard for residential slabs and reaches about 3500 PSI. Use 1:1:2 for thin sections or high-traffic surfaces, and 1:3:5 for footings where 3000 PSI is enough. Thicker slabs can handle a leaner mix.
For a 1:2:4 mix, sand is two-sevenths of the dry volume and gravel is four-sevenths. On a 45 ft³ wet pour, that’s about 22 ft³ of sand and 44 ft³ of aggregate, or 0.81 and 1.61 cubic yards.
Bag manufacturers quote yield per bag of premixed concrete, which already includes sand and aggregate. This tool calculates raw cement separately, so the two figures aren’t directly comparable. Use a premix calculator if you’re buying bagged concrete, not loose cement.
At a 0.5 water-to-cement ratio, an 80 lb bag of cement takes about 40 lbs of water, or roughly 4.8 gallons. Damp sand and aggregate reduce that figure, so start low and add gradually.
Ten percent covers most pours. Use 5 percent for a flat slab on a prepared base and 15 percent for footings dug into rough ground. Running short mid-pour is far more expensive than one extra bag.
Yes. Length, width, and depth accept meters, centimeters, and millimeters. Density can be entered as kg/m³, and bag size as kg. Bag weights of 25 kg and 50 kg are common outside the US.
Yes. Enter the footing’s length, width, and depth in whatever units you’re working with. Footings are usually deeper than slabs, so expect a higher bag count per linear foot. The mix ratio may need to shift toward 1:2:3 for structural work.
The tool estimates cement, sand, aggregate, water, and cost. Reinforcement is a separate line item with its own spacing and sizing rules. Use a rebar calculator for that, then add the cost to the “Additional Cost” field here.
This cement calculator produces estimates based on standard mix ratios, a fixed 1.54 dry-volume factor, and 94 lb/ft³ cement density. Real-world yields vary with moisture content, aggregate grading, and how the mix is placed. For structural work, load-bearing footings, or anything governed by a building code, have a qualified engineer or inspector review the numbers before you order. The cost output is a planning figure and doesn’t include delivery, taxes, or labor.
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