Work out how much concrete a slab, footing or patio needs. Enter the dimensions, pick bag size or ready-mix, then get volume, bag count, weight and cost.
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Bagged concrete is convenient until you need 158 of them. At that point a ready-mix truck starts looking cheaper, faster, and a lot kinder to your back. This concrete calculator estimates both paths from the same set of dimensions so you can see where the crossover sits.
Feed it the length, width, and thickness of your slab, footing, or patio. Set how many you’re pouring. Pick a bag size, confirm the density, and plug in local prices. The tool returns total volume in cubic feet, yards, and meters, the concrete weight, the bag count, and the estimated bill for each method.
Contractors use it for quick quotes. Homeowners use it before ringing a supplier. Everything runs in the browser, so your numbers stay private and the page keeps working when the signal drops.
Select your currency from the top dropdown.
Enter the length of the pour, then choose a unit: ft, in, m, cm, mm, or yd.
Do the same for width.
Add the thickness or depth. Slabs are usually measured in inches.
Set the number of slabs, areas, or footings you’re pouring.
Adjust the waste factor if 10 percent doesn’t fit your pour.
Pick a concrete strength, then confirm density and bag size.
Enter your price per bag and price per cubic yard of ready-mix.
The Summary tab leads with total volume. The Detailed Breakdown tab lists every intermediate value, from yield per bag to the ready-mix conversion.
Concrete volume is length times width times thickness, multiplied by how many pours you’re making, then inflated by the waste factor. Everything below that is unit conversion and pricing.
Formula: Volume per slab = Length × Width × Thickness (all in feet)
Formula: Total volume with waste = Volume per slab × Count × (1 + Waste %)
Formula: Bags needed = ⌈ Total volume with waste ÷ (Bag size in lb ÷ 133) ⌉
The 133 constant represents pounds of mixed concrete per cubic foot in the bag. An 80 lb sack yields about 0.60 ft³. A 60 lb sack yields roughly 0.45 ft³. Divide your total volume by the yield and you get the bag count, rounded up.
Formula: Ready-mix cost = Total volume with waste (yd³) × Price per yd³
If your price is entered per cubic meter, the tool converts it by multiplying by 0.764555 before the cost runs. Concrete weight comes from the density field: total volume with waste times density in lb/ft³. The default of 145 lb/ft³ matches normal-weight concrete.
The recommendation logic is a single threshold. Above 1 cubic yard, the tool labels ready-mix as the recommended method. At or below, bagged concrete gets the nod. Total cost then follows whichever method was recommended, plus any additional cost you entered.
One note on the strength dropdown. It records the PSI you’re targeting and displays it in the breakdown, but it doesn’t shift the bag yield or the volume math. Bag yield comes from bag weight alone.
Validation is minimal. If length, width, or thickness is zero or blank, the results clear. No upper bound exists, so a very large pour returns a very large number.
Say you’re pouring a driveway apron 16 ft long, 11 ft wide, and 6 inches thick. Waste factor is 8 percent. Bag size is 80 lb. Price per bag is $6.50, ready-mix is $150 per cubic yard.
Thickness in feet: 6 ÷ 12 = 0.5 ft
Volume per slab: 16 × 11 × 0.5 = 88.00 ft³
Total volume: 88.00 ft³, or 3.259 yd³
With 8% waste: 95.04 ft³, or 3.520 yd³
Concrete weight: 95.04 × 145 = 13,781 lbs (about 6,251 kg)
Yield per 80 lb bag: 80 ÷ 133 = 0.6015 ft³
Bags needed: 95.04 ÷ 0.6015 = 158.0, rounded to 158
Cost using bags: 158 × $6.50 = $1,027.00
Cost using ready-mix: 3.520 × $150 = $528.00
Recommended method: Ready-Mix Delivery
The volume clears one cubic yard by a wide margin, which is why the tool flags ready-mix. Here it’s roughly half the price of bagging it yourself, and that gap doesn’t count the 158 trips from the truck or the rental mixer you’d need to keep up.
Multiply length by width by thickness, all in the same unit, then add 5 to 15 percent for waste. A 10 ft × 12 ft slab at 4 inches thick comes to 40 ft³, or about 1.5 cubic yards. Enter those numbers above and the tool does the rest.
About 45 bags of 80 lb mix, or 60 bags of 60 lb mix. An 80 lb bag yields roughly 0.60 ft³ of finished concrete. A cubic yard is 27 ft³, so the division works out to just under 45.
Under one cubic yard, bags make sense. Over that, ready-mix usually costs less and saves hours of mixing. Access decides it in practice. If a truck can’t reach the pour, bags may stay competitive even past the one-yard mark.
Most US markets run $130 to $180 per cubic yard for standard ready-mix. Short-load fees apply to orders under about 5 yards. Type your quoted number into the price field and the tool gives you a running total.
Cement is the powder that binds everything together. Concrete is the finished mix of cement, sand, aggregate, and water. You buy bags of concrete mix for small pours, not bags of cement.
Forty-five bags, roughly. Each one yields about 0.60 ft³ once mixed, so 45 bags gets you to 27 ft³, which is one cubic yard. Order a few extra if the pour is uneven or you’re working in wind.
Four inches works for patios and walkways. Driveways need 6 inches. Footings go deeper, usually 12 to 24 inches depending on frost depth and load. Thickness is the biggest single driver of your concrete order.
No. The strength dropdown records the PSI you’re targeting, and it shows up in the detailed breakdown. Bag yield and volume math depend on dimensions, waste factor, bag weight, and density, not on PSI.
Yes. Set the length and width to the footing dimensions and use the depth of the trench as thickness. Change the count field to “footings” so the breakdown reads correctly. Long runs of narrow footings add up quickly.
An 80 lb bag covers about 0.60 ft³, which is roughly 2 ft² at 4 inches thick. That’s why a small patio eats bags fast. A 6 ft × 6 ft slab at 4 inches needs around 20 of them.
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