Concrete Area Calculator
Pouring a slab starts with two questions: how big is the area, and how much concrete does it take to fill it? Answer the first one wrong and the second one is wrong too, which is how pours run short with the forms already full.
Enter length, width, and thickness to get the area, the volume in cubic feet and cubic yards, and the bag count for 40, 60, or 80 lb bags with an optional waste allowance built in. A second mode works backwards: enter the cubic yards you have and the thickness, and it returns the area that volume will cover.
Rectangular slabs only. Split L-shaped or irregular pours into rectangles, run each one, and add the volumes. Bag yields follow the QUIKRETE Concrete Mix data sheet.
Quick Answer
Key takeaways
- Area is length x width; volume is area x thickness in feet; cubic yards are cubic feet divided by 27.
- The waste allowance is inside the math here, not a note underneath it: yardage and bags both grow by the percentage you set.
- Bag count is volume divided by yield, rounded up: 0.60 ft3 for 80 lb, 0.45 for 60 lb, 0.30 for 40 lb.
- Past about one cubic yard, price ready-mix before committing to bags: the worked example is nearly five tons of bagged mix.
How to use the concrete area calculator
Stay in the default Area and volume mode for a planned pour. Enter length and width in feet (or switch both to meters), set the thickness in inches or centimeters, pick the bag size you would actually buy, and set a waste allowance: 10% unless your subgrade is laser-flat and your forms are stiff. The result lists the area, the bare volume, the waste-adjusted volume, the yardage to order, and the bag count, in that order, so you can see exactly where the allowance entered.
Switch to Coverage from volume when the concrete is already decided: a leftover yard on a neighbor's order, a minimum-load delivery, or a bag stash in the garage. Enter the cubic yards and the pour thickness and it returns the square footage that volume covers, in square feet and square meters. If you already know your yardage and want to price or verify a specific pour shape instead, the concrete volume calculator covers additional shapes beyond the rectangle.
The concrete formulas behind the numbers
Every figure on this page is geometry plus one division, and the calculator shows its intermediate steps so you can audit it with a tape measure and a phone.
Thickness is the input people most often fumble, because it arrives in inches and the formula wants feet. Four inches is 0.333 feet, and forgetting that division inflates the volume twelvefold, an error that is obvious in a bag count (1,476 bags for a patio) but dangerously plausible in yardage if you are tired. The calculator converts inches and centimeters internally, which is the entire reason to use it instead of a napkin.
Bag yields are manufacturer figures, not conventions. The QUIKRETE Concrete Mix data sheet lists approximately 0.60 cubic feet for an 80 lb bag, 0.45 for 60 lb, and 0.30 for 40 lb, and those are the three numbers hard-wired into the bag math here. Check the yield printed on any specialty mix (fast-setting and high-strength bags differ slightly) before trusting a count to the last bag.
Worked example: a 20 by 10 patio at 4 inches
The calculator is pre-filled with this pour, and every figure below is what it returns.
| Measure | Result | How it is derived |
|---|---|---|
| Area | 200.00 sq ft | 20 x 10 |
| Volume | 66.67 ft3 | 200 x (4 / 12) |
| Volume with 10% waste | 73.33 ft3 | 66.67 x 1.10 |
| Order quantity | 2.72 yd3 | 73.33 / 27 |
| Bags needed (80 lb) | 123 bags | 73.33 / 0.60, rounded up |
Two reality checks on that example. First, the bag count is a warning label: 123 bags at 80 lb is 9,840 lb of dry mix to haul, open, mix, and place before the first batches start to stiffen, which is why the usual advice caps bag work at roughly a cubic yard and why this pour, at 2.72 yards, is a ready-mix job for most people. Second, the waste line is doing real work: without it you would order 2.47 yards, and a subgrade that dips half an inch across a fifth of the slab quietly eats more than that. Once the yardage is settled, the concrete driveway cost per square foot calculator turns contractor quotes for jobs like this into comparable per-foot and per-yard prices.
Coverage reference: what a yard buys at each thickness
These figures come straight out of the coverage formula, and they are the fastest sanity check on any quote or delivery ticket.
| Thickness | Area per cubic yard | Yards per 100 sq ft | Typical use |
|---|---|---|---|
| 3 in | 108 sq ft | 0.93 | Thin overlays, light-duty pads |
| 4 in | 81 sq ft | 1.23 | Patios, walkways, shed pads |
| 5 in | 64.8 sq ft | 1.54 | Driveways with heavier vehicles |
| 6 in | 54 sq ft | 1.85 | Driveways, garage floors, RV pads |
Drainage belongs in the same planning pass as thickness. An outdoor slab needs a slight fall so water runs off instead of pooling, and the concrete slope calculator works out the drop over your slab's length before the forms go in, when changing it is still free.
What competing calculators miss
The established concrete calculators are good at the central division and weak at everything around it. Calculator.net and InchCalculator return yardage cleanly, but the waste allowance lives in their advice text ("order 5 to 10% extra") rather than in the arithmetic, so the number on screen is never the number to order. Several bag calculators quietly assume 80 lb bags, which is the size many DIY buyers cannot comfortably lift all day.
Miss 1: waste as prose, not math. Here the allowance is an input, applied before the yardage and bag conversions, and the result shows both the bare and adjusted volumes so the size of the cushion is visible.
Miss 2: one direction only. "How much area will the concrete I have cover?" is the second question on every real job, and almost no top-ranking tool answers it. The coverage mode here runs the formula backwards with the same thickness logic.
Miss 3: bag size and metric treated as afterthoughts. Three bag sizes with the manufacturer's own yields, plus meters and centimeters as first-class inputs with a square-meter readout, cover the buyers the big tools leave converting by hand.
Frequently Asked Questions
Multiply length by width in the same unit. A 20 by 10 foot patio is 200 square feet. For L-shaped or irregular slabs, split the shape into rectangles, calculate each one, and add the areas before you work out the volume.
Multiply the area by the thickness in feet, then divide cubic feet by 27. The calculator does this for you and adds your waste allowance before converting, so the yardage it shows is the order quantity, not the bare geometry.
Divide your cubic feet by 0.60, the yield of an 80 lb bag, and round up. A 66.67 cubic foot pour needs 112 bags before waste; with a 10% waste allowance the adjusted volume needs 123 bags.
Yield. An 80 lb bag makes about 0.60 cubic feet of concrete, a 60 lb bag about 0.45, and a 40 lb bag about 0.30, per the QUIKRETE Concrete Mix data sheet. One cubic yard therefore takes 45, 60, or 90 bags respectively. Smaller bags cost more per cubic foot but are far easier to lift and mix.
Four inches suits patios, walkways, and shed pads. Go 5 to 6 inches for driveways and garage floors that carry vehicles, and follow local code or an engineer for anything structural. Thickness multiplies straight through the volume, so 6 inches needs 50% more concrete than 4 inches on the same footprint.
Yes. Order 5 to 10% extra for spillage, uneven subgrade, and forms that bow slightly under load. The waste input in this calculator adds the allowance before the yardage and bag math, so you do not have to inflate the numbers yourself and risk double-counting.
At 4 inches thick, one cubic yard covers 81 square feet. At 6 inches it covers 54 square feet. Use the coverage mode with your exact thickness for the precise figure, in square feet and square meters.
Yes. Set the length and width unit to meters and the thickness unit to centimeters, and the tool converts before calculating. It then reports the area in square feet and square meters, with the volume still in cubic feet and cubic yards for ordering.
Bags usually win under about half a cubic yard, where a delivery minimum and short-load fees dominate. Above roughly one cubic yard, ready-mix is typically cheaper per yard and far less labor: the 123 bags in the worked example weigh close to five tons and have to be mixed before the first batch starts to set.
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References
- QUIKRETE, "Concrete Mix No. 1101" technical data sheet - the manufacturer source for the bag yields used in the bag math: approximately 0.60 cubic feet per 80 lb bag, 0.45 per 60 lb, and 0.30 per 40 lb. Quikrete.com product literature. Accessed October 2026.
- Federal Highway Administration, "Concrete Pavement Design" (Long-Life Concrete Pavements chapter) - pavement thickness design context and the role of slab thickness in load capacity. FHWA: Concrete Pavement Design. Accessed October 2026.
- National Frame Building Association, "Slab on Grade" design guidance - the Portland Cement Association thickness design method for slabs on ground, including vehicle load examples. NFBA: Slab on Grade (PCA method). Accessed October 2026.
- City of Maple Heights Codified Ordinances, "Specifications for Concrete Work" - a worked example of local code specifying driveway and sidewalk thickness, joints, base, and curing. Maple Heights Code: Concrete Work Specifications. Accessed October 2026.
- Practical DIY, "Concrete: Designing and Planning" - residential slab thickness, sub-base, slope, and expansion joint guidance for patios, pads, and driveways. Practical DIY: Concrete Design and Planning. Accessed October 2026.