Concrete Slab Calculator - Cubic Yards & Bags

How to use this calculator

Pick the shape. Most patios, pads and shed floors are rectangles. Use Circle for round pads such as hot tub bases or fire pit areas.
Enter the size in feet. Measure the inside of your forms, not the outside of the boards.
Enter the thickness in inches. See How thick should the slab be? below.
Enter your prices. The ready-mix price, delivery fee and bag price start with example numbers. Replace them with quotes from your supplier and local store. A cleared field shows as not entered, and the bags vs. ready-mix comparison waits until all three are filled in. Enter 0 for the delivery fee if your supplier charges none. The default 10% waste is the top of NRMCA’s 4-10% range.

For an L-shaped slab, split it into two rectangles, calculate each one and add the results. If you are pouring several identical pads, set Number of slabs.

The formula

Concrete is sold by volume, so start with the volume of your slab:

Volume (cu ft) = length (ft) × width (ft) × thickness (in) ÷ 12

Cubic yards = cubic feet ÷ 27

For a round slab, the area is π × radius², and the radius is half the diameter. The calculator adds the waste percentage to this volume before it rounds anything.

To count bags, divide the volume by the yield of one bag and always round up. You can’t buy part of a bag.

Bag size	Yield per bag	Bags per cubic yard
80 lb	0.60 cu ft	45
60 lb	0.45 cu ft	60
40 lb	0.30 cu ft	90

These yields come from the Quikrete Concrete Mix (No. 1101) data sheet, revised October 2022, and Quikrete’s own calculator also tells you to round up to whole bags. High-strength, fast-setting, fiber-reinforced and other brands’ mixes list their own yields. Check the bag and adjust if yours differs.

Ready-mix is rounded up to the next ¼ cubic yard, and the cost uses that order plus your delivery fee. Suppliers set their own order steps and minimum loads, so confirm both when you call.

Worked example: a 12 × 16 ft patio

A 12 × 16 ft patio poured 4 inches thick, with 10% extra:

Volume: 12 × 16 × 4 ÷ 12 = 64 cubic feet
With 10% waste: 64 × 1.10 = 70.4 cubic feet
In yards: 70.4 ÷ 27 = 2.61 cubic yards → ready-mix order 2.75 yd³
In 80 lb bags: 70.4 ÷ 0.60 = 117.3 → 118 bags (or 157 × 60 lb, or 235 × 40 lb)
Control joints: at most 8 ft apart, 1 inch deep → 2 × 2 panels of 8 × 6 ft, 28 ft of joints

At the calculator’s example prices ($165/yd³, $150 delivery, $6.50 per bag), ready-mix comes to 2.75 × $165 + $150 = $604 and bags to 118 × $6.50 = $767. Your own quotes may change the answer. Either way, 118 bags is 9,440 lb of dry mix to lift and mix, a big job for a small crew.

Bags or ready-mix?

The calculator compares the two at your prices. Price isn’t everything, though:

Labor. Every 80 lb bag has to be carried, opened and mixed. Quikrete says you can mix by hand in a suitable container or in a drum or mortar mixer. With a mixer, allow at least 1 cubic foot of mixer capacity for each 80 lb bag in a batch.
One continuous pour. Quikrete says to work continuously from one end to the other and avoid partial-depth fills, which can lead to cold joints. Make sure your crew and mixer can keep up with the slab size.
Ready-mix logistics. Ask about delivery or short-load fees, the minimum load, how close the truck can get and how much time you get to unload. NRMCA advises re-checking the remaining volume near the end of a pour and avoiding short loads to finish it.

How thick should the slab be?

Use	Thickness	Source
Sidewalks	at least 4 in	Quikrete, Sidewalks and Small Slabs
Patios	4 in is generally suitable	Quikrete, Concrete Patios
Small foundations, bases, pads	4-6 in, depending on the load	Quikrete, Sidewalks and Small Slabs
Driveway, passenger cars only	4 in	Quikrete, Driveways and Curbs
Driveway with trucks or heavy vehicles	5-6 in	Quikrete, Driveways and Curbs
Concrete floor slab of a house	at least 3½ in	IRC R506.1

These are minimums for typical conditions, not a design. Soft, expansive or poorly drained soil, heavy loads (RVs, equipment, hot tubs) and local rules can call for a thicker slab, reinforcement or an engineered design. Quikrete notes that the part of a driveway between the street and the sidewalk is usually controlled by the local municipality. Check with your building department before you pour.

Tips for a good pour

Prepare a firm base. Quikrete says to remove sod and soil, then tamp and compact the sub-base until firm. For larger jobs or poor drainage, dig deeper and add several inches of gravel or crushed stone. Under a house floor slab below grade, the IRC requires a 4-inch base of clean graded sand, gravel or crushed stone, unless the slab sits on well-drained or sand-gravel soil classified as Group I. Our topsoil & dirt calculator works out excavation and fill volumes.
Know what reinforcement does. NRMCA points out that wire mesh won’t prevent cracking. It helps keep cracks and joints from opening up. If you use mesh or rebar, support it so it stays inside the slab instead of lying on the ground. Quikrete says at mid-depth, and the IRC (for house floor slabs) says between the middle and the upper third. Stop the mesh, or cut alternate wires, at control joints. If the slab is structural, sits on poor soil or carries heavy loads, have a professional or your local code specify the reinforcement.
Plan control joints. NRMCA recommends spacing them no more than 24-36 times the thickness and never more than 15 ft apart. Keep panels near square (length ≤ 1.5 × width) and cut grooves at least ¼ of the thickness deep, but not less than 1 inch. Quikrete says 6-8 ft for a 4-inch slab. The calculator uses the stricter value (8 ft for 4 inches) and suggests a panel layout. Tool joints early in finishing, or saw them within 4-12 hours after finishing (1-4 hours with an early-entry saw). Separate the slab from walls, steps and existing slabs with isolation-joint material.
Cure it. Start curing as soon as you can and keep it going for 5 days at 70 °F or warmer, or 7 days at 50-70 °F, per the Quikrete data sheet. Use a curing compound, plastic sheeting laid flat and sealed at the edges, wet burlap or continuous sprinkling. Don’t apply curing compound if rain or temperatures below 50 °F are expected within 24 hours.
Cold weather. Quikrete says to protect the concrete from freezing for the first 48 hours, with plastic sheeting and insulating blankets if temperatures are expected to drop below 32 °F. Never pour onto frozen ground, snow or ice. NRMCA warns that frozen fresh concrete can lose more than half its potential strength.
Hot, dry or windy weather. NRMCA advises avoiding placing and finishing during the hottest part of the day, dampening the subgrade and forms without leaving puddles, and having enough helpers to place and finish quickly. Start curing as soon as finishing is done. Don’t sprinkle water on the surface to make finishing easier.

Frequently asked questions

How many 80 lb bags of concrete do I need for a 10x10 slab?

You need 62 bags of 80 lb concrete for a 10 × 10 ft slab 4 inches thick, with 10% extra for waste and uneven ground. The slab is 33.3 cubic feet, and Quikrete's 80 lb Concrete Mix yields about 0.60 cubic feet per bag. With nothing extra, that is 56 bags.

How many bags of concrete make a yard?

A cubic yard takes 45 bags of 80 lb, 60 bags of 60 lb or 90 bags of 40 lb Quikrete Concrete Mix (No. 1101). One cubic yard is 27 cubic feet, and those bags yield 0.60, 0.45 and 0.30 cu ft each. High-strength, fast-setting and other brands' mixes have their own yields, so check the bag.

Is it cheaper to buy bags or order ready-mix?

It depends on your local prices and on any delivery or short-load fee, so there is no single break-even size. Enter your quotes: the calculator prices ready-mix on the rounded-up order plus the fee, and bags on the whole number you would buy. Also count the labor, because every 80 lb bag has to be lifted and mixed.

How thick should a concrete slab be?

Plan on at least 4 inches for sidewalks, patios and car-only driveways. Go to 5 to 6 inches for a driveway that trucks or other heavy vehicles will use. Those are Quikrete's numbers: at least 4 inches for sidewalks, 4 inches generally suitable for a patio, and 4 inches for a driveway used only by passenger cars. The International Residential Code requires at least 3½ inches for a concrete floor slab inside a house. Soft or poorly drained soil, heavy loads and local rules (especially for the part of a driveway in the public right-of-way) can require more, so check with your building department.

How far apart should control joints be?

For a 4-inch slab, space control joints no more than 8 feet apart. NRMCA recommends a maximum of 24 to 36 times the slab thickness, never more than 15 feet, with panels close to square (length no more than 1.5 times the width). NRMCA also calls for joints at least ¼ of the slab thickness deep, but not less than 1 inch. Quikrete says 6 to 8 feet for a 4-inch slab, and the calculator uses the stricter 24 times the thickness.

How much extra concrete should I order?

Order at least 4 to 10% more than the measured volume, as NRMCA recommends, to cover waste, spillage, form bulging and uneven ground. Small errors add up: a slab that ends up ⅛ inch thicker than 4 inches needs about 3% more concrete. Running short mid-pour can leave you with an unplanned construction joint.

Sources

Last reviewed Oct 8, 2026. See how we calculate. Results are planning estimates - confirm with your supplier and local code.