🏗️ Free Concrete Calculator

A concrete calculator is a free online tool that estimates the volume of concrete you need for a pouring job—slabs, columns, or tubes—and turns that volume into practical numbers: cubic feet, cubic yards, and the number of 80 lb bags to buy. Instead of doing geometry by hand and hoping you ordered enough, you enter a few dimensions and get an answer in seconds.

It is used for everyday projects such as patios, walkways, shed bases, fence-post footings, and small foundations. The tool is just as useful before the pour, when you are comparing the cost of bagged mix versus a ready-mix delivery, as it is on the day of the job, when you need to know exactly how many bags to bring to the mixer.

Anyone can use it: homeowners planning a weekend project, landscapers laying paths, contractors quoting multiple jobs, and students checking their math. You only need the shape of your pour and its dimensions in feet and inches—the calculator handles the unit conversions and the bag math for you.

Because the tool returns three views of the same job—cubic feet, cubic yards, and bag count—it works no matter how you plan to buy the material. A small repair calls for bags, a medium slab might justify a short load from a local plant, and a large foundation is usually ordered by the truckload. Seeing all three numbers side by side makes that decision immediate and removes the guesswork from the most expensive part of the project.

The calculator uses the standard geometry for each shape. For a slab, it multiplies length × width × thickness, with thickness converted from inches to feet by dividing by 12. For a column or a tube, it uses the area of the circle—π × radius²—and multiplies that by the height or depth. The radius is half the diameter you enter, and the height is also converted to feet when needed.

Once the volume is known in cubic feet, the tool applies two practical conversions. First, it divides by 27 to give cubic yards, because ready-mix concrete is sold by the cubic yard (1 cu yd = 27 cu ft). Second, it divides by 0.6 to estimate how many 80 lb bags are required, since one 80 lb bag of standard mix yields about 0.6 cu ft. The quantity field simply multiplies the result when you are pouring several identical shapes.

The result is an estimate of volume, not of strength. The PSI rating of the finished concrete depends on the mix you buy and the water you add, so always match the mix design to the job and follow the bag or delivery ticket instructions.

People reach for a concrete calculator for the same reason on every job: concrete is expensive, heavy, and hard to return, so guessing the quantity is a costly mistake. A few common situations:

• Patios and walkways: running short mid-pour ruins the finish, while over-ordering leaves you with hardened leftovers you cannot return.
• Footings and fence posts: getting the volume right keeps posts stable and avoids a second trip to the store for extra bags.
• Budget planning: comparing the bag total with a ready-mix quote before the job starts saves money on larger pours.
• Quoting work: contractors price multiple projects quickly with consistent, defensible numbers instead of rough guesses.
• Learning and checking: students and DIYers verify their own geometry and unit conversions against a reliable reference.

Whatever the project, the alternative—mental arithmetic or a spreadsheet—is slower and more error-prone, especially once inches, feet, cubic yards, and bag yields are all mixed together.

Example 1: a 10×10 ft patio slab, 4 inches thick.

1. Area: 10 ft × 10 ft = 100 sq ft.
2. Thickness: 4 in ÷ 12 = 0.333 ft.
3. Volume: 100 × 0.333 = 33.3 cu ft.
4. Cubic yards: 33.3 ÷ 27 = 1.23 cu yd.
5. 80 lb bags: 33.3 ÷ 0.6 = 55.6, so buy 56 bags—or about 62 with a 10% waste allowance.

Example 2: a round footing, 12 inches in diameter and 4 feet deep.

1. Radius: 12 in ÷ 2 = 6 in = 0.5 ft.
2. Circle area: 3.14 × 0.5 × 0.5 = 0.785 sq ft.
3. Volume: 0.785 × 4 ft = 3.14 cu ft.
4. 80 lb bags: 3.14 ÷ 0.6 = 5.2, so buy 6 bags for one footing.

Both examples show why the conversion steps matter: the slab is over a cubic yard and is a good candidate for ready-mix, while one footing uses barely six bags and is far cheaper to mix by hand.

Even with a calculator, small input errors change the answer a lot. Watch for these common problems:

• Mixing units. Entering the thickness as 4 instead of 4 inches gives 100 cu ft instead of 33.3 cu ft—a threefold overestimate. Always enter inches where the field asks for inches.
• Forgetting the ÷12 for thickness. Thickness is usually measured in inches, but volume needs feet. The tool converts it, so enter inches in the thickness field.
• Ordering without the ÷27. Ready-mix is quoted per cubic yard. Forgetting to convert cubic feet to cubic yards can make a small patio look like a truckload order.
• Skipping the waste allowance. Subgrade is rarely perfectly level and spillage happens. Order 5–10% extra, or you may run short mid-pour.
• Confusing diameter and radius. For columns and tubes, the calculator needs the diameter; using the radius instead quadruples the volume.
• Ignoring subgrade condition. Loose or low spots swallow concrete. Compact the base and re-measure after leveling, not before.

1. Select shape (Slab, Column, or Tube).
2. Enter dimensions in feet and inches.
3. Set quantity for multiple identical pours.
4. Click Calculate for volume and bag count.

How many bags of concrete do I need for a slab?

Multiply the length by the width by the thickness to get the volume in cubic feet, then divide by the yield per bag—about 0.6 cu ft for an 80 lb bag. For example, a 10×10 ft slab at 4 inches thick is 33.3 cu ft, which is about 56 bags. Round up and add roughly 10% for waste.

What is the difference between cement, concrete, and mortar?

Cement is the binding powder; concrete is cement mixed with aggregate (sand and gravel) and water; mortar is cement mixed with sand and water, used to bond bricks or blocks. The words are often used interchangeably, but they are different materials with different jobs.

How much concrete for a 10×10 slab at 4 inches thick?

About 1.23 cubic yards (33.3 cu ft). Order roughly 10% extra—around 1.35 to 1.4 cubic yards—to cover spillage and an uneven subgrade, so you do not run short during the pour.

Should I mix my own or order ready-mix?

For pours under about 1 cubic yard, bagged mix is practical and needs no delivery truck. Larger pours are usually cheaper and more consistent with ready-mix delivery, because hand-mixing dozens of bags is slow and hard to keep uniform.

How do I convert cubic feet to cubic yards?

Divide the cubic feet by 27, because 1 cubic yard equals 27 cubic feet. For example, 54 cu ft ÷ 27 = 2 cu yd. Ready-mix concrete is sold by the cubic yard, so this conversion matters when you order a delivery.

What strength concrete (PSI) should I use?

3000–4000 PSI suits most slabs, patios, and walkways, while driveways and footings often use 4000–5000 PSI to handle heavier loads. Check the bag label or ask the ready-mix supplier, and match the mix to the job before you pour.

Use the same unit of measure everywhere, keep a note of the dimensions on your phone, and measure the pour twice before you order. A few habits that make the estimate reliable:

• Add 5–10% for waste and uneven subgrade; round bag counts up to a whole number.
• For pours under about 1 cu yd, bagged mix is practical. For larger pours, compare the bag total with a ready-mix quote—it is usually cheaper and more consistent.
• Choose the right strength: 3000–4000 PSI suits most slabs and patios, while driveways and footings often use 4000–5000 PSI. Check the bag label or ask the supplier.
• Cure the concrete properly: keep it moist for about 7 days after the pour so it reaches full strength and resists cracking.
• Use the quantity field for repeated shapes, such as several fence-post footings, to total the job in one calculation.