What Is the Concrete Calculator?
The Concrete Calculator estimates the volume of concrete needed for a rectangular pour. It takes the length, width and thickness of a slab, pad or footing and returns the volume in cubic yards, cubic feet and cubic meters, along with the number of premixed bags that volume represents.
Concrete is ordered by volume. Ready-mix trucks deliver by the cubic yard, and bagged mix states its yield in cubic feet. Ordering too little is a serious problem, because a pour that stops halfway leaves a weak cold joint. Ordering far too much is expensive, and leftover concrete has to go somewhere. An accurate estimate with a sensible margin avoids both.
How the Calculator Works
The tool converts length and width to feet and thickness to inches, whatever units you enter. It multiplies the three to get cubic feet, adds your waste percentage, and divides by 27 to get cubic yards.
Bag counts come from typical yields. An 80 pound bag of standard concrete mix yields about 0.60 cubic feet. A 60 pound bag yields about 0.45 cubic feet. The calculator divides the total volume by each yield and rounds up to whole bags.
The default waste allowance is 10 percent. That covers uneven subgrade, forms that bow slightly under load, and spillage. On a carefully formed slab over a well compacted base you can reduce it to 5 percent.
The Concrete Formula
Volume (cubic feet) = Length (ft) × Width (ft) × Thickness (in) ÷ 12
Cubic yards = Cubic feet ÷ 27
Bags = Cubic feet ÷ Yield per bag, rounded up
Where 27 comes from
A cubic yard is a cube 3 feet on each side. Three times three times three is 27 cubic feet. This is the single most useful number in concrete estimating.
Step-by-Step Explanation
- Measure the length and width of the formed area in feet.
- Decide the thickness in inches and divide it by 12 to express it in feet. Four inches is 0.333 feet.
- Multiply length by width by thickness in feet. The answer is cubic feet.
- Multiply by 1.10 to add 10 percent for waste.
- Divide by 27 for cubic yards.
- For bags, divide the cubic feet by 0.60 for 80 pound bags or 0.45 for 60 pound bags and round up.
Worked Example
A patio slab measures 10 feet by 10 feet and will be 4 inches thick.
- Thickness in feet: 4 ÷ 12 = 0.333.
- Volume: 10 × 10 × 0.333 = 33.3 cubic feet.
- With 10 percent waste: 33.3 × 1.10 = 36.7 cubic feet.
- Cubic yards: 36.7 ÷ 27 = 1.36.
- 80 pound bags: 36.7 ÷ 0.60 = 61.1, so 62 bags.
- 60 pound bags: 36.7 ÷ 0.45 = 81.5, so 82 bags.
Sixty-two bags weigh almost 5,000 pounds and need mixing one or two at a time. At this size most people order 1.5 cubic yards of ready-mix instead, if the supplier’s minimum load allows it.
How to Use the Calculator
- Enter the slab length and choose the unit.
- Enter the width.
- Enter the thickness. Inches is the default, but centimeters and millimeters are available.
- Adjust the waste allowance. Ten percent is a sound starting point.
- Press Calculate and read the volume and bag counts.
- For a slab with a thickened edge or several sections, calculate each part and add the results.
For shapes that are not flat slabs, such as trenches or round holes, the Cubic Yard Calculator may be simpler. The base under the slab is a separate order, and the Gravel Calculator gives it in tons. If the slab is the base for a patio surface, the Paver Calculator covers the pavers and bedding sand. For walls built on a concrete footing, see the Concrete Block Calculator and the Mortar Calculator.
Practical Tips
- Check the forms before you order. Measure the inside dimensions, not the outside of the boards.
- Measure depth in several places. A base that dips by half an inch over a large slab adds a surprising volume.
- Round ready-mix orders up to the nearest quarter yard. Suppliers usually sell in quarter yard steps.
- Ask about minimum loads and short-load fees before deciding between bags and delivery.
- Have a place for excess. A small form for a stepping stone or a downspout pad puts leftover concrete to use.
- Pour in mild, dry weather where you can. Hot wind and freezing nights both damage fresh concrete.
- Order the right mix. A 3,000 psi mix suits most paths and patios, while driveways are often specified at 4,000 psi.
Common Mistakes
Entering thickness in feet. Typing 4 into a field that expects feet gives twelve times the real volume. This calculator labels thickness in inches to avoid that.
Ignoring the waste factor. The geometric volume assumes perfect forms and a perfectly flat base. Real sites have neither.
Forgetting thickened edges. Many slabs are deeper around the perimeter. Calculate the edge as a separate strip and add it.
Assuming all bags yield the same. High-strength and fast-setting mixes can have different yields. Check the bag.
Underestimating labor for bagged mix. Thirty bags is a hard day for two people. Sixty is rarely sensible without a mixer and help.
Not planning joints. Control joints do not change the volume, but forgetting them leads to random cracks. Plan them before the truck arrives.
Bags or Ready-Mix?
Bagged concrete suits fence posts, small pads and repairs. It lets you work at your own pace and needs no delivery. The break-even point for most homeowners is around one cubic yard, which is 45 bags of the 80 pound size.
Ready-mix arrives consistent and all at once. That is an advantage for finishing quality and a pressure on timing. You need the forms, base, reinforcement and helpers ready before the truck arrives.
Thickness Guidelines
Four inches is standard for walkways and patios. Driveways for passenger cars are usually 4 to 5 inches, and 6 inches where heavy vehicles park. Shed and garage slabs often have a thickened edge of 8 to 12 inches. Local codes and soil conditions can change these figures, so confirm them before you build forms.
If you still need the area of an irregular slab, split it into rectangles with the Square Footage Calculator and then enter each rectangle here.
Preparing the Base
Concrete needs uniform support. Remove topsoil and organic material, then place 4 inches of crushed stone and compact it in layers. A plate compactor is worth renting for anything larger than a small pad. Dampen the base before the pour so it does not pull water out of the mix.
Reinforcement
Welded wire mesh or number 3 rebar on an 18 inch grid is common in residential slabs. Reinforcement holds cracks tightly closed so they stay hairline. It must sit in the middle of the slab’s thickness to work, so support it on chairs. Fibre-reinforced mixes are an alternative for small slabs.
Curing
Concrete gains strength by hydration, which needs moisture. Keep the surface damp for the first week by misting, covering it with plastic, or applying a curing compound. Concrete reaches about 70 percent of its design strength in 7 days and is considered fully cured at 28 days. Keep vehicles off a new driveway for at least a week.
Estimating Round Footings and Post Holes
A cylinder’s volume is 3.1416 times the radius squared times the depth. A 12 inch diameter hole 3 feet deep holds 2.36 cubic feet, which is four 80 pound bags. For a fence line, multiply by the number of posts. The Fence Calculator estimates the bag count for typical post holes automatically.
What a Yard of Concrete Covers
One cubic yard covers 81 square feet at 4 inches thick, 65 square feet at 5 inches and 54 square feet at 6 inches. These figures are a quick way to sanity check any estimate. If your slab is 400 square feet at 4 inches, you should expect a little under 5 cubic yards before waste.
Weather and Timing
The best pouring temperatures are between 50 and 80 degrees Fahrenheit. In hot weather, start early and have extra hands for finishing because the surface sets quickly. In cold weather, protect the slab with insulating blankets and never pour onto frozen ground.