Concrete Countertop Calculator

1. Enter the top length, width and thickness in inches.
2. Set the number of pieces for identical tops or sections, and a 10 percent waste allowance.
3. Pick the bag size that matches your mix yield.
4. Read the result: bags to buy, with the cubic feet, weight and surface area to plan the pour.

Here is what each result line means:

A concrete countertop calculator turns the size of a cast top into the concrete volume and the number of bags to buy. It is for countertops and vanity tops cast from a bagged mix, whether poured in a mould or in place.

A top is a thin slab, so its volume is length times width times thickness, worked in inches because tops are small. The tool converts to cubic feet, divides by the bag yield and adds a waste allowance for the mix left behind and the edge build-up. To size the mix water for a strong, low-shrink top use the water cement ratio calculator, and for the binder the cement calculator.

Because a top is a thin rectangular slab, its volume is length times width times thickness. The tool keeps every dimension in inches and divides by 1728, the number of cubic inches in a cubic foot.

1. Multiply length by width by thickness in inches for the volume of one top in cubic inches.
2. Divide by 1728 for cubic feet, then multiply by the number of pieces.
3. Add the waste allowance for the mix left in the mixer and the edge build-up.
4. Divide by the bag yield for the bag count, and multiply cubic feet by 150 for the weight in pounds.

A thicker decorative edge is extra concrete beyond the flat slab; size that part with the concrete bag calculator and add it.

Suppose you are casting a kitchen top 72 inches long, 25 inches deep and 1.5 inches thick, with 10 percent waste.

Calculation: the volume is 72 x 25 x 1.5 = 2,700 cu in, which is 2700 / 1728 = 1.56 cu ft. Adding 10 percent waste gives 1.72 cu ft, about 3 eighty-pound bags and roughly 258 lb over 12.5 square feet of surface.

The lesson: a modest kitchen top is only a few bags, but it still weighs over 250 pounds, so plan the support and the lift.

Thickness

Thickness multiplies the whole surface, so it moves the volume most. A 2 inch top uses a third more concrete than a 1.5 inch one of the same size.

Surface Size

Length and width scale the volume directly. A large island holds far more than a small vanity top.

Edge Build-up

A thick drop edge or an apron adds concrete below the flat slab. Work that part out separately and add it to the slab volume.

Sink and Cooktop Cut-outs

A large undermount sink or cooktop opening removes some concrete. For a rough estimate you can ignore small cut-outs; subtract a big one from the area.

Cast tops can be made from a general concrete mix or a purpose-made countertop mix, which changes the yield and the finish.

Set the bag size to the yield printed on your mix. For the delivered price if you order ready-mix for a big run, the concrete cost calculator in the related tools prices it by the yard.

Buying Bags for a Cast Top

Turn the top size into a bag count before you shop, so you buy enough mix in one trip with a little to spare.

Planning the Support

The weight tells you how much the finished top weighs, so you can build cabinets and a base that carry it.

Sealing and Finishing

The surface area sizes the sealer and the polishing work after the pour.

1. Mixing Up Inches and Feet

Enter every dimension in inches, as the fields are labelled. A top is small, so a feet-and-inches mix-up throws the bag count badly off.

2. Ignoring the Edge Build-up

A thick drop edge can add as much as the slab itself on a small top. Add the edge volume separately.

3. Using the Wrong Bag Yield

A countertop mix may not yield the same as a general mix. Set the bag size to the yield printed on the bag you are buying.

4. Forgetting Waste

Some mix stays in the mixer and on the tools, and a cast top is unforgiving if you run short mid-pour. Keep the waste allowance.

5. Treating the Estimate as a Mix Design

Volume is not a recipe. The mix, reinforcement and sealing for a food-safe top come from the product data sheet and your fabricator, not from this tool.

The arithmetic is exact for a flat rectangular top; the usefulness depends on the dimensions and bag yield you enter.

Multiply the length by the width by the thickness in inches and divide by 1728 for cubic feet, then add about 10 percent waste. A 72 by 25 inch top at 1.5 inches is 2,700 cubic inches, about 1.56 cubic feet, or roughly 3 eighty-pound bags with waste.

It depends on the size and the mix yield. A standard 72 by 25 inch kitchen top at 1.5 inches thick is about 3 eighty-pound bags with waste. The tool gives the bag count for your top and the bag size you select, rounded up.

Cast tops are commonly 1.5 to 2 inches thick, with a thicker look created by a drop edge rather than a thicker slab. This tool uses whatever thickness you enter; a thinner slab usually needs reinforcement to span.

About 150 pounds per cubic foot, so a 72 by 25 inch top at 1.5 inches weighs roughly 258 pounds with waste, or about 18 to 20 pounds per square foot at that thickness. The tool shows the weight so you can plan the cabinets and the lift.

A purpose-made countertop mix has fine aggregate that flows into thin forms and finishes smoother, while a general concrete mix suits thicker, rustic tops. Either works; set the bag size to the yield printed on the mix you buy.

For a rough estimate you can ignore small cut-outs and keep the margin. For a large undermount sink or a cooktop, subtract its area from the top to avoid over-ordering by a bag or two.

Yes, at least 10 percent. Some mix stays in the mixer and on the tools, the edge is built up, and a cast top is unforgiving if you run short mid-pour, so the allowance matters more than on a big slab.

No. The calculation runs entirely in your browser and nothing you enter is stored or sent anywhere.

• QUIKRETE Concrete Mix 1101 data sheet (QUIKRETE; bag yields 0.60, 0.45 and 0.30 cu ft).
• National Ready Mixed Concrete Association (NRMCA; concrete materials and ordering).
• American Cement Association (American Cement Association; concrete basics).