Concrete Calculator

45 bags. An 80 lb bag of standard concrete mix yields 0.60 cubic feet, and a cubic yard is 27 cubic feet, so 27 divided by 0.60 is exactly 45. For the other common sizes it is 60 bags at 60 lb (0.45 cu ft each) and 90 bags at 40 lb (0.30 cu ft each).

Those yields are the manufacturers' own printed figures and they are consistent across the major brands. They are yields, not weights: 45 x 80 lb is 3,600 lb of dry mix, while a cured cubic yard weighs closer to 4,000 lb, because you add roughly three to four quarts of water per 80 lb bag.

Around half a cubic yard, which is 13.5 cubic feet or about 23 eighty-pound bags. Under that, bags are simpler: no minimum order, no delivery window, no truck on your driveway, and you can pour in stages.

Over that, the math turns. Twenty-three bags is 1,840 lb to lift twice, plus a rented mixer, plus the very real risk of a cold joint if you cannot keep up with your own pour. Ready-mix arrives in one load at a consistent mix design. The catch is that plants usually have a minimum order, often one cubic yard, and a short-load fee below roughly three to five yards. Those numbers vary by region and by plant, so call two and compare rather than trusting a national average.

The common residential figures are 4 inches for a patio, shed pad, sidewalk or standard garage floor, and 5 to 6 inches where vehicles heavier than a car will sit, such as a driveway used by a truck or RV, or a garage with a lift.

Thickness is only half the answer. A 4 inch slab on a poorly compacted base cracks faster than a 4 inch slab on 4 inches of compacted crushed stone. Your local building department sets the requirement for anything structural, including footing depth below the frost line, and that requirement wins over any figure on a website. Check before you order.

Almost always because of hidden overage. Some calculators quietly add 5% or 10% to the volume and show you only the final number, so their figure looks bigger than the raw geometry. Others round the cubic yards up to the next quarter yard, which is how plants actually sell it, without saying so.

The geometry itself is not in dispute: a 20 ft x 20 ft slab at 4 inches is 400 sq ft x 0.3333 ft = 133.33 cubic feet, and 133.33 / 27 = 4.94 cubic yards. Every honest calculator gets 4.94. This page shows the base volume, the overage and the total as three separate lines so you can see which number you are looking at.

For a common 4x4 post in a 10 inch diameter hole dug 3 feet deep, the hole holds about 1.64 cubic feet. Subtract the volume the post itself takes up, roughly 0.29 cubic feet for a 4x4 at 3 feet, and you are near 1.35 cubic feet of concrete, which is about 3 eighty-pound bags per post.

Use the Column / Tube mode above with your real hole diameter and depth, and set the quantity to your number of posts. This calculator gives you the full hole volume, so treat it as a slight over-estimate for posts, which is the direction you want to be wrong in.

No, and it should not. Rebar and welded wire mesh displace a fraction of a percent of the volume, which is far smaller than the variation in your actual excavated depth. A vapor barrier sits under the slab and displaces nothing.

The real variable is the subgrade. A slab formed at a nominal 4 inches over a hand-dug base routinely comes out at 4.5 inches in places, and that alone is a 12% increase in volume. That is what the overage line is for, and it is why the allowance should go up when the base is rough.

Use the Stairs mode. The formula for a solid poured staircase with N steps is rise x run x N x (N + 1) / 2 x width. For 4 steps at a 7 inch rise and an 11 inch run, 4 feet wide, that is 0.583 x 0.917 x 10 x 4 = 21.39 cubic feet, or 0.79 cubic yards.

The assumption baked into that formula is a solid block poured down to grade. If your stairs sit on compacted fill, are formed over a void, or are cantilevered from a wall, you need less, sometimes far less. Measure what you are actually filling and use the slab or footing mode for the pieces if the shape is unusual.

A sonotube is a cardboard forming tube used for round footings and pier columns. It is sold by inside diameter, commonly 8, 10, 12, 16 and 18 inches. The concrete volume is simply the volume of the cylinder: pi x radius squared x height.

A 12 inch tube has a 6 inch radius, which is 0.5 ft, so each foot of height takes 0.785 cubic feet. A 4 foot tube holds 3.14 cubic feet, about 0.12 cubic yards or 6 eighty-pound bags. Remember to measure the full height including the part below grade, and check your local frost depth requirement before you dig.