Concrete Calculator

How Much Concrete Do You Need?

Concrete is sold by volume, not by area. A patio is not "100 square feet of concrete", it is 100 square feet at a chosen thickness, and that thickness is what decides the order. Get the volume right and everything else, bag counts, truck size, cost, falls out of it. Get it wrong and you either run short halfway through a pour or pay for material that sits in the driveway hardening.

Every concrete quantity, whatever the shape, comes from the same idea: find the volume in cubic feet, divide by 27 to get cubic yards, then add a waste allowance. Ready mix suppliers quote in cubic yards. Bagged mix is labeled in cubic feet of yield. Those are the only two units that matter.

The inches to feet unit trap

This is the single most common arithmetic error in concrete estimating. Length and width come naturally in feet. Thickness comes naturally in inches. Multiplying 10 x 10 x 4 gives 400, which is not cubic feet of anything, and dividing that by 27 suggests almost 15 cubic yards for a small patio. The real answer is closer to one and a quarter yards. Convert first: 4 inches divided by 12 equals 0.3333 feet, 5 inches equals 0.4167 feet, 6 inches equals 0.5 feet, 8 inches equals 0.6667 feet, 12 inches equals 1 foot.

A Worked Example: A 10 ft by 10 ft Slab at 4 Inches

Take a square patio slab, 10 feet by 10 feet, poured 4 inches thick.

• Convert the thickness: 4 divided by 12 equals 0.3333 feet.
• Volume: 10 x 10 x 0.3333 equals 33.33 cubic feet.
• Cubic yards: 33.33 divided by 27 equals 1.23 cubic yards.
• Add 10 percent waste: 1.23 x 1.10 equals 1.36 cubic yards.

In bags, work from the cubic feet, not the yards. With waste, the job needs 36.67 cubic feet. A 60 lb bag yields 0.45 cubic feet, so 36.67 divided by 0.45 equals 81.5, round up to 82 bags. An 80 lb bag yields 0.60 cubic feet, so 36.67 divided by 0.60 equals 61.1, round up to 62 bags. Always round up. Half a bag of dry mix is useless once the pour is underway.

Note what 82 bags actually means: nearly 5,000 pounds of material, mixed a bag or two at a time, in a window short enough that the first batch has not set before the last one goes in. That is the real argument against bagging a slab this size, and it is a scheduling problem before it is a cost problem.

Slabs, Footings, Columns and Steps

Slabs and pads

Rectangular slabs are the simple case: length x width x thickness. For an L shape, split it into two rectangles, calculate each, then add. Do not average an irregular shape into a single rectangle, because the error runs in whichever direction hurts most.

Footings and trenches

A footing is a long rectangular prism, so the formula is width x depth x total linear feet. For 40 linear feet of footing 16 inches wide and 8 inches deep: 1.333 x 0.6667 x 40 equals 35.56 cubic feet, or 1.32 cubic yards before waste. Footings poured against earth take the biggest waste allowance of any concrete work, because trench walls are never straight and every bulge is concrete you paid for.

Round columns and post holes

A cylinder is 3.1416 x radius in feet squared x height in feet. Radius is half the diameter, so a 10 inch tube has a radius of 5 inches, which is 0.4167 feet. For a 10 inch diameter tube 4 feet tall: 3.1416 x 0.4167 x 0.4167 x 4 equals 2.18 cubic feet, about four 80 lb bags. Deck footings and mailbox posts are usually estimated per hole and then multiplied by the number of holes.

Steps

Treat a flight of steps as stacked rectangular blocks and add them. Three steps 4 feet wide with an 11 inch tread and a 7 inch rise works out as a bottom block 33 inches deep, a middle block 22 inches deep and a top block 11 inches deep, each 7 inches high and 4 feet wide. That totals about 12.8 cubic feet, roughly 0.48 cubic yards, or about 24 bags of 80 lb mix with waste included. If the steps sit on compacted fill rather than solid concrete all the way down, subtract the fill volume.

Why a Waste Allowance Exists

The calculated volume is the volume of a perfect void. Real pours are not perfect. Material disappears into several predictable places.

• Chute and drum residue. Concrete clings to the mixer drum, the chute and the wheelbarrow. On a small load that residue is a measurable share of the order.
• Spillage and handling. Some goes over the edge of the forms, some ends up on the ground between the truck and the pour.
• Uneven subgrade. A subgrade that dips half an inch across a 10 by 10 pad quietly swallows extra cubic feet. Subgrade dips always cost concrete, they never save it.
• Form flex and overdig. Forms bow outward under the weight of wet concrete. Hand dug trenches and post holes are always wider than the shovel intended.

Ten percent is the sensible default for most residential work. Drop to 5 percent only when forms are braced properly, the subgrade has been screeded flat and compacted, and the shape is simple. Push to 15 or 20 percent for hand dug footings, post holes and anything poured directly against soil, where overdig is unavoidable.

Bag Yields and Bags Per Cubic Yard

Bagged concrete mix is labeled by weight, but what matters is yield in cubic feet. These figures are consistent across the major mixes sold in US home centers.

The bags per 100 square feet column has no waste added, so add 10 percent to it before ordering. Mixing water runs roughly 3 quarts per 80 lb bag, scaled down for smaller bags, but the printed instruction on the bag is the number to follow.

Bags or Ready Mix?

The crossover point in practice sits around 1 cubic yard, and by 2 cubic yards it is not close. One cubic yard is 45 bags of 80 lb mix, 3,600 pounds to lift, open, mix and place, usually inside about 90 minutes if the slab is to be monolithic. Two people with a good mixer can do it. Two people with a wheelbarrow and a hoe will regret it.

Below about half a yard, bags win easily. There is no delivery to schedule, no short load fee, no waiting truck, and small batches can be mixed at whatever pace the work allows. Post holes, small pads, steps, repairs and setting a few footings are all bag jobs.

Above roughly 1 to 1.5 yards, price the ready mix. Suppliers usually set a minimum load and charge a short load fee under 4 or 5 yards, which makes the per yard cost of a small delivery look high, but it is still often cheaper than the bag count once labor is honest. Ready mix also arrives at a consistent, tested strength, which 45 separately mixed bags never quite do.

How Thick Should the Slab Be?

• 4 inches: patios, walkways, shed pads, standard residential garage floors and basement floors. This is the default for anything carrying foot traffic or a passenger car.
• 5 to 6 inches: driveways, particularly where a pickup, van or delivery truck will park or turn.
• 6 inches with reinforcement: heavy vehicles, RV pads, workshop floors with equipment loads, and any slab over soft or poorly draining soil.

Thickness is only half the story. A 4 inch slab on a well compacted base outlasts a 6 inch slab poured on loose fill. Compact the subgrade, keep the thickness uniform, and put control joints in at roughly 24 to 30 times the slab thickness in feet, so about every 8 to 10 feet on a 4 inch slab. Concrete cracks. Joints decide where.

Strength, Water and Dusty Concrete

Most bagged concrete mix sold for general use is rated 3,000 to 4,000 psi at 28 days, which covers essentially all residential slabs, footings and posts. Higher strength mixes exist for structural work and for cold weather pours that need to gain strength faster, but for a patio or a garage floor a standard mix is not the limiting factor.

Adding water beyond the printed ratio to make the mix flow is the fastest way to produce weak, dusty, surface scaling concrete. Extra water raises the water to cement ratio, and strength falls off sharply as that ratio climbs. It also drives bleed water to the surface, which is what leaves a chalky layer that dusts off underfoot a year later. If the mix is too stiff to place, the fix is better placement technique, a plasticizer, or a smaller batch, not a bucket of water.

Curing matters as much as mixing. Concrete gains strength through hydration, not drying, so keeping the surface damp or covered for the first several days is worth more than any change to the mix design.

Costly Mistakes to Avoid

1. Multiplying by thickness in inches instead of feet, which inflates the answer by a factor of twelve.
2. Forgetting to divide cubic feet by 27, which produces a yardage figure that is 27 times too large.
3. Ordering with no waste allowance and coming up short on the last few square feet of a slab.
4. Averaging an L shaped or irregular area into one rectangle instead of splitting it into parts.
5. Using the slab area to estimate bags without converting to volume first.
6. Ignoring subgrade dips, which quietly absorb concrete across the whole pour.
7. Bagging a job over about 1.5 cubic yards, then losing the pour because early batches set before later ones arrive.
8. Adding water to loosen the mix, trading long term strength for short term workability.
9. Skipping control joints, then treating the resulting random cracks as a material defect.
10. Rounding bag counts down. Always round up, and keep one or two spare bags on site.