How Much Concrete Do I Need? A Guide for Slabs, Footings, Columns, and Stairs

Every concrete project, no matter how big or small, starts with the same practical question: how much concrete do I need to buy? Order too little and a pour has to stop partway through while more material is sourced, which can leave a visible cold joint or a weak seam. Order too much and money is wasted on concrete that never gets used. Getting the estimate right the first time comes down to knowing which shape formula applies to the job and working through it carefully.

This guide walks through that process for five common concrete shapes: a rectangular slab, a set of round fence-post footings, a circular slab, a curb and gutter run, and a flight of stairs. Each section below includes a full worked example with real numbers, checked by hand, so the arithmetic can be followed and verified rather than taken on faith. Think of each worked example as its own small concrete estimator: a concrete slab calculator, a post hole concrete calculator (also a concrete footing or concrete column calculator), a circular slab calculator (or sonotube calculator, for the hollow version), a curb and gutter calculator, and a concrete stair calculator (or concrete step calculator). For any of these five shapes, the concrete calculator automates the same math instantly, converting a raw volume into both cubic yards and 60 lb / 80 lb bag counts.

The one constant across every shape: cubic feet, then cubic yards, then bags

Regardless of which shape a pour happens to be, the estimating process always follows the same three stages.

Stage one: find the volume in cubic feet. Every shape below has its own formula for this, covered in its own section, but the output is always a single number measured in cubic feet.

Stage two: convert to cubic yards. There are 27 cubic feet in a cubic yard, so dividing the cubic-foot figure by 27 gives the cubic-yard equivalent. Ready-mix concrete delivered by truck is bought and priced by the cubic yard, so this conversion matters most for larger pours.

Stage three: convert to bag counts, if buying bagged concrete. Pre-mixed bagged concrete most commonly comes in 60 lb and 80 lb bags. Based on widely published manufacturer figures (Quikrete, one of the most common brands, publishes yields close to these), one 60 lb bag of concrete yields approximately 0.45 cubic feet of mixed concrete, and one 80 lb bag of concrete yields approximately 0.60 cubic feet. Dividing the needed cubic-foot volume by a bag's yield gives the bag count, and that count always rounds up, since a store does not sell part of a bag. A useful reference point for a full truckload comparison: a complete cubic yard, 27 cubic feet, needs about 60 bags total (the answer to how many 60lb bags of concrete in a yard), or about 45 bags total (the answer to how many 80 lb bags of concrete in a yard).

With those three stages fixed, the only real work left for any given job is stage one: finding the correct volume formula for the shape being poured.

Worked example 1: a rectangular slab

A rectangular slab, footing, or wall uses the simplest formula of the five: length times width times thickness, the same formula as the volume of any rectangular box. The one wrinkle is unit consistency. Length and width are almost always measured in feet, but thickness is almost always specified in inches, a "4 inch slab," so the thickness needs converting to feet before the multiplication makes sense. Dividing the thickness in inches by 12 does that.

Take a classic reference size: a patio slab 10 feet by 10 feet, poured 4 inches thick.

1. Convert the thickness: 4 inches divided by 12 equals 0.333 feet.
2. Multiply: 10 feet times 10 feet times 0.333 feet equals 33.33 cubic feet.
3. Convert to cubic yards: 33.33 divided by 27 equals 1.234 cubic yards.
4. Convert to bags: 33.33 divided by 0.45 equals 74.07, which rounds up to 75 bags of 60 lb mix. Using 80 lb bags instead, 33.33 divided by 0.60 equals 55.55, which rounds up to 56 bags.

So a 10 by 10 foot, 4 inch slab needs about 1.234 cubic yards of concrete, 75 bags of 60 lb pre-mix, or 56 bags of 80 lb pre-mix. Scaling this formula up or down for a different footprint or a different thickness is just a matter of plugging in new numbers; a 6 inch thick pour over the same 10 by 10 foot footprint, for comparison, needs exactly 1.5 times as much concrete as the 4 inch version, since 6 divided by 4 equals 1.5 and every other factor in the formula stays the same.

Worked example 2: fence-post footings (a cylinder, multiplied by quantity)

A round fence-post hole, a deck support footing, or a round column form is a cylinder, and a cylinder's volume is pi times the radius squared, times the height. Since holes and footings are usually specified by diameter rather than radius, the diameter gets divided by two first.

Say a fence line needs six identical post holes, each 12 inches (1 foot) in diameter and 3 feet deep.

1. Find the radius: 1 foot divided by 2 equals 0.5 feet.
2. Find one hole's volume: pi times 0.5 squared times 3 equals pi times 0.25 times 3, which is 0.75 times pi, or about 2.356 cubic feet.
3. Multiply by the quantity: 2.356 times 6 equals about 14.14 cubic feet. Rounded to two decimal places, that is 14.14 cubic feet.
4. Convert to cubic yards: 14.14 divided by 27 equals about 0.524 cubic yards.
5. Convert to bags: 14.14 divided by 0.45 equals about 31.42, which rounds up to 32 bags of 60 lb mix. Using 80 lb bags, 14.14 divided by 0.60 equals about 23.57, which rounds up to 24 bags.

Six fence-post footings at 12 inches by 3 feet each need a little over half a cubic yard of concrete in total, about 32 bags of 60 lb mix or 24 bags of 80 lb mix. Notice that the quantity is applied to the single-hole volume before converting to yards and bags, not after, since bag counts do not scale in a simple linear way once rounding up is involved for each individual calculation; applying the quantity first and rounding once, at the very end, gives the correct total.

Worked example 3: a circular slab or a hollow tube

A circular slab, a round patio, a hot tub pad, or a decorative circular stepping pad, is also a cylinder, just a short, wide one instead of a tall, narrow one. Some circular pours are hollow in the middle instead of solid, a sonotube concrete pier casing left in place, or a ring-shaped footing, in which case the volume is the outer cylinder's volume minus the inner cylinder's volume, an annulus (a ring shape) extruded by the height.

Take a circular slab with an outer diameter of 4 feet, an inner diameter of 2 feet (a ring-shaped footing rather than a solid pad), and a height of 0.5 feet (6 inches).

1. Find the outer radius: 4 feet divided by 2 equals 2 feet. Find the inner radius: 2 feet divided by 2 equals 1 foot.
2. Find the ring's area: pi times (2 squared minus 1 squared) equals pi times (4 minus 1), which is 3 times pi, or about 9.425 square feet.
3. Multiply by the height: 9.425 times 0.5 equals about 4.712 cubic feet, which rounds to 4.71 cubic feet.
4. Convert to cubic yards: 4.71 divided by 27 equals about 0.174 cubic yards.
5. Convert to bags: 4.71 divided by 0.45 equals about 10.47, which rounds up to 11 bags of 60 lb mix. Using 80 lb bags, 4.71 divided by 0.60 equals 7.85, which rounds up to 8 bags.

If that same slab were solid instead of a ring, entering an inner diameter of 0 removes the subtraction entirely: pi times 2 squared times 0.5 equals about 6.28 cubic feet, noticeably more than the ring version, since a full solid disc obviously contains more material than one with a hole cut out of its middle.

Curb and gutter: an L-shaped cross-section

A concrete curb and gutter run is not a simple rectangle in cross-section; it is an L-shape, a vertical curb face rising up on one side and a horizontal gutter flag extending out along the ground on the other. The volume formula decomposes that L into its two rectangular pieces, finds each piece's area, adds them together, and multiplies by the length of the run.

Take a curb depth of 8 inches, a curb height of 18 inches, a gutter width of 24 inches, a flag thickness of 6 inches, and a run length of 100 feet.

1. Convert every cross-section dimension to feet: 8 inches is 0.667 feet, 18 inches is 1.5 feet, 24 inches is 2 feet, and 6 inches is 0.5 feet.
2. Find the curb rectangle's area: 0.667 feet times 1.5 feet equals almost exactly 1.0 square foot.
3. Find the gutter flag's area: 2 feet times 0.5 feet equals 1.0 square foot.
4. Add them together: 1.0 plus 1.0 equals 2.0 square feet of cross-section.
5. Multiply by the length: 2.0 square feet times 100 feet equals 200 cubic feet.
6. Convert to cubic yards: 200 divided by 27 equals about 7.407 cubic yards.
7. Convert to bags: 200 divided by 0.45 equals about 444.4, which rounds up to 445 bags of 60 lb mix, or 200 divided by 0.60 equals about 333.3, rounding up to 334 bags of 80 lb mix.

A 100 foot curb and gutter run at these dimensions needs a little over 7.4 cubic yards of concrete, which is well past the point where ordering ready-mix by the truck makes more sense than buying hundreds of individual bags.

Stairs: a stepped profile, not a simple box

Stairs are the most involved of the five shapes, because the concrete needed forms a stepped, staircase-shaped profile rather than a single box. The calculation breaks that profile into three pieces, each measured across the stair's full width: a flat landing block at the top, a triangular wedge of material under the treads, and a small correction rectangle for the bottom step, which needs a full riser's worth of concrete rather than the thin sliver a pure triangle would give it at that point.

Take a flight with 3 risers, a run of 10.8 inches (0.9 feet) per step, a rise of 7.2 inches (0.6 feet) per step, a stair width of 3 feet, and a landing platform depth of 3 feet.

1. Find the total rise: 0.6 feet times 3 risers equals 1.8 feet.
2. Find the landing block's area: platform depth times total rise, 3 feet times 1.8 feet equals 5.4 square feet.
3. Find the stepped triangle's area: run times total rise times (risers minus 1), divided by 2. That is 0.9 times 1.8 times 2, divided by 2, which equals 1.62 square feet.
4. Find the bottom-step correction: run times rise, 0.9 times 0.6 equals 0.54 square feet.
5. Add the three pieces: 5.4 plus 1.62 plus 0.54 equals 7.56 square feet of cross-section.
6. Multiply by the width: 3 feet times 7.56 square feet equals 22.68 cubic feet.
7. Convert to cubic yards: 22.68 divided by 27 equals exactly 0.84 cubic yards.
8. Convert to bags: 22.68 divided by 0.45 equals 50.4, which rounds up to 51 bags of 60 lb mix, or 22.68 divided by 0.60 equals 37.8, rounding up to 38 bags of 80 lb mix.

This 3 riser flight needs 22.68 cubic feet, 0.84 cubic yards, 51 bags of 60 lb mix, or 38 bags of 80 lb mix. Doubling the number of risers does not simply double the volume, because the triangular term grows with the square of the riser count while the landing and correction terms grow more slowly, so a taller flight of stairs needs proportionally more concrete per step than a shorter one, not the same amount per step throughout.

Why buy a little more than the exact calculated figure

Every worked example above produces a precise number, but precise numbers assume perfectly even ground, perfectly accurate measurements, and zero material loss during mixing and pouring. Real job sites rarely offer all three. Subgrade is rarely perfectly flat, a form can bow slightly under the weight of wet concrete, and some mix is inevitably lost to the mixer, the wheelbarrow, or the float during finishing.

For that reason, many contractors add 5 to 10 percent to the calculated volume before placing an order, whether that order is a stack of bags or a call to a ready-mix supplier. Running short of concrete in the middle of a continuous pour is a serious problem, since starting and stopping mid-slab can create a visible weak seam called a cold joint, while a small amount of leftover mixed concrete is, at worst, a minor inconvenience. This margin matters most for a delivery by truck, where a follow-up trip for a shortfall usually costs far more per yard than simply rounding the original order up.

When bagged concrete makes sense, and when it does not

Every worked example above works equally well whether the concrete comes from bags or a ready-mix truck; the volume calculation itself does not care which source the concrete comes from. But the practical choice between the two options does depend heavily on the total volume involved.

Bagged concrete suits small jobs well: a handful of fence-post footings, a single step repair, a small patio section, or anything under roughly a cubic yard or two in total. Mixing dozens of bags by hand or with a small portable mixer becomes impractical fast as the volume grows, both in terms of the physical labor involved and the difficulty of achieving a consistent mix across many separate batches. For a driveway, a full slab foundation, a long curb and gutter run, or any pour that adds up to several cubic yards or more, ordering ready-mix concrete delivered by truck is almost always more economical per cubic yard and saves enormous amounts of labor, even accounting for most suppliers' minimum order requirements, which commonly sit around one cubic yard.

Curing time matters as much as the pour itself

Getting the volume estimate right only solves half the problem; giving a fresh pour the time it needs to cure properly matters just as much for a durable, crack-resistant result. Concrete typically firms up enough to walk on within a day or two of pouring, is usually ready for light vehicle traffic after about a week, and continues gaining strength for roughly 28 days, the benchmark most concrete mix designs are tested and rated against. Keeping a new pour damp and shielded from extreme heat, freezing temperatures, or heavy rain during that first week of curing helps it develop its full design strength evenly, rather than curing unevenly and cracking prematurely.

Getting the pour right: mix ratios, reinforcement, and cold-weather concrete

Getting the volume estimate right only sets up the pour; a few other decisions matter just as much for a durable result. The concrete mix ratio is the starting point: a general-purpose mix is commonly described by the 1:2:3 concrete ratio, roughly 1 part cement, 2 parts sand, and 3 parts gravel by volume, with just enough water to stay workable without weakening it; a leaner cement to sand ratio, closer to 1:3, suits less demanding fill work. Bagged pre-mix already has that ratio built in.

Matching the PSI rating to the job matters just as much. The typical PSI for a sidewalk or a patio runs around 3,000 to 3,500, the PSI rating for a driveway is usually closer to 4,000 because it carries vehicle weight and must resist repeated freeze-thaw damage, and the PSI for a footing under a deck post or fence can often be lower, around 2,500 to 3,000. Where freeze-thaw damage is a real concern, suppliers often add an air-entraining admixture, tiny air bubbles that give freezing water somewhere to expand into instead of cracking the slab.

Reinforcement matters for anything wider than a narrow footing. Rebar for a concrete slab, or welded wire mesh where loads are lighter, controls cracking as concrete shrinks while curing; correct rebar spacing and support chairs keep it within the slab rather than resting on bare ground. A vapor barrier under a slab, typically 6 mil polyethylene sheeting laid before the pour, blocks ground moisture from wicking into a slab that will later carry flooring. Control joints cut across a large slab give shrinkage cracks a place to happen on purpose, in a straight line, not at random.

Cold weather concreting needs its own precautions. Once forecasts call for pouring concrete in cold weather near or below 40 degrees Fahrenheit, guidance calls for insulating blankets or a heated enclosure over the fresh pour, since concrete that freezes before it cures loses strength permanently. A little extra concrete overage beyond the 5 to 10 percent already covered above is worth ordering on an irregular or hand-dug subgrade, where how much extra concrete to order is hard to judge from plans alone.

How much concrete do I need, and other phrasings of one process

People search for this same question in a lot of different ways: how to calculate concrete, how to figure concrete, figuring yards of concrete, how do you figure concrete in yards, how to figure yards of concrete, how to calculate yards of concrete, and how to estimate concrete yards all describe the exact same three-stage process covered in this guide. However the question happens to be phrased, the underlying math from cubic feet, to cubic yards, to bag counts, does not change.

Try the calculations yourself

Every formula walked through above, the rectangular slab, the cylinder for a hole or column, the annulus for a circular slab or tube, the L-shaped curb and gutter cross-section, and the stepped staircase profile, has its own dedicated tool on the concrete calculator, effectively a full concrete yardage calculator for five shapes in one place, each converting a raw volume instantly into cubic yards and both 60 lb and 80 lb bag counts. Working through the math by hand once, the way the five examples above did, is a useful check on what a calculator reports; for anyone still typing "how much concrete do I need" into a search bar for an actual job, using the calculator once the shape and formula are familiar is the faster way to get there.