Concrete Calculator

Slab, square footing, or wall

Concrete volume for a rectangular slab is simply length multiplied by width multiplied by thickness, the same formula used for the volume of any rectangular box. The one adjustment needed is unit consistency: length and width are almost always measured in feet, but slab thickness is almost always specified in inches, so the thickness has to be converted to feet before the multiplication makes sense. Dividing the thickness in inches by 12 does that conversion.

Take a classic reference size: a 10ft by 10ft patio slab, poured 4 inches thick. The thickness in feet is 4 ÷ 12 = 0.333 ft. Multiplying that by the 10ft by 10ft footprint gives 10 × 10 × 0.333 = 33.33 cubic feet of concrete, which is 1.234 cubic yards, about 75 bags of 60 lb mix or 56 bags of 80 lb mix. This same tool also works for a square footing or a poured wall, since both are just a rectangular volume measured the same way.

Hole, column, or round footing

A cylindrical pour, a fence-post hole, a round deck-support footing, or a round column form, uses the volume formula for a cylinder: pi times the radius squared, times the height.

Since the calculator asks for a diameter rather than a radius (the way these are usually specified and measured on site), it divides the entered diameter by two internally before squaring it. A quantity field lets you calculate several identical holes or columns, such as a run of six fence-post footings, in one pass rather than multiplying the single-hole result by hand.

Circular slab or tube

A round, flat pour, a circular patio, a hot tub pad, or a decorative circular stepping pad, is also a cylinder, just a short, wide one. This tool goes a step further and supports a hollow center too, for a tube-shaped pour like a sonotube concrete pier or a ring-shaped footing: the volume becomes the outer cylinder's volume minus the inner cylinder's volume, an annulus (a ring shape) extruded by the height. Entering 0 for the inner diameter removes the hole entirely, leaving a plain solid circular slab.

Curb and gutter barrier

A concrete curb and gutter run has an L-shaped cross-section rather than a simple rectangle: a vertical curb face rises up on one side, and a horizontal gutter flag extends out along the ground on the other. This calculator finds that L-shaped area by splitting it into its two rectangular pieces, curb depth times curb height for the upright curb, and gutter width times flag thickness for the flat flag, adding the two pieces together, and then multiplying the combined cross-sectional area by the length of the run to get a total volume.

Stairs

A flight of concrete steps is treated as a stepped, staircase-shaped profile filled solid all the way down to the ground, then multiplied by the stair's width to turn that cross-section into a volume. The shape breaks into a flat landing block at the top, a stepped wedge under the treads that is approximated as a triangle, and a small correction for the bottom-most step, which needs a full riser's worth of material rather than the sliver the triangle alone would give it. Entering the run and rise of a single step, the stair width, the depth of the landing platform, and the number of risers is enough to work out the whole flight's volume.

Converting volume to bag counts with the concrete calculator

Pre-mixed bagged concrete is sold in a few standard weights, most commonly 60 lb and 80 lb bags. Each bag size yields a different volume of mixed concrete once water is added, based on published manufacturer figures (Quikrete, one of the most widely sold brands, publishes yields close to these): a 60 lb bag yields approximately 0.45 cubic feet of concrete, and an 80 lb bag yields approximately 0.60 cubic feet.

Dividing the total volume needed by a single bag's yield gives the number of bags, and that number always gets rounded up, since a store does not sell a fraction of a bag. Continuing the 10x10, 4-inch slab example above, which needs 33.33 cubic feet of concrete: 33.33 ÷ 0.45 = 74.07, which rounds up to 75 bags of 60 lb pre-mix. Using 80 lb bags instead, 33.33 ÷ 0.60 = 55.55, which rounds up to 56 bags. Every calculator on this page, not just the slab tool, converts its own cubic-foot volume into 60 lb and 80 lb bag counts the same way.

Notice that switching to the larger bag size does not simply scale the count down in proportion to the weight. An 80 lb bag is about 33 percent heavier than a 60 lb bag, but it only needs 56 bags against 75, a reduction of about 25 percent, because the yield per pound is not identical between the two bag sizes. Comparing total weight and total cost for both options, rather than assuming the heavier bag is automatically the better deal, is worth the extra minute before buying.

Bags for common slab sizes

Used as a concrete slab calculator, the first card gives cubic feet, cubic yards and bag counts for any slab. The table lists common sizes at 4 inches thick so you can check a quick estimate before you enter exact dimensions.

General concrete-estimating tips

Bagged concrete is really meant for small jobs: a fence post footing, a small patio repair, a single step, or anything under roughly a cubic yard or two. For larger pours, a driveway, a slab foundation, or a large patio, ordering ready-mix concrete delivered by truck is usually far more economical than buying dozens of individual bags, and it saves the considerable labor of mixing each bag by hand or with a small mixer. Most ready-mix suppliers set a minimum order size, often around one cubic yard, and price per cubic yard drops as the order size grows, so it is worth calling a local supplier for a quote once a job's volume climbs much past what a pallet of bags can reasonably cover.

Always buy slightly more concrete than the calculated figure suggests. Many contractors add 5 to 10 percent extra to cover uneven subgrade, spillage during mixing and pouring, and ordinary measurement error, since running short partway through a pour is far more disruptive than having a little extra mix left over. This is especially worth doing for a truck delivery, where a second, smaller trip usually costs more per yard than simply rounding the original order up.

Give freshly poured concrete time to cure properly before putting it to full use. It typically reaches enough strength to walk on within a day or two, is usually safe for light vehicle traffic after about a week, and continues gaining strength for roughly 28 days, the industry-standard benchmark most concrete mix designs are rated against. Keeping a new pour damp and protected from extreme heat, freezing, or heavy rain during that first week helps it cure evenly and reach its full design strength.