Free Concrete Calculator Online

Concrete Calculator: Volume and Bag Count for a Slab, Footing, or Column

Concrete is bought two ways: by the cubic yard for a ready-mix truck, or by the bag for a smaller job you're mixing yourself, and both depend on getting the volume right first. This calculator takes the dimensions of a slab, footing, or round column and works out exactly how much concrete that shape requires — in cubic feet, cubic yards, and cubic meters — then converts that volume into a bag count based on the premix bag size you tell it you're using. All of it runs as client-side math the moment you type, so there's no waiting on a server and no file involved.

The reason a dedicated calculator earns its keep here is that concrete volume is a three-dimensional multiplication most people don't do often enough to have memorized, and the unit mismatch is easy to get wrong by hand: a slab is measured in feet across but poured in inches thick, and translating "4 inches" into a fraction of a foot before multiplying is exactly the kind of step that produces an order of magnitude error if you rush it. This tool does that conversion internally so you can enter thickness the way you'd actually say it out loud.

How to Estimate Concrete Volume with This Tool

The calculator adapts its fields to the shape you're pouring:

1. Choose the shape: Slab or Footing (both treated as a rectangular prism) or Column (a round pier or post footing).
2. Pick the unit for the horizontal dimensions — feet, meters, or inches — using the unit toggle. This applies to length and width (or diameter and height for a column); it does not apply to slab thickness, which is discussed next.
3. For a slab or footing, enter length, width, and thickness. Thickness is always entered in inches regardless of which unit you picked for length and width, because slab thickness is almost universally spoken and specified in inches on job sites.
4. For a column, enter diameter and height instead — both use the unit you selected in step two.
5. Pick a bag size, 60 lb or 80 lb, and adjust the waste percentage if you want something other than the 5% default.
6. Read the results: cubic yards, cubic feet, and the bag count update together, with cubic meters and the waste-adjusted volume shown just below the main numbers.

Why People Need to Calculate Concrete Volume

Concrete volume calculations come up whenever a project involves a poured foundation element:

• Pouring a patio, walkway, or shed slab. Ready-mix suppliers price and deliver concrete by the cubic yard, and ordering short means a second, more expensive partial-truck delivery mid-pour — ordering long wastes money on unused material.
• Mixing bagged concrete for a small job. Fence posts, a small pad, or a set of stepping stones are usually mixed by hand from bags rather than trucked in, and knowing the bag count ahead of a hardware store run avoids either running short or hauling home far more than needed.
• Setting deck or pergola footings. A round column pour — the footing under a deck post — uses a different formula than a flat slab, since it's based on a circle's area rather than a rectangle's.
• Budgeting a foundation or driveway project. Concrete is one of the pricier line items in many builds, and a volume estimate is usually the number a contractor uses to generate the material portion of a quote.
• Double-checking a contractor's quote. Running your own dimensions through the same formula a supplier would use is a quick sanity check against a quoted cubic-yard figure before you sign off.

The Math and the Assumptions Behind the Bag Count

Volume for a slab or footing is length times width times thickness, with thickness converted from inches to feet before multiplying. Volume for a column is the area of a circle — pi times the radius squared, where radius is half the diameter you entered — multiplied by the column's height. Either way, the result starts life in cubic feet, then gets converted to cubic yards by dividing by 27 (since a cubic yard is 3 feet on each side, or 27 cubic feet) and to cubic meters using the standard 0.0283 conversion factor.

Quantity	Example: 10ft × 10ft slab, 4in thick
Volume (cubic feet)	10 × 10 × (4 ÷ 12) = 33.33 cu ft
Volume (cubic yards)	33.33 ÷ 27 = 1.235 cu yd
Volume (cubic meters)	≈ 0.944 m³
With 5% waste added	33.33 × 1.05 = 35 cu ft
60 lb bags (yield ~0.45 cu ft each)	35 ÷ 0.45 → 78 bags (rounded up)
80 lb bags (yield ~0.60 cu ft each)	35 ÷ 0.60 → 59 bags (rounded up)

Two assumptions in that table are worth stating plainly, because they're exactly the numbers a DIYer needs to sanity-check rather than trust blindly. First, the bag yields: this tool assumes a 60 lb bag of premix concrete mixes up to roughly 0.45 cubic feet, and an 80 lb bag to roughly 0.60 cubic feet, which are the manufacturer-typical figures printed on common premix bags like Quikrete's standard mix — but yields vary slightly by brand and mix design, so check the bag you're actually buying if the count needs to be precise. Second, the waste percentage: the tool adds 5% to the raw volume by default before computing the bag count, covering the concrete you inevitably lose to spillage, an uneven subgrade, or a form that isn't perfectly square. You can raise that percentage for a rougher site or lower it if you're confident in your forms, and the tool always shows you the exact volume without waste too, so you can see precisely how much cushion is built in.

A second worked example, at a non-round set of numbers, shows the same math applied to a footing: a 20-foot by 1-foot strip footing poured 8 inches thick comes to 13.33 cubic feet, or 0.494 cubic yards (about 0.378 cubic meters). With the default 5% waste that's 14 cubic feet, which needs 32 bags of 60 lb premix. And for a round deck post footing 2 feet in diameter and 8 feet deep — using the column formula instead of the slab formula — the volume works out to about 25.13 cubic feet, or 0.931 cubic yards (roughly 0.712 cubic meters); with 5% waste that's 26.39 cubic feet, needing 59 bags of 60 lb premix.

Mistakes That Throw Off a Concrete Estimate

The most common error is entering slab thickness in the wrong unit — because the length and width fields follow whatever unit you picked (feet, meters, or inches), it's tempting to also type thickness in that same unit, but the thickness field is always inches no matter what. Entering "0.33" thinking you're specifying a third of a foot, instead of "4" for four inches, silently shrinks the estimated volume to a fraction of the real pour. For columns, mixing up diameter and radius has the same effect here as it does with a circular area calculation — the field wants the full diameter (distance straight across), not the radius, and the tool halves it internally. It's also easy to forget that ready-mix trucks typically have minimum order quantities well above what a small slab or footing needs, so a volume under roughly a half cubic yard is usually more economical — and sometimes the only option — as bagged concrete rather than a delivery. Finally, treat the bag-yield figures as typical rather than exact: a partially set bag, a different brand's mix design, or mixing to a wetter-than-recommended consistency can all shift the real yield enough to matter on a large pour, so buying a few extra bags beyond the calculated count is cheap insurance against a mid-pour shortage.

Manual Calculation vs. Using This Tool

Concrete volume math is simple in principle but easy to fumble under the time pressure of a hardware store aisle:

• Doing the arithmetic yourself. Length times width times thickness is one multiplication, but converting inches to feet, then cubic feet to cubic yards, then applying a waste percentage before dividing by a bag yield is four separate steps where a dropped decimal point compounds through the rest.
• Bag-yield charts printed on the packaging. Manufacturers print approximate coverage per bag on the label, which is a fine reference but doesn't do the volume calculation for your specific slab or footing dimensions.
• Asking a ready-mix supplier. Suppliers will often estimate cubic yards for you over the phone if you give them dimensions, though that's slower than an instant browser calculation and still relies on you providing accurate measurements.
• A general-purpose volume calculator. Plenty of geometry calculators compute rectangular-prism or cylinder volume, but few also carry it through the unit conversions and bag-count math specific to premix concrete.
• This calculator's metered API. For a contractor's quoting software or a batch job that needs to price out several pours at once, the same slab/footing/column math is exposed as a metered API endpoint returning cubic feet, cubic yards, cubic meters, and bag count as JSON — see the API documentation for details.

Should slab thickness really always be entered in inches?

Yes, regardless of whether you've set the length/width unit to feet, meters, or inches. Slab and footing thickness is a fixed field that always expects inches, because that's how thickness is conventionally specified on job sites and in building plans.

Are the 60 lb and 80 lb bag yields exact for every brand?

No — they're typical figures (about 0.45 cubic feet for a 60 lb bag and about 0.60 cubic feet for an 80 lb bag) based on common premix products. Actual yield can vary slightly by manufacturer and mix design, so treat the bag count as a close estimate and round up a bit further if precision matters for your specific product.

What does the 5% waste percentage actually cover?

It's a buffer added to the raw volume before the bag count is calculated, meant to account for spillage while mixing and pouring, an uneven or slightly over-dug subgrade, and minor form imperfections. You can adjust it higher for a rough or uneven site, or lower if you're confident in tight forms and a clean pour.

Why does a small footing still need so many bags relative to its size?

Bagged concrete yields a relatively small volume per bag — well under half a cubic foot — so even a modest pour of a few cubic feet adds up to dozens of bags quickly. This is exactly why larger pours are almost always more economical as a ready-mix delivery rather than bag after bag of premix.

Does this tool account for reinforcement like rebar or wire mesh?

No — the volume calculation is for the concrete itself only. Rebar, wire mesh, and other reinforcement reduce the true concrete volume by a negligible amount that most estimating guides ignore, but they're a separate materials and labor line item this calculator doesn't compute.

Related Construction Tools

Concrete work rarely stands alone. Start with the square footage calculator if you still need the footprint area of the slab you're pricing, or head to the fence calculator if the concrete you're estimating is for setting fence posts rather than a slab. Once the pour is done and it's time to finish the surface, the tile calculator and mulch and gravel calculator cover what typically comes next on a patio or walkway project.