Concrete Calculator — slabs, footings & columns

This concrete calculator works out exactly how much concrete you need — in cubic yards and cubic metres — for a slab, a footing, or a round column or post, then converts that into the number of pre-mixed bags and the total cost. Whether you're pouring a patio slab, a fence-post footing, or a deck support column, getting the quantity right the first time avoids both a mid-pour shortage and paying for concrete you'll never use.

Concrete is sold and priced in a way that punishes guessing — ready-mix trucks charge minimum order fees, and bagged concrete is priced per bag with a fixed yield, so an inaccurate estimate either means an expensive emergency top-up or bags of leftover mix going hard in the garage. Arb Digital built this calculator, part of a full library of free everyday tools, to make that estimate quick and reliable.

What This Concrete Calculator Does

Select your shape — slab, footing, or column — and enter the relevant dimensions. For a slab or footing, that's length, width, and thickness (or depth). For a column or post, that's diameter and height, since the calculator treats a column as a cylinder. The tool computes the total volume in cubic feet, converts it to both cubic yards (the standard unit ready-mix concrete is sold in) and cubic metres, applies your waste percentage for spillage and uneven excavation, and then works out how many 40, 60, or 80 lb bags of pre-mixed concrete that volume requires, along with the total materials cost.

How to Use the Concrete Calculator

1. Choose your unit system and shape. Select US or metric units, then pick slab, footing, or column depending on what you're pouring.
2. Enter your dimensions. For a slab or footing, enter length, width, and thickness. For a column or post, enter the diameter and height.
3. Set the quantity and waste percentage. If you're pouring multiple identical footings or posts, enter the total count. The default 10% waste allowance covers minor spillage and uneven ground.
4. Choose a bag size and confirm the yield and cost. Selecting 40, 60, or 80 lb auto-fills a typical published yield and price, both of which you can edit to match your exact product.
5. Read your results. The calculator shows cubic yards, cubic metres, bags needed, and total estimated cost.

The Formula Behind the Concrete Calculator

For a slab or footing, volume is Length × Width × Thickness, with thickness converted to the same unit as length and width (inches divided by 12 for feet, or centimeters divided by 100 for meters). For example, a 10 ft × 10 ft slab poured 4 inches thick is 10 × 10 × (4 ÷ 12) = 33.33 cubic feet, which is 33.33 ÷ 27 = 1.23 cubic yards — a figure widely cited by concrete suppliers for a 100 sq ft slab at 4-inch thickness. For a column or post, volume uses the cylinder formula π × radius² × height, where radius is half the diameter. Whichever shape you choose, the raw volume is multiplied by your quantity and by (1 + waste% ÷ 100), then divided by 27 for cubic yards or multiplied by 0.0283168 for cubic metres. Bags needed is the waste-adjusted volume in cubic feet divided by the yield per bag, rounded up to a whole bag, and cost is bags multiplied by price per bag. Bag yield figures used as defaults here reflect commonly published averages from major concrete-mix manufacturers, consistent with general guidance from the National Ready Mixed Concrete Association on estimating pour volumes — always confirm against your specific bag's label since yields vary slightly by brand and mix design.

Bagged Concrete vs. Ready-Mix Delivery

Bagged concrete makes sense for small jobs — fence post footings, a small patch, a single column base — where the total volume is under roughly half a cubic yard to one cubic yard. Beyond that threshold, most contractors switch to ready-mix delivery, where a truck brings already-mixed concrete measured directly in cubic yards. Ready-mix is almost always cheaper per cubic yard once volume climbs past that point, and it saves substantial labor, since mixing dozens of 60 or 80 lb bags by hand or with a mixer is slow and physically demanding. Most ready-mix suppliers charge a minimum order fee regardless of how little you order, so it's worth calling ahead to compare the minimum-order price against the bagged-concrete total this calculator gives you before deciding which route to take.

One practical middle ground for medium-sized jobs is a small towable mixer with bagged mix, which speeds up mixing dozens of bags without the cost or scheduling of a full ready-mix delivery. Whichever method you choose, always order slightly more than your bare calculated minimum — running short mid-pour on a slab means a visible cold joint where old and new concrete meet, which is both a cosmetic flaw and a potential weak point.

Slabs, Footings, and Columns — What's Different

A slab is a flat, relatively thin pour — patios, sidewalks, and small pads are typically 3.5 to 4 inches thick, while a driveway or anything supporting vehicle weight often needs 5 to 6 inches or more with reinforcement. A footing is a deeper, often narrower pour that transfers load from a structure above down into stable soil — fence post footings and deck support footings are common examples, and their required depth typically depends on your local frost line, so check your area's building code rather than guessing. A column or post is usually a round or square vertical pour supporting a deck, porch, or fence post directly, calculated here as a cylinder using its diameter and height. All three shapes use the same underlying volume math, but the depth or height requirement for footings and columns is often dictated by local code rather than personal preference, so confirm your area's frost-depth requirement with your local building department before finalizing footing depth.

Concrete Curing and Common Additives

Beyond the raw mix quantity, most concrete pours also need reinforcement (rebar or wire mesh) for anything load-bearing, and proper curing time before the surface can bear weight — typically 24 to 48 hours before light foot traffic and roughly 28 days to reach full design strength. Weather matters too: concrete cures faster in warm conditions and much more slowly in cold weather, and pouring in freezing temperatures without precautions can seriously weaken the final cure. None of these factors change the raw volume this calculator produces, but they're worth planning for alongside your material order, especially for structural pours like footings and columns that need to reach adequate strength before load is applied.

Multiple Footings and Columns in One Calculation

Deck and fence projects rarely need just one footing or one column — a typical deck might need six, eight, or more identical post footings, and a fence might need a footing at every post along the run. Rather than calculating each one separately and adding the numbers by hand, enter the dimensions for a single footing or column and set the quantity field to the total number you need. The calculator multiplies the per-unit volume by that quantity before applying waste, giving you one combined total for bags and cost across the whole project. This is particularly useful for comparing footing diameter options — a slightly wider footing might only add a modest amount to each one individually, but multiplied across ten posts it can meaningfully change your total bag count and budget, so it's worth running a quick comparison before finalizing your post spacing and footing size.

Keep in mind that quantity should reflect genuinely identical pours. If your project mixes footing sizes — for example, larger footings at corner posts and standard footings elsewhere — run the calculator once per size group and add the resulting totals together, rather than trying to average an odd mix of dimensions into a single quantity entry.

Common Mistakes to Avoid

• Skipping the waste allowance — uneven ground, formwork gaps, and minor spillage almost always mean the true pour needs slightly more than the bare geometric volume.
• Guessing bag yield instead of checking the label — yields vary between manufacturers and mix types, and using the wrong figure throws off both bags needed and total cost.
• Ignoring local frost-depth codes for footings — pouring a footing shallower than your area requires can lead to frost heave and structural problems later.
• Mixing diameter and radius for columns — always enter the full diameter; the calculator halves it to get the radius automatically.
• Not comparing bagged cost against ready-mix delivery for larger pours, where ready-mix is often the cheaper and far less labor-intensive option.

Related Free Tools From Arb Digital

Tiling the slab once it's cured? Check out the tile calculator for tiles, boxes, and cost. Planning the rest of your project budget? Try the electricity bill calculator or the road trip cost calculator. The area converter and length converter are useful for converting measurements between unit systems. Browse everything at our free online tools hub.

Frequently Asked Questions

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