Concrete Calculator
Find out how many cubic yards of concrete — and how many pre-mix bags — you need for a slab, footing, or post hole.
Pour dimensions
Results
For a 10 ft x 10 ft pour at 4 in thick, you'll need about 1.23 cubic yards of concrete — roughly 75 bags if using 60 lb pre-mix.
Concrete needed
1.23 cubic yards
That's a typical amount for a patio, walkway, or single-car pad.
This is around the threshold where a ready-mix truck often becomes cost-competitive with bags — get a quote from a local supplier and compare it against buying and mixing this many bags by hand before deciding.
Equivalent bags: 75 x 60 lb bags
Raw volume: 0.94 m³
How this works
Volume is calculated as length x width x thickness (or depth), multiplied by how many identical pours or holes you're doing, giving a raw volume in cubic feet that's converted to cubic yards for ready-mix ordering (1 cubic yard = 27 cubic feet, an exact geometric conversion, not a sourced figure). For bagged pre-mix, that same volume is divided by a single bag's yield — approximately 0.45 cubic feet for a 60 lb bag or 0.6 cubic feet for an 80 lb bag of standard general-purpose concrete mix — and rounded up to the next whole bag, since bags are sold as whole units and running short mid-pour risks a visible cold joint where an interrupted pour meets freshly mixed concrete. This calculator uses a rectangular (length x width x depth) volume model throughout, which is exact for square and rectangular slabs and footings, and a conservative overestimate for round post holes if you enter a hole's diameter as both length and width (see the FAQ above for the more precise round-hole adjustment). It does not account for sub-base irregularities, structural reinforcement, or ready-mix delivery minimums, all of which are separate planning steps beyond a volume estimate. Bag yield figures (60 lb ≈ 0.45 cu ft, 80 lb ≈ 0.6 cu ft): Quikrete Concrete Mix technical data / yield chart, corroborated by NRMCA consumer guidance on approximate bags per cubic yard.
Worked example
A 10 ft x 10 ft patio slab, 4 inches (0.333 ft) thick: volume is 10 x 10 x 0.333 = 33.3 cubic feet, which is about 1.24 cubic yards (33.3 / 27). Using 60 lb bags at roughly 0.45 cu ft of yield each, that's about 33.3 / 0.45 = 74 bags, rounded up to 74 whole bags (since it divides evenly here) — or, if going with 80 lb bags at roughly 0.6 cu ft yield each, about 33.3 / 0.6 = 55.5 bags, rounded up to 56 bags.
A second example: six fence post holes
Six round post holes, each approximated as a 1 ft x 1 ft x 2 ft-deep square-sided volume (a common simplification for an auger-dug hole roughly a foot across, 2 ft deep): one hole's volume is 1 x 1 x 2 = 2 cubic feet, so six holes total 12 cubic feet — about 0.44 cubic yards. Using 60 lb bags at roughly 0.45 cu ft yield each, that's about 12 / 0.45 = 26.7 bags, rounded up to 27 bags. Note that for a truly round hole, the actual volume is closer to pi/4 of this square-sided estimate (about 9.4 cubic feet total), so treating the square-sided figure as a built-in safety buffer for post holes is a reasonable, conservative default.
Common mistakes
Forgetting that concrete is priced and sold by volume, not by area
It's tempting to think of a slab in square feet the way you might for flooring, but concrete is a volume — square footage alone tells you nothing without a thickness. A 10 ft x 10 ft slab at 4 inches thick needs about half the material of the same footprint at 8 inches, so always confirm your project's actual depth before ordering.
Mixing up cubic feet and cubic yards when getting a ready-mix quote
Ready-mix trucks price and order by the cubic yard, not the cubic foot (1 cubic yard = 27 cubic feet), so requesting '30' when you mean 30 cubic feet instead of cubic yards would be more than 27 times too much concrete. Always double check which unit a supplier is quoting in before placing an order.
Not accounting for ground that isn't perfectly level or compacted
A prepared sub-base that isn't perfectly flat means your actual poured thickness varies across the slab — low spots use more concrete than the nominal depth suggests. Budgeting a small buffer (ordering slightly more than the calculated minimum, especially for ready-mix where a mid-pour shortage is a serious problem) is standard practice for exactly this reason.
Buying exactly to the bag, with zero buffer
Spillage during mixing, a slightly generous trowel-off, and minor sub-base irregularities all use a bit more material than the pure geometric calculation. This calculator rounds up to the next whole bag, which provides a small buffer, but for anything beyond a very small, simple pour, many experienced DIYers add one extra bag as insurance against a mid-project store run with a partially set batch already on the ground.
Mixing bag sizes without recomputing
If you buy a mix of 60 lb and 80 lb bags (common when a store is out of one size), don't just add the bag counts from two separate calculator runs and assume they're equivalent — each bag size covers a different volume, so combine them by volume (cubic feet), not by bag count, when confirming you have enough.
Skipping rebar, mesh, or control joints because they weren't in the volume estimate
This calculator estimates material volume only — it doesn't account for structural reinforcement (rebar or wire mesh) or the control joints needed to manage cracking as the slab cures, both of which are typically required for anything beyond a very small, thin pour. Confirm your project's reinforcement and joint-spacing needs separately, ideally against local code or manufacturer span tables, before pouring.
When this calculator's model stops applying
- Sloped or crowned slabs (like a driveway with drainage pitch): this tool assumes a uniform, flat thickness entered once. For a slab with a meaningful slope or crown, use the average thickness across the pour as a reasonable approximation, understanding the true figure will vary slightly by location on the slab.
- Round post holes and footings: entering a hole's diameter as both length and width overestimates volume by roughly 21.5% relative to the true circular volume (a circle inscribed in a square uses pi/4 of the square's area) — multiply the result by about 0.785 for a more precise round-hole figure, or accept the overestimate as a safety buffer.
- Very large commercial or structural pours: at this scale, engineering-specified mix strength, reinforcement schedules, and pour sequencing (multiple truck loads, expansion joints, curing time between sections) all matter more than a simple volume estimate — this calculator is a starting-point tool for DIY and small-contractor jobs, not a substitute for engineered drawings.
- Cold-weather or hot-weather pours: extreme temperatures affect cure time and may require special additives or curing blankets, which don't change the volume of concrete needed but do change scheduling and technique — check the mix manufacturer's temperature guidance before pouring outside moderate conditions.
- Footings below the frost line or with specific engineering depth requirements: local building codes often specify a minimum footing depth based on regional frost depth, which is a code question, not a calculator question — confirm required depth with your local building department before finalizing the thickness/depth figure you enter here.
- Pours requiring a specific finish or color additive: integral color and some decorative finishes are added at the mixing stage and can slightly change the effective yield per bag depending on the additive — check the specific product's technical data if you're using a colored or specialty mix rather than standard general-purpose pre-mix.
FAQ
Does this calculate ready-mix truck cost or delivery minimums?
No — this calculator gives you a volume (cubic yards) and, for smaller pours, an equivalent bag count. Ready-mix trucks typically have a minimum order (often around 1 cubic yard, sometimes higher plus a short-load fee below that), so for small pours under about a yard, pre-mix bags are usually more practical than ordering a truck. For larger pours, use the cubic-yard figure to get a ready-mix quote.
Why does the calculator ask for thickness or depth separately from length and width?
Concrete volume is three-dimensional — length x width x thickness — so a slab and a footing with the same footprint but different thickness need different amounts. A 4-inch patio slab needs roughly half the concrete of an 8-inch driveway slab of the same length and width, which is why thickness has its own field rather than being assumed.
Should I use 60 lb or 80 lb bags?
80 lb bags deliver more concrete per bag (fewer bags to carry and mix for the same volume) but are noticeably heavier to lift and handle alone. 60 lb bags are the common choice for one-person DIY jobs like fence post holes or small patches; 80 lb bags are more common on larger pours with two people or when minimizing the number of bags mixed matters more than per-bag weight.
Why does the calculator round the bag count up?
Bags are sold whole, not fractionally, so a raw calculated need of 14.3 bags means you buy 15. This calculator always rounds the final bag count up, never to the nearest whole number, since running short mid-pour is a serious problem — concrete that's already mixed and placed starts setting, and you can't pause a pour to make a store run without risking a visible cold joint where the old and new concrete meet.
Does this account for post hole shape (round vs. square)?
No — this calculator uses a rectangular volume model (length x width x depth), which is exact for square or rectangular footings and a reasonable approximation for round post holes if you enter the hole's diameter as both length and width. For a more precise round-hole figure, multiply by pi/4 (about 0.785) afterward to correct from a square to a circular cross-section, since a circle inscribed in a square uses about 21.5% less material.
How much does concrete shrink or expand as it cures?
Properly mixed concrete doesn't meaningfully expand, but it does undergo very slight drying shrinkage (a small fraction of a percent) as it cures over roughly 28 days, which is accounted for by control joints and rebar/mesh reinforcement in the original design rather than by over-ordering material. This calculator estimates the wet, as-poured volume you need to buy, not a shrinkage-adjusted figure.
Can I use this for a footing that isn't a simple rectangle, like a curved patio edge?
For irregular shapes, break the area into simpler rectangular sections, run this calculator for each section separately, and add the results together — that's more accurate than trying to average an irregular footprint into one length and width. For a strongly curved or circular area, calculate the area separately (using the pi x radius^2 formula) and treat it as an equivalent rectangle of the same area and your desired depth.
Does the 'count' field help with multiple post holes?
Yes — if you're pouring several identical post holes or footings (say, six fence posts of the same diameter and depth), enter one hole's dimensions and set count to 6 rather than running the calculator six separate times. If your holes vary in size, calculate each size group separately and add the totals.
Is bagged concrete the same strength as ready-mix from a truck?
Standard bagged pre-mix (like the widely available general-purpose mix this calculator's bag yield is based on) is formulated for typical structural strength (commonly around 4,000 psi at 28 days for general-purpose mixes), suitable for most DIY slabs, footings, and post holes. Structural work with higher engineering requirements (load-bearing foundations, retaining walls) should use an engineer-specified mix, which is usually ordered from a ready-mix supplier rather than bagged.
Why do I need extra water and how does that affect the volume?
Bag yield figures (like the 0.45 or 0.6 cu ft used in this calculator) already assume the manufacturer's recommended water ratio, so you don't need to adjust the bag count for water — you just need to follow the water quantity printed on the bag. Adding too much water increases workability but reduces final strength and slightly changes the cured volume, so it's not a substitute for buying more bags if you're short.
Related tools
Rebar Calculatorif your slab or footing needs reinforcement, work out the rebar grid — bar count, spacing, and stock lengths — before you pour.Concrete Block Calculatorif your project also includes a block foundation wall or garden wall alongside the poured concrete, work out block quantity separately.Epoxy Floor Calculatoronce a new slab has cured, estimate how many epoxy kits or gallons you need to coat it.More concrete tools (curing schedule estimator) are planned but not yet published.