Concrete Calculator
How many bags of concrete mix you need for a slab or a row of post holes, using the yields on the maker's data sheet, with control joints and any steel for a slab.
Why our numbers are more realistic
Concrete bags are easy to miscount. Here's what changes:
| Typical calculator | This calculator | Why it matters |
|---|---|---|
| One post-hole calculator we reviewed uses the wrong yield for a bag | The bag maker's data-sheet yield for each bag size | That calculator lists a 50 lb bag at 0.30 cu ft, which is the 40 lb yield, and that makes the bag count about 25% too high. |
| Quikrete's calculator allows nothing for waste | Adds 10% (5% for level, tight forms) and rounds up once for the whole project | Real pours lose some concrete to uneven ground, low spots and spillage, so the exact volume can leave you short partway through. Quikrete's own calculator says its figures don't allow for uneven ground or waste. |
| Slab calculators count a flat slab | An optional thickened edge, at the width and depth you enter | A 12 × 12 ft shed slab at 4 in with a 12 × 12 in edge needs about 60% more concrete than the flat slab: 142 bags of 80 lb instead of 88. |
| Quikrete's calculator offers slab thickness of only 4 or 6 in | Any thickness from 2 in up | Driveways often use 5 in. Rounding a 5 in slab down to 4 leaves you 20% short, and up to 6 buys 20% too much. |
| Rebar tools divide the total length by the bar length; mesh tools count by area | Bars cut to fit from whole lengths, and mesh laid out in rows with 6 in overlaps | An offcut too short for another bar is scrap. A 12 × 16 ft patio with #3 bars on an 18 in grid takes 21 bars of 20 ft, not the 14 that dividing the length gives. Mesh by area says 4 sheets; laid out with overlaps it takes 6. |
| Most slab calculators we checked say nothing about control joints | Joint layout, spacing and depth for your slab | Concrete cracks as it shrinks. Joints cut into the slab make it crack along straight lines you chose instead of at random. |
| Post-hole calculators count concrete only | Concrete, a concrete collar over packed gravel, or packed gravel only | Packed gravel needs no concrete, and a collar needs about a third less. The gravel is counted in 0.5 cu ft bags. |
How this works
Slab: cubic feet = length × width × thickness (in inches ÷ 12), plus 10% (5% for level, tight forms). Bags = cubic feet ÷ the bag's yield, rounded up. A thickened edge adds a band the edge width wide around the whole slab, as deep as the edge goes below the rest. We count it with straight sides, so a trench with a sloped inner side takes a little less.
Post holes: concrete per hole = π × radius² × concrete depth, minus the post (width × width × depth). The concrete depth is the post's depth in the ground, less 3 in if you top the hole with soil. Times the posts, plus 10%, ÷ bag yield, rounded up once for the job.
- Waste: no standard sets a figure, so the 10% (5% for level, tight forms) is a planning allowance. Running out partway through a pour is worse than a spare bag, and tight, level forms lose less.
- Yields (Quikrete Concrete Mix No. 1101 data sheet): 40 lb = 0.30 cu ft, 60 lb = 0.45, 80 lb = 0.60. A 50 lb fast-setting bag makes about 0.375.
- Hole size and depth: 3 times the post width across, rounded up to an even inch (12 in for a 4×4, the size the setting guides give). You enter how deep the post goes; about a third of its length is typical. We put 4 in of gravel under it, so the hole is that much deeper (guides give 3 to 6 in). This is the fence calculator's hole method, so the concrete per hole matches. Both add 10% for spillage; the fence calculator also sets corner, gate and end posts deeper.
- Top of the hole: Quikrete's setting-posts steps stop the concrete 3 to 4 in below ground level and finish with soil; we use 3 in, about 8% less concrete on a 36 in post. Others fill to ground level and slope the top so water runs off. Both are common, so you pick.
- Setting the posts: "Concrete collar" packs gravel around the lower third of the post's depth and fills the top two-thirds with concrete. "Packed gravel only" fills the hole with crushed gravel tamped up to ground level. Gravel around a post is the hole minus the post, plus 20% because loose stone packs down when it's tamped (highway figures run 25 to 30%; hand tamping packs less).
- Control joints: no more than twice the slab thickness apart, in feet (8 ft for a 4 in slab; the bag maker says 6 to 8 ft), cut a quarter of the thickness deep, with panels no longer than 1.5 times their width. We split the slab into the fewest panels that meet those rules.
- Wire mesh: 6×6 10 gauge, in 5 × 10 ft sheets or 5 × 150 ft rolls, laid in rows with 6 in overlaps (guides give 6 to 9 in), in whichever direction takes fewer pieces.
- Rebar: a grid at the spacing you pick, stopping 3 in from the edges, never wider apart than you picked. Each bar is cut from whole lengths, and bars longer than a length are spliced with an overlap of 40 bar widths (15 in for #3, 20 in for #4). Weight uses 0.376 lb per ft for #3 and 0.668 for #4.
- Chairs: about one every 4 ft each way, half the slab thickness tall. The bag maker puts the steel at the middle of the slab; other guides say the upper third.
Common questions
How many bags of concrete are in a cubic yard?
90 bags of 40 lb, 72 of 50 lb, 60 of 60 lb, or 45 of 80 lb, with nothing extra. With our 10% for waste, a cubic yard of slab takes 50 bags of 80 lb.
Do I need rebar or wire mesh in a concrete patio?
Usually not for a 4 in patio or walk on a well-packed base with control joints. Steel doesn't stop concrete from cracking; it holds the cracks tight so the pieces stay level. Slabs over 5 in thick should get rebar or mesh, and it only works if it sits up in the concrete on chairs, not on the ground.
Can I set a post in gravel instead of concrete?
Yes, for plain fence-line posts in firm soil that drains. University guidance recommends packed crushed rock because water drains away from the post. Tamp crushed gravel with mixed sizes in layers of 4 to 5 in. Gate, corner and end posts, and posts in loose or sandy soil, are better in concrete. Don't set deck, porch or pergola posts in gravel: they need the footing your local code requires.
When should I order ready-mix instead?
From about 1 cubic yard, and almost always above 2 (a trade rule of thumb, not a standard). A yard is about 4,000 lb of bags to mix by hand. Small loads often cost extra, so ask about minimums, and enter the ready-mix price under "Add your prices" to compare.
What this doesn't cover
- Form lumber, and steel for footings, walls and structural slabs, which follow your local code. That includes bars in a thickened edge: the steel count covers the flat slab only, and your building department or plan sets the edge bars.
- Separate footings, walls, steps and round columns.
- Mix strength, curing, and hot or cold weather pours.
- Local frost depth and code rules for posts and footings. Before you dig, call 811 (free; most states require it) so buried lines get marked.
Sources
- Quikrete Concrete Mix No. 1101 data sheet: bag yields and the 2 in minimum pour.
- Sakrete Fast-Setting Concrete data sheet: 50 lb yield of 0.38 cu ft.
- Quikrete, setting posts: hole 3 times the post width, depth, gravel at the bottom, concrete stopped 3 to 4 in short, and fast-setting water.
- Ask Extension, setting fence posts: half the above-ground height buried, and packed crushed rock drains better than concrete.
- Quikrete, concrete sidewalks and small slabs: control joints 6 to 8 ft apart in a 4 in slab, about a quarter of the thickness deep, and mesh at the middle of the slab.
- Concrete Decor, control joint spacing: joints twice the slab thickness apart in feet, and the looser rule of three.
- Concrete Network, slab reinforcement: steel for slabs over 5 in, and when it's optional.
- Hunker, reinforcing slabs with wire mesh: overlapping sheets and dobies about every 4 ft.
- Ernest Maier, welded wire mesh roll: 6×6 10 gauge mesh in 5 × 150 ft rolls.
- Inch Calculator rebar tool and Raken rebar calculator: reviewed for the rebar comparison.
- Ask the Builder, fence posts: crushed gravel tamped in layers for line posts, concrete at corners and gates.
- InterNACHI, inspecting footings (IRC R403): footings at least 12 in wide and at least 12 in below the ground.
- New Prague, MN, garage and accessory structure handout: a slab thickened around the whole perimeter, at least 12 in below grade.
- Concrete Network, driveway thickness: 4 in minimum, 5 to 6 in for heavier loads.
- Inch Calculator post-hole tool and Quikrete calculator: reviewed for the comparison above.
Numbers last reviewed: October 9, 2026.