Concrete Slab Calculator

What goes into a slab

A slab on the ground is three layers of material: a compacted gravel base, reinforcement (wire mesh or a rebar grid, if you use any), and the concrete. The calculator works out each one from the same length and width.

Example: a 12 × 10 ft patio slab, 4 in thick

1. Concrete: 12 × 10 × 4 ÷ 12 = 40 cu ft = 1.48 cu yd
2. With 10% extra: 44 cu ft = 1.63 cu yd
3. 80 lb bags: 44 ÷ 0.60 = 73.3 → 74 bags; or 60 lb bags: 44 ÷ 0.45 = 97.8 → 98 bags
4. Base: 12 × 10 × 4 ÷ 12 = 40 cu ft compacted; × 1.4 = 56 cu ft = 2.07 cu yd of crusher run, about 2.7–3.3 tons
5. Mesh: 3.5 × 7 ft sheets overlapped 6 in cover 3 × 6.5 ft each; two sheets along the 12 ft side and four across the 10 ft side = 8 sheets

That is 74 bags and 5,920 lb of dry mix to mix by hand, so price a ready-mix delivery too.

Example: a 20 × 20 ft slab with #4 rebar

1. Concrete: 20 × 20 × 4 ÷ 12 = 133.33 cu ft; + 10% = 146.67 cu ft = 5.43 cu yd (245 bags of 80 lb)
2. Base: 133.33 cu ft × 1.4 = 186.67 cu ft = 6.91 cu yd, about 9.0–11.1 tons
3. Rebar, 3 in from the edges: clear run 19.5 ft; 19.5 ÷ 1.5 = 13 gaps, + 1 = 14 bars each way, 28 bars in all
4. 28 × 19.5 ft = 546 ft × 0.668 lb/ft = 365 lb; every run fits one bar, so 28 bars of 20 ft. With 3.5 × 7 ft mesh sheets instead: 21 sheets

The rebar calculator handles other spacings, bar sizes, 10 ft bars and lap splices.

Bags of concrete for common slab sizes

All at 4 inches thick with 10% extra. Standard concrete mix yields 0.60 cu ft per 80 lb bag and 0.45 cu ft per 60 lb bag (QUIKRETE and SAKRETE print the same figures).^[1][2]

NRMCA advises ordering 4–10% more than the calculated volume. The calculator uses 10%, the top of that range, because a small error in depth costs a lot of concrete: NRMCA points out that a 4 in slab poured just ⅛ in thicker than planned uses 3% more concrete.^[3][4] Ready-mix trucks carry 8 to 12 cubic yards.^[3]

How thick should the slab be?

For driveways, carports, patios and walks, the Ohio Ready Mixed Concrete Association says 3½ to 4 inches is generally enough, unless heavy trucks will park on it regularly.^[9] See concrete slab thickness for more detail.

The gravel base

For house floor slabs on the ground, IRC R506.2.2 calls for a 4 in base course of clean graded sand, gravel or crushed stone where the slab is below grade, unless one of its exceptions applies.^[8] That rule is for floors in dwellings, not patios. For walks and slabs, SAKRETE suggests a 2–4 in drainage base.^[10] The calculator’s 4 in default, measured after compaction, is a starting point for a patio, not a code requirement for one.

Crusher run (also sold as road base or dense-graded aggregate) is full of fines that pack into the gaps. A compacted base takes about 1.33 to 1.5 times its volume in loose stone: 1.33 in Penn State’s 8 in to 6 in rule, about 1.4 from Tennessee DOT’s weights and 1.34–1.52 in Wisconsin DOT’s tests. FHWA reports a volume loss of 30% or more on gravel roads.^{[11][12][13][14]} It is a dense-graded stone with fines, not the clean graded stone R506 names for house floors. The calculator orders 1.4 times the compacted volume and weighs it at 1.30–1.60 tons per loose cubic yard.^[12][13] Clean #57 stone (1.25–1.45 tons per cubic yard) barely compacts, so the calculator orders it at the finished volume; if you run a plate compactor over it, add about 10–15%.^[12][15]

Call 811 before you dig so buried utility lines are marked. Dig deep enough for both layers. A 4 in slab on 4 in of base needs 8 in of depth below the top of the slab, before any height you want the slab to stand above the ground.

Wire mesh or rebar

Welded wire mesh. The Wire Reinforcement Institute lists 6x6-W1.4xW1.4 (10 gauge, about 21 lb per 100 sq ft) and 6x6-W2.9xW2.9 (6 gauge, about 42 lb) as common styles for light construction.^[16] Home centers sell it in 3.5 × 7 ft sheets and 5 × 150 ft rolls.^[17] The calculator counts sheets with a 6 in overlap at each seam; that overlap is our assumption, so use your supplier’s or your plans’ lap if it differs.

A roll is cut into strips laid side by side, each overlapping the next by 6 in, so a 5 ft strip adds 4.5 ft of width. The calculator runs the strips whichever way uses less mesh and counts how many strips each 150 ft roll yields. For the 12 × 10 ft slab that is 3 strips of 10 ft (30 ft of mesh), cut from 1 roll; the 20 × 20 ft slab takes 5 strips of 20 ft (100 ft), also 1 roll. Strips longer than 150 ft are made by joining rolls end to end with the same overlap. A roll left over from a small slab is a lot of spare mesh, so compare it with the price of sheets.

Rebar. The calculator’s rebar options are #3 or #4 bars no more than 18 in apart each way, 3 in from the edges, in 20 ft lengths, with Grade 40 lap lengths where a run is longer than one bar. The 18 in spacing is a common practice, not a code value; we did not find it in a code or an industry standard.

Where it sits. IRC R506.2.4 says slab reinforcement should be supported so it stays between the middle and the upper third of the slab during the pour.^[18] WRI places mesh in the middle of a 4 in slab and 2 in below the top of a 5 or 6 in slab, on supports 2 to 3 ft apart.^[16] Count the chairs or supports too.

For reference, #3 rebar weighs 0.376 lb per foot and #4 weighs 0.668 lb.^[19]

Control joints

Joints give shrinkage cracks a planned place to form. NRMCA sets the maximum joint spacing at 24 to 36 times the slab thickness, never more than 15 ft, and the joint depth at a quarter of the thickness but not less than 1 in.^[20] The Ohio RMCA gives one spacing per thickness, with 12 ft as its maximum.^[9]

QUIKRETE uses 6 to 8 ft for 4 in slabs, closer than either.^[6] With a 10 ft maximum, the 12 × 10 ft slab above needs 1 joint, halfway along the 12 ft length, making two 6 × 10 ft panels; the 20 × 20 ft slab needs 1 joint each way, making four 10 × 10 ft panels.

Related calculators

• Concrete calculatorFootings, posts, steps and curbs as well as slabs
• Rebar calculatorAny spacing, bar size, 10 or 20 ft bars and laps
• Concrete slab thicknessHow thick to pour a patio, walk or driveway
• Crushed stone calculatorCrusher run and #57 for the base
• Bags vs ready-mixWhen a truck makes more sense than bags
• Project materials listPrint everything you added from each calculator

Prepared by the Yard & Build editors. The formulas behind this calculator are checked against hand-worked answers every time the site is built. Bag yields, base factors and code values checked September 26, 2026.