Concrete Slab Calculator
Enter your slab dimensions and get real cubic yards, bag counts, ready-mix trucks, cost, a mesh or rebar estimate, and the gravel base underneath -- all in one place.
Slab dimensions
Applied to bag counts, trucks, and cost -- not to the raw volume shown above them.
You'll need
1.23 cubic yards
(33.3 cubic feet · 0.94 cubic meters)
With 10% waste, order
1.36 cu yd
60lb bags needed
82
80lb bags needed
62
Ready-mix and cost
Ready-mix trucks (10 cu yd each)
1
Most suppliers charge a short-load fee well under a full truck -- confirm the minimum with your supplier before ordering a partial load.
Estimated cost: $203.70
Bag cost
Estimated cost: $403.00
Reinforcement
Wire mesh sheets (5×10 ft, 6" overlap)
3
Estimate assumes sheets tiled with the 10ft edge along the length. For a slab this size, the rebar calculator sizes a bar grid instead if you're reinforcing with rebar rather than mesh.
Rebar grid
220 linear ft · 161.7 lb (waste included)
Base gravel (4" compacted base)
1.23 cu yd · 1.73 tons
Most slabs sit on 4" of compacted gravel, not bare ground. For a different base depth, the gravel calculator sizes the base layer on its own.
For a footing or a pour that isn't a flat slab, the concrete calculator handles cubic yards and bags for any shape.
Keep going
- For a pour that isn't a flat rectangular slab -- a footing, a pier, an odd shape -- the concrete calculator handles cubic yards and bags for any form. Concrete Calculator
- This page estimates the standard 4-inch base automatically; for a thicker or thinner base under the same slab, the gravel calculator sizes that layer on its own. Gravel Calculator
- Want a closer look at the bar grid -- different spacing, a bigger bar size -- before ordering? The rebar calculator breaks that estimate out on its own page. Rebar Calculator
- A slab often needs a thickened edge or perimeter footing around it; the footing calculator sizes that strip separately from the flat pour. Footing Calculator
How it's calculated
Volume starts as plain rectangular geometry: cubic feet = length (ft) × width (ft) × (thickness (in) ÷ 12). Dividing by 12 converts thickness from inches to feet so all three dimensions are in the same unit before multiplying. Cubic feet becomes cubic yards by dividing by 27 (there are 27 cubic feet in a cubic yard), since ready-mix and most bag charts are priced by the yard.
Waste is applied to that volume as a straight percentage before bag counts are rounded -- ordered cubic feet = raw cubic feet × (1 + waste% ÷ 100) -- because rounding up first and adding waste after can land you a whole bag short. Bag counts are ceil(ordered cubic feet ÷ bag yield), using each bag size's published yield (an 80lb bag covers about 0.6 cubic feet, a 60lb bag about 0.45 -- standard bag-manufacturer coverage figures, e.g. Quikrete and Sakrete). Ready-mix trucks are ceil(ordered cubic yards ÷ 10), since a standard mixer truck carries about 10 cubic yards.
Wire mesh sheets (5×10 ft, with a 6" overlap between adjacent sheets) are counted the same way a strip of overlapping tiles is: the first sheet covers its own full width, and every additional sheet only adds its width minus the overlap. Rebar uses the standard grid formula: bars in one direction are spaced across the perpendicular dimension at floor(dimension ÷ spacing) + 1 bars (one at each end, plus one every full spacing interval), each running the slab's full length or width. The gravel base uses the same volume formula as the slab itself, at a standard 4-inch compacted depth.
Worked example
Using the calculator's own defaults -- a 10×10 ft slab, 4 inches thick, 1 slab, 10% waste, and the default $150/cu yd and $6.50/bag pricing:
- Raw volume: 10 × 10 × (4 ÷ 12) = 33.33 cubic feet = 1.23 cubic yards (0.94 cubic meters).
- With 10% waste, order 1.36 cubic yards -- that rounds up to 1 ready-mix truck (10 cu yd capacity), for an estimated $203.70 at $150/yd.
- 36.67 cubic feet of ordered volume needs 62 bags at 80lb (36.67 ÷ 0.6 = 61.1 → 62), or 82 bags at 60lb (36.67 ÷ 0.45 = 81.5 → 82). At $6.50/bag, 62 bags of 80lb runs about $403.
- Wire mesh: 3 sheets of 5×10 ft with a 6" overlap covers the 10×10 ft area.
- Rebar: #4 bar at 12" on-center gives 11 bars each direction, 220 linear feet before waste, 242 ft after 10% waste, for about 161.7 lb of #4 bar.
- Base gravel at the standard 4" depth: the same 1.23 cubic yards, about 1.73 tons.
Common slab sizes
Raw cubic yards and 80lb bag counts (no waste applied) for common footprints, at the two most common residential thicknesses.
| Slab size | 4" -- cu yd | 6" -- cu yd | 4" -- 80lb bags | 6" -- 80lb bags |
|---|---|---|---|---|
| 8 x 10 ft | 0.99 | 1.48 | 45 | 67 |
| 10 x 10 ft | 1.23 | 1.85 | 56 | 84 |
| 10 x 12 ft | 1.48 | 2.22 | 67 | 100 |
| 12 x 12 ft | 1.78 | 2.67 | 80 | 120 |
| 12 x 16 ft | 2.37 | 3.56 | 107 | 160 |
| 16 x 20 ft | 3.95 | 5.93 | 178 | 267 |
| 20 x 20 ft | 4.94 | 7.41 | 223 | 334 |
| 24 x 24 ft | 7.11 | 10.67 | 320 | 480 |
Standard trade advice is to order 5-10% extra to cover spillage, form variance, and subgrade irregularity -- running short mid-pour is a much bigger problem than a little leftover mix.
Frequently Asked Questions
How thick should a concrete slab be?
4 inches is the standard depth for a shed floor or a patio. A driveway or any slab that carries a car needs at least 4-5 inches, and 6 inches is the common step-up for a slab that will see trucks, RVs, or heavy equipment. Thicker isn't automatically better -- it's mostly about the load the slab actually needs to carry.
Do I need rebar, wire mesh, or neither?
A thin decorative slab under 4 inches with no real load on it is sometimes poured plain. Once you're at 4 inches or more, or the slab sees vehicle weight, most installers reinforce it with one or the other: wire mesh is quicker to lay out but has to be pulled up to mid-depth during the pour to actually do anything, while rebar holds its position more reliably and carries more strength for wider cracks or heavier loads. Check your local code for anything load-bearing.
How many bags make a cubic yard of concrete?
A cubic yard is 27 cubic feet. An 80lb bag yields about 0.6 cubic feet, so it takes 45 bags to make a yard (27 / 0.6). A 60lb bag yields about 0.45 cubic feet, so it takes 60 bags to make a yard (27 / 0.45) -- the smaller bag needs more of them for the same volume, not fewer.
Should I order ready-mix or buy bags?
Ready-mix -- a truck ordered by the cubic yard -- is usually cheaper and faster to place once you're above roughly 1 cubic yard, but most suppliers charge a short-load fee for orders well under a full truck, so check that minimum before you order a small amount. Bagged concrete makes more sense for small, precise pours (a shed pad, post holes, patch repairs) where a delivery minimum wouldn't be worth it.
How long does a slab need to cure before I can use it?
Standard trade guidance is roughly 24-48 hours before light foot traffic and about a week before heavier loads like a vehicle, with the mix reaching most of its rated strength around 28 days. Exact timing depends on the specific mix, temperature, and any additives used, so treat this as general guidance and check your product's own instructions rather than a fixed rule.