How to Calculate Concrete for a Slab
How thick the slab should be, how much concrete that takes, and how to turn it into bags or a ready-mix order — with worked examples you can check by hand.
Two questions decide every slab: how thick, and how much. They are the same calculation, because volume runs in direct proportion to thickness — pick 4″ instead of 3½″ on a 100 ft² slab and you have bought another 4 ft³ of concrete. This guide takes the thickness question first, then the volume math, then the order.
How thick should a concrete slab be?
Start with what the code actually says, because almost every guide online skips it and then quotes a thickness as if it were law. IRC 2021 R506.1 states that concrete slab-on-ground floors shall be designed and constructed per that section or ACI 332, and that floors shall be a minimum 3½ inches (89 mm) thick. That is the only thickness the residential code gives you, and it is a floor — a bottom line for a structural slab inside the envelope, not a recommendation for your patio.
Outside that minimum, thickness is a load-and-soil question, which is why the numbers you meet in the field are practice rather than code:
| Use | Common thickness | Basis |
|---|---|---|
| Interior slab-on-ground (house, basement) | 4″ typical | IRC R506.1 minimum 3½″ |
| Shed or garden-building floor | 4″ | IRC R506.1 minimum 3½″ |
| Patio, walkway, footpath | 4″ | Not IRC-prescriptive (R506.2.3 exempts flatwork from the vapor retarder) |
| Driveway (cars, light trucks) | 5–6″ over a compacted base | Not IRC-prescriptive — load and soil driven (ACI 332 / engineering) |
| Garage floor | 5–6″ | Not IRC-prescriptive — load and soil driven |
| Heavy vehicle, RV or equipment pad | 6″+ , often engineered | Engineering |
Read the right-hand column honestly: apart from the 3½″ floor, none of those numbers is a code requirement. Contractor sources reviewed for this page disagree with each other on the vehicle cases — some write 4″, some 5″, some 6″ for a driveway or garage floor — and none of them cite a code section. The reason they disagree is that thickness alone does not decide whether a slab survives. A 4″ slab on 6″ of well-compacted, well-drained granular base will outlast a 6″ slab poured straight onto soft clay, and the code handles that part through the requirements below rather than through a bigger number in R506.1.
The rest of R506 — the requirements that travel with the thickness
A slab is a system. The thickness rule is one line of R506, and the other lines explain why two slabs of the same thickness behave differently:
- Site preparation (R506.2). All vegetation, topsoil and foreign material must be removed from inside the foundation walls. Organics under a slab decompose and leave a void.
- Fill (R506.2.1). Fill must be free of vegetation and foreign material and compacted for uniform support — and unless the building official approves otherwise, fill depth is capped at 24″ for clean sand or gravel and 8″ for earth. Deeper fill needs approval, i.e. it is a design decision.
- Base course (R506.2.2). Where the slab sits below grade, a 4-inch-thick base course of clean graded sand, gravel, crushed stone, crushed concrete or crushed blast-furnace slag passing a 2″ sieve goes on the prepared subgrade. There is an exception where the subgrade is well-drained or a sand-gravel Group I soil.
- Vapor retarder (R506.2.3). A minimum 10-mil (0.010″) vapor retarder meeting ASTM E1745 Class A, joints lapped not less than 6 inches, placed between the slab and the base course or prepared subgrade. The code exempts garages, utility buildings and other unheated accessory structures; unheated storage rooms under 70 ft² and carports; and driveways, walks, patios and other flatwork not likely to be enclosed and heated later.
- Reinforcement support (R506.2.4). Where reinforcement is used, it must be supported so it stays in the middle to upper one-third of the slab for the duration of the pour. Mesh lying on the subgrade is doing nothing.
The formula — turning a footprint into a volume
A slab is a box: length × width × depth. Keep the units straight — measure the footprint in feet and the depth in inches, then divide by 12.
Volume (ft³) = Length (ft) × Width (ft) × Depth (in) ÷ 12
Example: a 10′ × 12′ slab at 4″ thick = 10 × 12 × (4 ÷ 12) = 40 ft³.
Add waste — yes, really
Ground is never perfectly level, forms flex, and mix sticks to the wheelbarrow. Add 10% on top of the theoretical volume. For the example above: 40 × 1.10 = 44 ft³. This is the number you buy.
Bags or ready-mix?
Bagged concrete yields are standardized:
- 80-lb bag ≈ 0.60 ft³ (0.022 yd³)
- 60-lb bag ≈ 0.45 ft³ (0.017 yd³)
- 40-lb bag ≈ 0.30 ft³ (0.011 yd³)
Divide your waste-adjusted volume by the bag yield and round up. For 44 ft³ with 80-lb bags: 44 ÷ 0.60 = 74 bags. That's 74 × 80 = 5,920 lb of concrete — you will feel every bag.
Ready-mix is ordered by cubic yard (1 yd³ = 27 ft³). 44 ft³ = 1.63 yd³, so you'd order 1.75 or 2 yards depending on the plant's minimum and your confidence in the forms. Most plants have a delivery minimum around 1–3 yd³ — below that, bags are cheaper.
Skip the arithmetic
Use the concrete calculator — enter your dimensions, depth, and waste, and it returns cubic feet, yards, bag counts, and a cost estimate.
Worked example — thickness, volume and order in one pass
A 10 ft × 10 ft shed slab at 4″ thick. Work it in the order you would on the job:
- Footprint. 10 × 10 = 100 ft².
- Depth in feet. 4 ÷ 12 = 0.333 ft.
- Raw volume. 100 × 0.333 = 33.3 ft³.
- Add 10% waste. 33.3 × 1.10 = 36.7 ft³.
- Cubic yards. 36.7 ÷ 27 = 1.36 yd³ — over the typical bag/ready-mix line, so this is a plant order.
- In bags, if you must. 36.7 ÷ 0.60 = 61.2 → 62 × 80-lb bags, about 4,960 lb to mix by hand.
Now the thickness decision, run the same way. At the R506.1 minimum of 3½″: 100 × (3.5 ÷ 12) = 29.2 ft³. At 4″: 33.3 ft³. The half inch costs you 4.2 ft³ — roughly seven 80-lb bags, or about a sixth of a yard. On a 300 ft² driveway the same half inch is 12.5 ft³, close to half a yard, which is where the argument about driveway thickness actually gets settled: on the order sheet.
Common mistakes
- Quoting a thickness as code when it isn't. The IRC gives one number — 3½″ (R506.1). Everything above that is practice. Say which one you mean.
- Forgetting waste — a 10% allowance is the difference between one more trip and a cold joint.
- Ordering ready-mix by feet — plants speak in yards. 27 ft³ = 1 yd³.
- Mixing depth units — 4″ is 0.333 ft, not 4 ft.
- Not accounting for grade — a sloped site needs more concrete at the low end; measure the average depth.
- Pouring an enclosed slab without a vapor retarder. R506.2.3 wants 10-mil ASTM E1745 Class A, lapped 6″. Skip it and you will meet the moisture from above, later, through a floor covering.
- Laying mesh on the ground. R506.2.4 puts it in the middle-to-upper third of the slab. On the subgrade it is a very expensive wire.
Concrete slab FAQ
How do I calculate how much concrete I need for a slab?
Measure the footprint in feet and the depth in inches, then compute Length × Width × Depth ÷ 12 for cubic feet. Example: a 10′ × 12′ slab at 4″ thick = 10 × 12 × (4 ÷ 12) = 40 ft³. Add 10% waste, so you'd order 44 ft³.
How thick should a concrete slab be?
IRC 2021 R506.1 sets the minimum slab-on-ground thickness at 3½″ (89 mm), or the slab may be designed to ACI 332. That is the structural floor minimum. In practice 4″ is ordinary for a patio, walkway or shed floor, and 5–6″ is common where cars park or drive. Driveway and garage thickness is not IRC-prescriptive — it is load- and soil-driven, so treat any single number there as practice, not code.
Do I need a vapor barrier under a concrete slab?
For an enclosed, heated slab-on-ground, yes: R506.2.3 requires a minimum 10-mil (0.010″) vapor retarder to ASTM E1745 Class A, joints lapped at least 6″. The code exempts garages, utility buildings and other unheated accessory structures, unheated storage rooms under 70 ft² and carports, and driveways, walks, patios and other flatwork not likely to be enclosed and heated later.
Does a thicker slab use more concrete?
Yes, in direct proportion. A 10′ × 10′ slab is 100 ft²: at the 3½″ minimum it takes 29.2 ft³, at 4″ it takes 33.3 ft³. That ½ inch is about 4 ft³ — roughly seven extra 80-lb bags, or a sixth of a cubic yard. Thickness and volume are the same decision, so make it before you call the plant.
How many bags of concrete do I need?
Divide your waste-adjusted volume by the bag yield and round up: 80-lb bag ≈ 0.60 ft³, 60-lb bag ≈ 0.45 ft³, 40-lb bag ≈ 0.30 ft³. For 44 ft³ with 80-lb bags: 44 ÷ 0.60 = 74 bags.
Bags or ready-mix — which is cheaper?
Under about 1 cubic yard, bagged mix usually wins on cost and control. Over about 2 cubic yards, ready-mix saves your back and your schedule. Most plants have a 1–3 yd³ delivery minimum, so below that bags are cheaper.
How many cubic feet are in a cubic yard of concrete?
27 cubic feet. Ready-mix is ordered by cubic yard, so 44 ft³ = 1.63 yd³, and you'd order 1.75 or 2 yards depending on the plant's minimum.
Before you pour
- Check your local frost depth — footings go below it.
- Fix the thickness first: 3½″ minimum (R506.1), 4″ for most patios, walks and shed floors, 5–6″ where vehicles drive.
- On an enclosed slab, plan the 10-mil vapor retarder and the base course before you dig (R506.2.2–R506.2.3).
- Call before you dig — utility locates are free.
Related pages
- Concrete calculator — volume in ft³ and yd³, bag counts and a cost estimate.
- How long does concrete take to cure? — when you can walk on it, load it and drive on it.
- Rebar size chart — bar diameter, area and weight per foot for a take-off.
- Rebar & footing calculator and the deck footing calculator — turning a bar list into pounds and sizing the pads underneath.