Thickness by use
| Use | Thickness | Source |
|---|---|---|
| Patio | 4 in | QUIKRETE[1] |
| Sidewalk or garden walk | at least 4 in | QUIKRETE[2]; SAKRETE, 4 in minimum over gravel[3] |
| Small slabs and pads | 4–6 in, by load | QUIKRETE[2] |
| Driveway, passenger cars only | 4 in | QUIKRETE[4] |
| Driveway or parking, cars and light trucks | 4 in | Michigan Concrete Assn.[5] |
| Driveway, trucks or heavier vehicles | 5–6 in | QUIKRETE[4]; Michigan Concrete Assn. (light-medium trucks)[5] |
| Heavy commercial trucks | 7 in or more | Michigan Concrete Assn.[5] |
| Floor slab on the ground, such as a garage floor | 3½ in minimum | International Residential Code R506.1[6] |
The Ohio Ready Mixed Concrete Association considers 3½–4 inches generally enough for residential driveways, carports, patios and walks, unless heavy trucks will park on them regularly.[7] The IRC figure is a legal minimum for floors, not a recommendation, and your jurisdiction may have adopted a different edition or its own amendments.[6] Sidewalks in the public right-of-way follow local rules: New Jersey’s site standards, for example, require at least 4 inches and 6 inches where vehicles cross.[8]
Don’t go thinner than the mix allows. QUIKRETE designs its Concrete Mix for pours 2 inches thick or more.[9]
How thickness changes the concrete you need
Concrete scales directly with thickness: a 6 inch slab takes half again as much as a 4 inch one. Per 100 sq ft, before extra for waste:
| Thickness | Cu ft per 100 sq ft | Cu yd per 100 sq ft | Sq ft per cu yd | 80 lb bags per 100 sq ft | Compared with 4 in |
|---|---|---|---|---|---|
| 3.5 in | 29.17 | 1.08 | 92.6 | 49 | −12.5% |
| 4 in | 33.33 | 1.23 | 81.0 | 56 | — |
| 5 in | 41.67 | 1.54 | 64.8 | 70 | +25.0% |
| 6 in | 50.00 | 1.85 | 54.0 | 84 | +50.0% |
“Sq ft per cu yd” is 324 ÷ thickness in inches; SAKRETE gives the same 81 sq ft for a cubic yard at 4 inches.[10] Measure the actual depth of your forms and subgrade: ⅛ inch of extra depth on a 4 inch slab leaves you about 3% short,[11] which is one reason the ready-mix industry advises ordering 4–10% extra.[12]
The base under the slab
For floors on the ground, IRC R506.2.2 calls for a 4-inch base course where the slab is below grade.[6] For walks and slabs, SAKRETE suggests a 2 to 4 inch gravel drainage base.[13]
A base you compact needs more stone than the finished depth suggests. Under 100 sq ft, 4 inches of compacted crusher run is 33.33 cu ft (1.23 cu yd) finished, so order about 1.73 cu yd loose (× 1.4). At Tennessee DOT’s loose weight of 2,900 lb per cubic yard, that is about 2.5 tons.[14] The gravel calculator does this for any size.
Control joint spacing
Slabs crack as they shrink, and joints decide where. The NRMCA guideline is a maximum spacing of 24 to 36 times the slab thickness, no more than 15 ft, cut at least a quarter of the thickness deep and never less than 1 inch.[15] The Ohio RMCA gives a single spacing for each thickness, up to 12 ft.[7]
| Thickness | NRMCA: 24–36 × thickness | Ohio RMCA | Minimum joint depth |
|---|---|---|---|
| 3.5 in | 7.0–10.5 ft | 8 ft | 1.00 in |
| 4 in | 8.0–12.0 ft | 10 ft | 1.00 in |
| 5 in | 10.0–15.0 ft | 12 ft | 1.25 in |
| 6 in | 12.0–15.0 ft | 12 ft | 1.50 in |
For the common 4 inch slab, joints every 8–10 ft satisfy both. QUIKRETE’s project sheets suggest 6–8 ft for 4 inch work, which is closer spacing and also fine.[2]
Questions people ask
How thick should a concrete driveway be?
4 inches for passenger cars (QUIKRETE), and 5–6 inches if trucks or heavier vehicles will use it (QUIKRETE, Michigan Concrete Association). Driveways for heavy commercial trucks need 7 inches or more and a proper design.
Is 3½ inches thick enough for a garage floor?
The IRC sets 3½ inches as the minimum for a concrete floor on the ground, and many jurisdictions build on the IRC. It is a legal minimum, not a recommendation; check what your building department requires and pour thicker if heavy vehicles will use the garage.
How far apart should control joints be in a 4 inch slab?
The NRMCA rule of 24–36 times the thickness gives 8–12 ft, and the Ohio RMCA recommends 10 ft. Joints every 8–10 ft satisfy both. Cut them at least 1 inch deep.
Related calculators and guides
Sources
- QUIKRETE, Project sheet: Concrete Patios. Checked 2026-09-26.
- QUIKRETE, Project sheet: Concrete Sidewalks and Small Slabs. Checked 2026-09-26.
- SAKRETE, High-Strength Concrete Mix technical data sheet. Bag yields match QUIKRETE at 40, 60 and 80 lb. Checked 2026-09-26.
- QUIKRETE, Project sheet: Concrete Driveways and Curbs. Checked 2026-09-26.
- Michigan Concrete Association, Concrete vs. Asphalt: Comparing Thickness and Strengths (2020). Light use 4 in, medium use 5–6 in, heavy use 7 in or more. Checked 2026-09-26.
- International Code Council, International Residential Code 2021, Section R506 Concrete Floors (on ground), via UpCodes. Minimum slab-on-ground thickness 3½ in. Checked 2026-09-26.
- Ohio Ready Mixed Concrete Association, Guidelines for Durable Driveways, Carports, Patios, Walks. Checked 2026-09-26.
- State of New Jersey (N.J.A.C., via Cornell LII), N.J. Admin. Code 19:31C-3.10 Site plan; subdivision standards. Public sidewalks at least 4 in thick, 6 in at vehicle crossings. Checked 2026-09-26.
- QUIKRETE, Concrete Mix No. 1101 data sheet (rev. Oct 2022). Bag yields: 40 lb 0.30, 50 lb 0.375, 60 lb 0.45, 80 lb 0.60 cu ft; water per bag. Checked 2026-09-26.
- SAKRETE, Pro Tip: Estimating Bagged Concrete Jobs. 45 bags of 80 lb per cubic yard; add 10%. Checked 2026-09-26.
- National Ready Mixed Concrete Association, CIP 8: Discrepancies in Yield. Checked 2026-09-26.
- National Ready Mixed Concrete Association, CIP 31: Ordering Ready Mixed Concrete. Truck capacity 8–12 cu yd; order 4–10% extra. Checked 2026-09-26.
- SAKRETE, All-Purpose Gravel technical data sheet. Checked 2026-09-26.
- Tennessee Department of Transportation, Roadway Design Guidelines, Chapter 4 (rev. 03/20/15). Loose and compacted weights of mineral aggregate base and crushed stone. Checked 2026-09-26.
- National Ready Mixed Concrete Association, CIP 6: Joints in Concrete Slabs on Grade. Checked 2026-09-26.