Key numbers
- Formula: thickness (in) = ft³ ÷ ft² × 12
- 1 yd³ = 4 in over 81 ft², 5 in over 65 ft², 6 in over 54 ft²
- 2 yd³ on 12 × 12 ft: 4.5 in exact, 4.1 in after 10% waste
- Each inch over 100 ft² is 0.31 yd³ (about 14 × 80 lb bags)
On this page
- How to use the Concrete Slab Thickness Calculator
- How to calculate slab thickness from the concrete you have
- How thick should a concrete slab be?
- What thickness will my concrete give?
- How much each extra inch costs in concrete
- Metric slab thickness
- How to measure slab thickness
- Common thickness mistakes
- Frequently asked questions
How to use the Concrete Slab Thickness Calculator
- 1Enter your numbersFill in slab length (ft), slab width (ft), concrete you have or ordered, unit or allowance for waste (%).
- 2Check the assumptionsThickness = volume ÷ area. The waste allowance comes off first, because spillage and low spots in the base use part of the load. The thickness you need comes from your plans and local code, not from this calculator.
- 3Read the resultsThe calculator shows average thickness this concrete gives, slab area, concrete placed after waste or needed for 4 / 5 / 6 in with waste, updated as you type.
- 4Use the resultRound up to what your supplier sells, and keep a little extra for waste.
Results update as you type. Back to the calculator ↑
How to calculate slab thickness from the concrete you have
Thickness is volume divided by area. Put the volume in cubic feet and the area in square feet, then multiply by 12 for inches:
Thickness (in) = volume (ft³) ÷ area (ft²) × 12
One cubic yard is 27 ft³ and an 80 lb bag of Concrete Mix yields about 0.60 ft³, so you can start from a delivery ticket or a pile of bags. The calculator above takes yards, cubic metres, cubic feet or 40, 60 and 80 lb bags.
It also removes a waste allowance first (10% by default). Spillage, form bulges and low spots in the base use part of every load, so the average finished thickness is a little less than the raw division suggests.
Worked example: 2 yd³ on a 12 × 12 ft slab
- Volume: 2 × 27 = 54 ft³. Area: 12 × 12 = 144 ft²
- 54 ÷ 144 × 12 = 4.5 in exact
- After 10% waste: 49.1 ft³ ÷ 144 × 12 = 4.09 in
- A full 4 in needs 1.78 yd³, or 1.96 yd³ with waste: the 2 yd³ order is right
How thick should a concrete slab be?
The thickness you need comes from the job: plans, local code and any equipment maker’s pad specification. The figures below are common practice for planning volumes, not a load rating.
| Use | Common thickness | cm | ft² per yd³ | yd³ per 100 ft² | 80 lb bags per 100 ft² (+10%) | Weight per ft² |
|---|---|---|---|---|---|---|
| Topping over sound concrete | 2 in | 5.1 | 162 | 0.62 | 31 | 25 lb |
| Light garden path | 3 in | 7.6 | 108 | 0.93 | 46 | 37.5 lb |
| Walk formed with 2×4s | 3.5 in | 8.9 | 92.6 | 1.08 | 54 | 44 lb |
| Patio, walkway, shed pad | 4 in | 10.2 | 81 | 1.23 | 62 | 50 lb |
| Driveway for cars | 4–5 in | 10–13 | 65–81 | 1.23–1.54 | 62–77 | 50–62.5 lb |
| Garage floor, heavy pickups | 5–6 in | 13–15 | 54–65 | 1.54–1.85 | 77–92 | 62.5–75 lb |
| RV or hot-tub pad | 6 in | 15.2 | 54 | 1.85 | 92 | 75 lb |
| Equipment pad | 8 in | 20.3 | 40.5 | 2.47 | 123 | 100 lb |
Weights use 150 lb/ft³ for normal-weight concrete. Bagged Concrete Mix is rated for pours 2 in thick or more, so thinner toppings need a product made for them. Thickness is only one part of slab performance: the base, joints, reinforcement and the mix strength (see concrete PSI) matter as much.
What thickness will my concrete give?
These are the questions people ask once the concrete is ordered or the bags are in the truck. Exact means the raw division; the second figure keeps 10% for waste.
| Area | Concrete | Thickness (exact) | Thickness after 10% waste |
|---|---|---|---|
| 10 × 10 ft (100 ft²) | 1 yd³ | 3.24 in | 2.95 in |
| 10 × 10 ft (100 ft²) | 1½ yd³ | 4.86 in | 4.42 in |
| 12 × 12 ft (144 ft²) | 1½ yd³ | 3.38 in | 3.07 in |
| 12 × 12 ft (144 ft²) | 2 yd³ | 4.50 in | 4.09 in |
| 20 × 20 ft (400 ft²) | 5 yd³ | 4.05 in | 3.68 in |
| 20 × 20 ft (400 ft²) | 6 yd³ | 4.86 in | 4.42 in |
| 24 × 24 ft (576 ft²) | 8 yd³ | 4.50 in | 4.09 in |
| 4 × 4 ft (16 ft²) | 10 × 80 lb bags | 4.50 in | 4.09 in |
| 4 × 8 ft (32 ft²) | 20 × 80 lb bags | 4.50 in | 4.09 in |
The 20 × 20 ft row is the one that catches people out: 5 yd³ sounds like plenty for a 4 in slab, but after normal waste it averages under 3.7 in. A 20 × 20 ft slab at 4 in should be ordered as 5½ yd³.
How much each extra inch costs in concrete
Every inch of thickness over 100 ft² is 0.31 yd³ (8.3 ft³), about 14 bags of 80 lb concrete. On bigger slabs it adds up quickly:
- 20 × 20 ft: 4 in → 6 in adds 2.47 yd³.
- 24 × 24 ft: 4 in → 5 in adds 1.78 yd³; 4 in → 6 in adds 3.56 yd³.
- 10 × 10 ft: 3.5 in → 4 in adds 0.15 yd³ (1.08 → 1.23).
Use the extra-inch figure to decide whether a thicker slab is worth it, then price the difference with concrete price per yard.
Metric slab thickness
In metric the calculation is simpler: thickness (m) = volume (m³) ÷ area (m²). One cubic metre covers 10 m² at 10 cm, 8 m² at 12.5 cm, 6.67 m² at 15 cm and 5 m² at 20 cm. So 1.5 m³ over a 3 × 4 m slab (12 m²) gives 12.5 cm before waste. Select cubic metres in the calculator’s unit menu to enter a metric volume; the result is shown in both inches and centimetres.
How to measure slab thickness
Before the pour, stretch a string tight across the tops of the forms and measure down to the base at three or four points, including the middle. Use the average, or the deepest reading if the spread is more than ½ in. A base ½ in low over 400 ft² adds 0.62 yd³.
For an existing slab, measure at an exposed edge or at a drilled hole, in several places. Slabs are often thicker at the edges than in the middle. When thickness matters for a structural decision, a testing lab can take a core.
Common thickness mistakes
- Trusting the lumber size. A 2×4 form gives a 3½ in slab, not 4 in.
- Ignoring waste in the reverse calculation. The raw division always overstates finished thickness.
- Assuming one reading is the whole slab. Check several points before and after the pour.
- Choosing thickness from volume. Order the volume your thickness needs, not the other way round. The concrete slab calculator gives the order for any size.
FAQ
Frequently asked questions
How thick will 1 yard of concrete be over 100 square feet?
3.24 in. One cubic yard is 27 ft³, and 27 ÷ 100 × 12 = 3.24 in. After a 10% allowance for waste it is about 2.95 in, so a 10 × 10 ft slab at a full 4 in needs 1.23 yd³ (order 1½).
How thick will 2 yards of concrete be on a 12×12 slab?
4.5 in exactly (54 ft³ over 144 ft²). Allowing 10% for spillage and low spots, plan on about 4.1 in. That is why 2 yd³ is the usual order for a 12 × 12 ft slab at 4 in.
How thick should a concrete slab be?
Patios, walks and shed pads are commonly about 4 in; driveways and garage floors 4–6 in; RV and hot-tub pads often 6 in. These are common practice, not code: follow your plans, local code and any maker's pad spec. Concrete Mix bags are rated for pours 2 in thick or more.
Is 3.5 inches of concrete enough for a slab?
It is common for light-duty floors and walks formed with 2×4 lumber, which is 3½ in tall. It uses 12.5% less concrete than 4 in: 1.08 yd³ per 100 ft² instead of 1.23. Check what your plans or local code require.
How much more concrete does a 6 inch slab need than 4 inch?
50% more. A 20 × 20 ft slab is 4.94 yd³ at 4 in and 7.41 yd³ at 6 in, an extra 2.47 yd³. A 24 × 24 ft slab goes from 7.11 to 10.67 yd³.
How thick will 10 bags of concrete be over a 4×4 area?
4.5 in. Ten 80 lb bags yield about 6 ft³ (0.60 ft³ each), and 6 ÷ 16 × 12 = 4.5 in. With 60 lb bags (0.45 ft³) ten bags give 3.4 in over the same area.
How do I measure the thickness of an existing slab?
Measure at an exposed edge, or drill a small hole and measure with a wire or depth gauge. Check several spots, because slabs are rarely uniform. A core taken by a testing lab gives the exact figure.
How many cubic metres for a 10 cm slab?
Area in m² × 0.10. One cubic metre covers 10 m² at 10 cm, 8 m² at 12.5 cm and 6.67 m² at 15 cm. A 4 × 4 m slab at 10 cm needs 1.6 m³.
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Last reviewed October 12, 2026 by M Bilal. Found an error? Tell us.