Ordering the right amount of concrete matters — too little means a second delivery mid-pour, too much wastes money. This calculator (also useful as a quick slab estimator) works out the volume of concrete needed for a flat slab (a patio, driveway, or footing) from its length, width, and thickness.
The formula
For a slab 5 m long, 4 m wide, and 10 cm thick: volume = 5 × 4 × (10 ÷ 100) = 5 × 4 × 0.1 = 2 cubic meters. In cubic yards (a common unit for ready-mix concrete orders in the US), that's approximately 2.62 yd³, since 1 m³ ≈ 1.308 yd³.
Worked examples
| Length | Width | Thickness | Volume |
|---|---|---|---|
| 3 m | 3 m | 10 cm | 0.90 m³ |
| 10 m | 0.6 m | 15 cm | 0.90 m³ |
| 6 m | 4 m | 12 cm | 2.88 m³ |
Step-by-step guide
- Measure the length and width of the area in meters.
- Decide on the slab thickness in centimeters — typically 10 cm for a patio, more for a driveway that carries vehicle loads.
- Multiply length × width to get the area, then multiply by thickness (converted to meters) to get volume.
- Add a small buffer, commonly 5–10%, to account for uneven ground and minor spillage — most suppliers recommend rounding up rather than ordering the exact calculated amount.
Common mistakes
- Forgetting to convert thickness to meters. Thickness is usually measured in centimeters but must be converted before multiplying with length and width in meters.
- Not ordering a buffer. Ground is rarely perfectly level, and a small buffer avoids a costly mid-project shortage.
- Confusing area with volume. Length × width alone gives area, not the concrete volume — thickness must be included.
This estimate is for general planning purposes. For structural work, always confirm quantities and specifications with a qualified contractor or engineer.
Frequently asked questions
How much concrete do I need for a slab?
Multiply the slab's length and width in meters by its thickness in meters (thickness in cm divided by 100) to get the volume in cubic meters.
Should I order extra concrete beyond the calculated amount?
Yes — a 5–10% buffer is common practice to account for uneven subgrade, formwork variance, and minor spillage during the pour.
How do I convert cubic meters to cubic yards?
Multiply cubic meters by approximately 1.308 to get cubic yards, the unit commonly used for ready-mix concrete orders in the United States.
References
- American Concrete Institute — Concrete mix and placement standards