← Home & Construction Calculators
Plan a rectangular concrete slab with unit conversion, waste allowance, bag and ready-mix estimates, optional granular subbase, approximate reinforcement length and supplier-ready costs.
Concrete slab symbols and formulas
The calculator converts all dimensions to feet before applying the formulas. Costs remain editable because supplier pricing and minimum-load fees vary by location.
Slab measurement diagram
Measure inside the forms. If the excavation depth varies, use several measurements and an appropriate average rather than the shallowest point.
Worked concrete slab example
For one 20 ft × 10 ft slab that is 4 in thick:
Thickness = 4 ÷ 12 = 0.3333 ft
Net volume = 200 × 0.3333 = 66.667 ft³
Net cubic yards = 66.667 ÷ 27 = 2.469 yd³
With 10% allowance = 2.716 yd³
Using 0.60 ft³ per 80 lb bag, the adjusted volume requires 123 bags. A ready-mix order may be more practical for this quantity.
Metric example
A 3 m × 2 m slab at 10 cm thick contains 0.60 m³ before allowance.
Common slab thicknesses
| Typical application | Common planning thickness |
|---|---|
| Walkway or light patio | 4 in (about 100 mm) |
| Residential shed slab | 4 in (about 100 mm) |
| Residential driveway | 4–6 in (about 100–150 mm) |
| Garage or heavier load | Project-specific engineering |
These are general planning ranges only. Soil, climate, loading, vehicle type and local codes can require a different design.
Premix reference
| Bag size | Approximate yield |
|---|---|
| 60 lb | 0.45 ft³ |
| 80 lb | 0.60 ft³ |
| 90 lb | 0.675 ft³ |
Subbase, forms and reinforcement
- Excavate organic or weak material and compact the prepared subgrade.
- Use the specified granular base depth and compact it in suitable lifts.
- Brace forms so they remain straight, level and square during placement.
- Support reinforcement at the designed elevation; do not leave it on the ground.
- Plan control-joint location and timing before concrete placement.
- Confirm drainage slope before ordering concrete.
The reinforcement output estimates a two-direction grid over the full slab area with a 10% allowance. It does not design bar size, laps, edge clearances or structural capacity.
Frequently asked questions
How many cubic feet are in one cubic yard?
One cubic yard equals exactly 27 cubic feet.
Should slab concrete be ordered exactly to the calculated volume?
No. A reasonable allowance helps cover uneven excavation, form movement and placement loss. The proper allowance depends on site conditions.
Does the bag estimate include partial bags?
The result rounds upward to a whole bag because concrete mix is normally purchased in complete bags.
Is the reinforcement estimate a structural design?
No. It is a material-planning estimate only. Use approved plans for bar size, spacing, laps, supports and cover.
Does ready-mix cost include every fee?
The estimate includes the editable delivery value but may not include tax, pump, waiting, environmental or short-load fees.
Common mistakes
- Entering a 4-inch thickness as 4 feet.
- Using outside dimensions instead of the inside form dimensions.
- Ignoring thickened edges or grade beams.
- Estimating reinforcement without required laps and clear cover.
- Ordering before access, labor, forms and subbase are ready.