Calculate effective span of slab, clear span, or effective depth from the other two values with support for feet, inches, meters, and cm.
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Effective Span of Slab Formula
The calculator uses the approximate slab effective span relationship:
Rearranged forms used when a different value is missing:
- ES = effective span of slab
- CS = clear span between the faces of supports
- d = effective depth of the slab
The calculator lets you enter any 2 of the 3 values. It then solves for the missing value after converting the units so the clear span, effective depth, and effective span are all compatible.
- Calculate effective span: adds the clear span and effective depth.
- Calculate clear span: subtracts the effective depth from the effective span.
- Calculate effective depth: subtracts the clear span from the effective span.
This is an approximate calculation. In reinforced concrete design, some codes define effective span as the lesser of clear span plus effective depth or center-to-center distance between supports. Use the rule required by your design standard when preparing final structural calculations.
Common Length Unit Conversions for Slab Span Calculations
| Unit | Equivalent in feet | Typical use in this calculator |
|---|---|---|
| 1 inch | 0.083333 ft | Small slab depths |
| 1 foot | 1 ft | Clear span and effective span |
| 1 yard | 3 ft | Longer span input |
| 1 centimeter | 0.0328084 ft | Metric depth or span input |
| 1 meter | 3.28084 ft | Metric clear span and effective span |
Typical Slab Depth Reference Values
| Slab condition | Typical effective depth range | Notes |
|---|---|---|
| Light residential slab | 4 in to 6 in | Used for short spans and lighter loads |
| One-way reinforced slab | 5 in to 10 in | Depth depends on span, load, reinforcement, and deflection limits |
| Heavier floor slab | 8 in to 12 in or more | Often controlled by load and serviceability requirements |
Example Calculations
Example 1: Find the effective span
Suppose the clear span is 12 ft and the effective depth is 6 in.
Convert the effective depth to feet:
Apply the formula:
The effective span is 12.5 ft.
Example 2: Find the effective depth
Suppose the effective span is 4.2 m and the clear span is 4.0 m.
The effective depth is 0.2 m, which is also 20 cm.
FAQ
What is the effective span of a slab?
The effective span of a slab is the design span used for structural calculations. For a simple approximate calculation, it is taken as the clear span plus the effective depth of the slab. Depending on the design code, the effective span may also be limited by the center-to-center distance between supports.
What is the difference between clear span and effective span?
Clear span is the open distance between the inside faces of supports. Effective span is the span used in design calculations. In this calculator, effective span is estimated by adding the slab effective depth to the clear span.
Why does unit consistency matter?
The formula only works when the span and depth are in the same length unit. For example, if the clear span is in feet and the effective depth is in inches, the depth must be converted to feet before adding or subtracting it.
