Floor Joist Span Calculator
Deflection-governed max span of rectangular timber joists at your spacing.
50×200 @ 400 c/c carries a residential floor about 3.3 m — matching IRC span tables within a few percent. Deflection, not strength, almost always governs joists.
Formula
Note: Deflection criterion only — confirm bending/shear and use graded design values for real construction.
Floor Joist Span Calculator is a free joist span for structural engineers, fabricators and site engineers — instant, accurate and 100% client-side, with the governing formula and reference shown next to the result so the number can be defended, not just quoted.
About Floor Joist Span Calculator
Deflection-governed max span of rectangular timber joists at your spacing. The calculation implements 5wL⁴/384EI = L/360 → L = ∛(384EI/1800w) (IRC span tables; IS 883). 50×200 @ 400 c/c carries a residential floor about 3.3 m — matching IRC span tables within a few percent. Deflection, not strength, almost always governs joists.
How to use Floor Joist Span Calculator
- 1Enter Joist breadth in mm.
- 2Enter Joist depth in mm.
- 3Enter Joist spacing in mm.
- 4Enter Floor load (DL+LL) in kN/m².
- 5Read Max span (L/360) instantly — no submit button needed.
- 6Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Floor Joist Span Calculator?
- ✓Implements the standard formula — 5wL⁴/384EI = L/360 → L = ∛(384EI/1800w)
- ✓Reference cited on-page: IRC span tables; IS 883
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Live worked example: the substitution recomputes from your numbers
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Floor Joist Span Calculator use?+
It computes 5wL⁴/384EI = L/360 → L = ∛(384EI/1800w), per IRC span tables; IS 883. The formula is displayed under the result along with a worked example substituted with your own inputs.
What should I keep in mind when using this calculator?+
50×200 @ 400 c/c carries a residential floor about 3.3 m — matching IRC span tables within a few percent. Deflection, not strength, almost always governs joists. Note: Deflection criterion only — confirm bending/shear and use graded design values for real construction.
Can I use this for real structural design?+
It implements the exact textbook/code formula cited below the result and is ideal for sizing, checking and learning. Final designs should be verified by a qualified engineer against the full code with all load cases.
Is the Floor Joist Span Calculator free to use?+
Yes — completely free, no sign-up, no limits. It runs client-side in your browser, so inputs stay private and results are instant even on slow connections.
Related Engineering tools
Reynolds Number Calculator
Compute Re from density, velocity, pipe diameter and viscosity — with instant laminar/turbulent classification.
● LiveLaminar vs Turbulent Flow Classifier
Classify hydraulic-oil or process-fluid flow regime from kinematic viscosity, line velocity and tube bore.
● LiveBernoulli Equation Solver
Solve downstream pressure between two points from velocities, elevations and fluid density (ideal, lossless).
● Live