Purlin Design Calculator
Load, moment and required Z for a roof purlin from sheeting load and spacing.
wL²/10 credits the continuity of multi-span purlins. Cold-formed Z-purlins also need sag rods and a biaxial check on slopes — gravity doesn't act along the web on a 20° roof.
Formula
Purlin Design Calculator is a free purlin design 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 Purlin Design Calculator
Load, moment and required Z for a roof purlin from sheeting load and spacing. The calculation implements w = q·s; M = wL²/10 (continuous); Z = M/σ (IS 800 / IS 801 cold-formed practice). wL²/10 credits the continuity of multi-span purlins. Cold-formed Z-purlins also need sag rods and a biaxial check on slopes — gravity doesn't act along the web on a 20° roof.
How to use Purlin Design Calculator
- 1Enter Roof load (DL+LL or wind) in kN/m².
- 2Enter Purlin spacing in m.
- 3Enter Purlin span (truss spacing) in m.
- 4Enter Allowable stress in MPa.
- 5Read Line load, Moment (continuous, wL²/10), Required Z instantly — no submit button needed.
- 6Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Purlin Design Calculator?
- ✓Implements the standard formula — w = q·s; M = wL²/10 (continuous); Z = M/σ
- ✓Reference cited on-page: IS 800 / IS 801 cold-formed practice
- ✓One-click SI ⇄ Imperial toggle — values convert in place, physics stays in SI
- ✓Runs entirely in your browser — nothing uploaded, free forever
Frequently asked questions
What formula does the Purlin Design Calculator use?+
It computes w = q·s; M = wL²/10 (continuous); Z = M/σ, per IS 800 / IS 801 cold-formed practice. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
wL²/10 credits the continuity of multi-span purlins. Cold-formed Z-purlins also need sag rods and a biaxial check on slopes — gravity doesn't act along the web on a 20° roof.
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 Purlin Design 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