Attic Ventilation Calculator (1/300 Rule)
Net free vent area for an attic, split between soffit intake and ridge exhaust.
More intake than exhaust is fine; the reverse sucks conditioned air through ceiling leaks. NFA is stamped on every vent — sum the stamps, not the hole sizes.
Formula
Attic Ventilation Calculator (1/300 Rule) is a free attic ventilation for HVAC designers, technicians and energy auditors — 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 Attic Ventilation Calculator (1/300 Rule)
Net free vent area for an attic, split between soffit intake and ridge exhaust. The calculation implements NFA = attic area / 300 (or /150), split 50/50 intake/exhaust (IRC R806 — roof ventilation). More intake than exhaust is fine; the reverse sucks conditioned air through ceiling leaks. NFA is stamped on every vent — sum the stamps, not the hole sizes.
How to use Attic Ventilation Calculator (1/300 Rule)
- 1Enter Attic floor area in m².
- 2Enter Rule denominator (300 with vapour barrier & balanced vents; 150 otherwise).
- 3Read Total net free area, Soffit (intake) share, Ridge (exhaust) share instantly — no submit button needed.
- 4Need US units? Flip the SI/Imperial toggle and every field converts.
Why use Attic Ventilation Calculator (1/300 Rule)?
- ✓Implements the standard formula — NFA = attic area / 300 (or /150), split 50/50 intake/exhaust
- ✓Reference cited on-page: IRC R806 — roof ventilation
- ✓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 Attic Ventilation Calculator (1/300 Rule) use?+
It computes NFA = attic area / 300 (or /150), split 50/50 intake/exhaust, per IRC R806 — roof ventilation. The formula is displayed under the result.
What should I keep in mind when using this calculator?+
More intake than exhaust is fine; the reverse sucks conditioned air through ceiling leaks. NFA is stamped on every vent — sum the stamps, not the hole sizes.
Is this based on ASHRAE methods?+
The calculations follow the standard ASHRAE/ISHRAE formulations cited under the result, simplified where industry practice does the same. For permit-grade design, confirm with the full code procedure.
Is the Attic Ventilation Calculator (1/300 Rule) 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