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Intake Exhaust Air path {{ splitLabel }}
Enter the total footprint or calculate it from a same-unit length and width.
Changing the unit converts the displayed value and preserves the same physical area.
The pair shares one unit; changing it converts both dimensions.
Choose the applicable ratio or enter a documented local denominator.
Allowed planning range: 1:50 through 1:1000.
1 :
Record whether the reduced ratio has been checked for this attic.
Set the low-intake share; exhaust receives the remainder.
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% intake
Choose continuous or discrete low intake ventilation.
Enter the manufacturer rating in the unit printed on the product data.
Choose a continuous or discrete high exhaust product.
Enter the manufacturer rating in the unit printed on the product data.
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Existing intake reduces only the additional intake quantity.
Existing exhaust reduces only the additional exhaust quantity.
Optional 0–25% reserve for planning and product selection.
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Visible vent size is not the same as usable vent opening. Louvers, mesh and the product body reduce the passage available to air, so passive attic ventilation is sized with net free area (NFA). Manufacturer NFA ratings are the dependable basis for comparing a soffit strip, ridge vent, roof vent or gable louver.

A working vented attic needs a continuous low-to-high path. Outdoor air enters through intake near the eaves, moves above the insulation and leaves through higher exhaust. Adding ridge exhaust without clear lower intake can pull replacement air through ceiling leaks instead of producing the intended path.

Vented area
The horizontal attic floor footprint served by the openings for a typical flat-ceiling attic.
Intake NFA
Usable opening low at soffits or eaves.
Exhaust NFA
Usable opening high at the ridge, roof or upper gable.
Vent ratio
The relationship between attic area and required NFA, commonly written 1:150 or conditionally 1:300.

The 1:150 basis requires twice as much NFA as 1:300. The reduced 1:300 path is conditional under building codes and may depend on vent distribution, vapor control, climate zone and local amendments. A balanced schedule commonly assigns 50% to 60% of total NFA to low intake and the remainder to high exhaust.

CheckWhy it matters
Use horizontal floor areaSloped roof surface can overstate the ratio area for a conventional attic.
Verify NFA, not rough openingGross dimensions ignore restriction from the vent product.
Inspect each airflow pathInsulation, paint, dust and framing can block installed capacity.
Keep compartments separateDisconnected attic spaces need their own intake and exhaust plan.

Vent count alone cannot diagnose condensation, ice dams or summer heat. Air sealing, insulation, roof assembly, bath-fan termination and indoor moisture may be more important. Unvented roofs, cathedral ceilings and powered attic systems need a design method suited to those assemblies.

How to Use This Tool:

Calculate the required opening first, then turn only the remaining intake and exhaust deficits into products.

  1. Enter the total attic floor area or use length and width. Choose units that match the survey.
  2. Select the 1:150, conditional 1:300 or documented custom ratio. Record whether the 1:300 conditions have been verified.
  3. Set the Intake share from 50% to 60%. Exhaust receives the remaining percentage.
  4. Choose continuous or discrete intake and exhaust products, then replace the preset NFA ratings with the manufacturer values.
  5. Enter measured existing intake and exhaust NFA. Add a planning margin only when the project needs extra ordering or field allowance.
  6. Review the Vent schedule and Balance review. Fix any invalid area, ratio, product rating or unit mismatch before ordering vents.

Interpreting Results:

Total NFA is the required intake-plus-exhaust opening after the optional planning margin. The intake and exhaust targets split that value; existing NFA is credited only to its own side.

Product quantity is based on the remaining deficit, not the total target. Discrete vents round up to whole pieces. Continuous metric products round up to 0.1 m, while products rated per foot round up to the next full foot.

A schedule that meets the numeric target can still fail in the field. Confirm that low intake is clear, high exhaust is weather-protected, NFA ratings match the selected unit and every attic compartment has a connected path.

Technical Details:

Ventilation ratios compare areas in the same units. The calculation normalizes the footprint to square metres and NFA to square centimetres, then converts product ratings as needed. One square metre equals 10,000 cm², one square foot equals 0.09290304 m² and one square inch equals 6.4516 cm².

Formula Core:

Let A be attic floor area in m², n the selected ratio denominator, m the planning margin percent and s the intake share percent:

Nbase=A×10000n Ntotal=Nbase×(1+m100) Nintake=Ntotal×s100 Nexhaust=Ntotal−Nintake

Existing NFA is subtracted separately from each target and never creates a negative deficit. Product quantity divides that deficit by the selected NFA rating:

Qraw=max(0,Ntarget−Nexisting)Rproduct

Rule Core:

SelectionExact calculation behavior
1:150Uses denominator 150.
Conditional 1:300Uses denominator 300; the verification choice is recorded but does not change the arithmetic.
Custom ratioAccepts a denominator from 50 through 1,000.
Intake splitAccepts 50% through 60%; exhaust receives 50% through 40%.
Planning marginAccepts 0% through 25% and increases total NFA before the split.
Discrete ventsAlways rounds up to the next whole vent.
Continuous ventsRounds up to 0.1 m for metric ratings or to one full foot for imperial ratings.

The included product presets are editable planning examples: continuous soffit 190 cm²/m, perforated soffit panel 420 cm² each, round soffit vent 130 cm² each, ridge vent 380 cm²/m, low-profile ridge vent 255 cm²/m, roof box vent 390 cm² each and gable louver 650 cm² each.

Planned NFA equals existing NFA plus the rounded product quantity times its rating. Overage is planned NFA minus target NFA, which makes the effect of purchase rounding visible.

Accuracy Notes:

The result is a passive-ventilation planning schedule, not code approval or an airflow measurement.

  • Confirm the adopted code edition and local amendments before using 1:300 or a custom ratio.
  • Replace preset ratings with manufacturer NFA data for the exact product and installation orientation.
  • Do not credit blocked, painted-over or disconnected openings as existing NFA.

Worked Examples:

Balanced metric schedule

A 120 m² attic at 1:300 needs 4,000 cm² total NFA. A 50/50 split assigns 2,000 cm² to intake and exhaust. With no existing vents, 190 cm²/m soffit and 380 cm²/m ridge products round up to 10.6 m and 5.3 m, providing 2,014 cm² on each side.

Imperial product ratings

A 30 ft by 42 ft attic is 1,260 ft². At 1:300 with a 50/50 split, 9 in²/ft intake and 18 in²/ft exhaust ratings require raw lengths of 33.6 ft and 16.8 ft; purchase lengths round up to 34 ft and 17 ft.

References: