Ridge Vent Calculator

Use attic area, ventilation rule, and product NFA to estimate ridge vent length and balanced intake needs in one place.

Find out how much ridge vent you need, whether your ridge is long enough, and whether your intake can support a balanced attic ventilation system.

Attic size

Use attic floor area here — not roof surface area. This tool sizes ventilation demand, not shingle coverage.

What attic number do you have?
Attic floor area (sq ft)
Enter attic area
Ventilation rule

Ridge vent product

Choose a common product preset or switch to custom NFA per linear foot.

Intake and feasibility

Balanced ventilation only works when intake can support the exhaust plan.

How do you want to describe intake?
Existing exhaust setup

Results are for estimating only. Verify local code and manufacturer specs before installation.

Built for contractors

Ridge vent sizing only works when ridge length, intake, and product NFA all agree.

This page helps you turn attic area into required net free area, split it into balanced intake and exhaust, and check whether the ridge you actually have can carry the plan.

Size the vent

Convert attic floor area into required ridge vent length using the ventilation rule and product NFA per foot.

Check the ridge

Do not assume every roof has enough ridge line for a ridge-only exhaust plan. This page flags shortfalls early.

Check the intake

Balanced ventilation breaks down when intake falls behind. This tool tells you whether intake is helping or hurting the plan.

What this tool is for

Use it before material ordering, before a customer conversation, or before finalizing a ventilation scope when the roof report still needs a quick ventilation reality check.

What this tool is not

It is an estimating tool, not a code-certification engine. Local code and manufacturer specs still need to be verified before installation.

How the ridge vent math works

  1. Start from attic floor area, not roof surface area.
  2. Apply the ventilation rule to get total required net free area.
  3. Split the requirement into balanced intake and exhaust targets.
  4. Divide the exhaust target by the ridge vent product NFA per foot.
  5. Compare that requirement to the ridge length you actually have on the roof.

Why intake matters

Ridge vent is only half the system. If soffit or intake capacity is short, the exhaust plan may look good on paper but still underperform in the field.

Why product NFA matters

Ridge vent products do not all move the same amount of air per linear foot. Product choice changes required footage, so generic rule-of-thumb counts are risky.

Common mistakes this page helps catch

  • Using roof area instead of attic floor area.
  • Ignoring intake and only sizing exhaust.
  • Assuming every ridge vent product provides the same NFA.
  • Forcing a ridge-only plan on a roof with limited ridge length.
  • Leaving box vents or power vents in place without checking mixed exhaust risk.

FAQ

Do I use attic floor area or roof area?+
Use attic floor area. Ridge vent sizing starts from how much attic space needs ventilation, not from how many squares of shingles are on the roof.
What is the difference between 1/300 and 1/150?+
1/300 is the lighter rule and works as the default practical baseline. 1/150 is more demanding and will increase the required total net free area.
Can ridge vent work without soffit vents?+
A ridge vent system performs best with balanced intake. If intake is missing or undersized, exhaust performance can suffer even if ridge footage looks sufficient.
Can I mix ridge vent with box vents?+
Mixed exhaust systems should be field-verified. This tool flags the risk, but it does not certify whether the final combination is acceptable for your project.
What if the roof does not have enough ridge length?+
That is exactly why this tool asks for available ridge length. If ridge-only exhaust comes up short, the roof may need a different exhaust plan or a field-verified compromise.
Why does product NFA per foot matter?+
Different ridge vent products provide different net free area per linear foot. The same attic can require very different ridge footage depending on the product you choose.
Use this in the full roofing material calculator →Need roof area first? Open the roof pitch calculator →