SettingsSettings for this calculationUS
The length of the eave line where rafter bays need baffles.
Measure along the eave, perpendicular to the rafters.
The on-center spacing between rafters.
16 in O.C. is most common in residential roof framing; 24 in O.C. is used with engineered rafters or trusses in some designs.
Baffle vents needed
30 baffles (one per rafter bay)
- Attic width
- 39 linear ft
They open the calculator with your figures already in it
Attic Baffle Vent Calculator: 30 baffles (one per rafter bay) — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- One baffle vent is installed per rafter bay; bay count = attic width along the eave / rafter spacing, rounded up
Inputs used
- Attic Width Along Eave
- 39 ft
- Rafter Spacing
- 16 in
Intermediate steps
- Attic width
- 39 linear ft
What this calculation does not cover
- The figure covers one eave line, because that is the only length asked for. A gable roof carries a soffit down each long side and needs the calculation run for each, so the 30 baffles returned for a 12 m eave at 16 in centres is 60 for the pair, and a hipped roof has eave on all four sides.
- The result counts bays, not the number of baffle sections a bay swallows. Rafter baffles are commonly sold in sections of about 1.2 m (4 ft), enough to carry the channel over the top plate and the first stretch of loft insulation, so where the vented channel must continue up the rafter — the vaulted assembly described in the FAQ, running soffit to ridge — a 4 m rafter run takes three or four sections in every bay and the quantity to order is that multiple of the number shown.
- Baffles protect an airflow path; they do not create one. Nothing here tests whether the soffit actually has vents in it, or whether their net free area meets the 1:150 or 1:300 attic rule, and a baffle fitted above a solid soffit or an uncut sheathing panel achieves nothing. Intake area also has to be balanced against ridge or gable exhaust before the channel will draw.
- The count says nothing about the depth of the channel a baffle holds open or its height above the finished insulation. A minimum clear air gap between the baffle and the underside of the sheathing is normally required — one inch is the figure most often quoted, but the clearance that applies is set by the code in force — and at a shallow eave that gap plus full insulation depth may not fit beneath the rafter at all, which is a raised-heel truss or insulation-depth question rather than a baffle-count one.
- Dividing the eave length by the rafter spacing assumes parallel, uninterrupted bays for the whole run. Hip corners with jack rafters, dormers, valleys, chimneys and the bay that dies against a gable wall all break that pattern, and the number neither removes bays with no soffit vent below them nor adds the short jack-rafter bays a hip creates.
Estimated cost — your price
This site holds no price list for this material — local prices vary too much to publish honestly. Enter your supplier's price and the result is costed with it.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-02 · in the site-wide review of 2026-09-06 · v1.0.1
Regulatory standards & verification citations1
- One baffle vent is installed per rafter bay; bay count = attic width along the eave / rafter spacing, rounded up
Cite this page
Your workspace
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Tools and safety for this job
To fit the eaves and ridge ventilation. Generic types, no brands, no prices.
Protection this work requires
- Mineral and glass wool irritate skin, eyes and airways: cover your arms, wear a P2/N95 respirator while cutting, and rinse in cold water — hot water drives fibres in.
- Most fatal falls on small jobs are from under three metres: use a tower or a ladder tied at the top, and never work off the top two rungs.
- Knives cause more site injuries than any power tool: cut away from your body, change blades often, and wear cut-resistant gloves to EN 388 level C for repeated cutting.
Fit the eaves and ridge ventilation: Fitting vents and trays is roofing. How much free vent area a roof needs at the eaves and the ridge is set by the regulations or code the work falls under and by the underlay's maker, and this site counts the parts rather than judging whether a roof is ventilated enough.
Show the 5 tools this job needsHide tools
Essential
Staple gun
Tape measure
Utility knife
Recommended
Cordless drill/driver
Crawl boards
Also needed as materials: drill and driver bits, replacement blades, staples.
what each roofing tool is for, and the spec that decides which to buy where one does.