OneTripDIY

Soffit & Fascia Calculator

Enter the run of the eaves and how far they overhang. You get the soffit area, the panels, the fascia and channel lengths, and how much ventilation vented panels will actually provide.

By The OneTripDIY Team · Updated

ft

Add up every side that has a soffit, gable ends included if they are boxed in.

in

Wall to fascia. The same as the overhang plus the fascia thickness.

ft

Vinyl and aluminium soffit usually comes in 12 ft lengths.

in

Most vinyl and aluminium soffit is 12 in wide. Check the box.

%
sq ft

Optional. Fill it in to check the ventilation against the 1-in-300 rule.

Check the manufacturer's net free area; it varies a lot between products.

Soffit panels — pieces
Soffit area — sq ft
Fascia — linear ft
J-channel — × 12 ft
F-channel — × 12 ft
Ventilation from these panels — sq in
Low vents needed — sq in
Ventilation check —

Estimates only. Check against your supplier's stated coverage before you buy.

Take this list to the store

We'll email the quantities above, what you typed in, and a link that reopens this calculator already filled in — so you can adjust it on your phone in the aisle.

We send the list you asked for. Anything else only if you tick the box, and you can unsubscribe in one click. What we do with it.

How to work it out

  1. Measure every eave

    Walk the house and add up each run that has a soffit. Boxed gable returns count; open rafter tails do not.

  2. Measure the width

    From the wall out to the back of the fascia. Soffit panels are sold in 12 ft lengths and cut to this width, so a 16 in soffit gets nine pieces out of each 12 ft length.

  3. Count the channels

    J-channel or F-channel goes on both edges — one at the wall, one behind the fascia. That is two runs of channel for every run of eave.

  4. Check the ventilation

    Soffit vents are the intake half of a roof's airflow. Enter the attic area and the calculator checks it against the usual 1 sq ft of free area per 300 sq ft of attic, split half low and half high.

Panels per 12 ft length

How many pieces you cut from one 12 ft soffit panel.

Soffit widthPieces per 12 ft panelPanels per 100 ft of eave
12 in129
16 in912
18 in813
24 in617
36 in425

Ventilation rule of thumb

The 1-in-300 rule with a balanced split, which is what most codes accept.

Attic areaTotal free area neededLow (soffit) half
1,000 sq ft480 sq in240 sq in
1,500 sq ft720 sq in360 sq in
2,000 sq ft960 sq in480 sq in
2,500 sq ft1,200 sq in600 sq in

How much soffit do I need?

Total eave run times the soffit width. A house with 120 ft of eave and a 16 in soffit has 160 sq ft of soffit to cover, which is about 15 twelve-foot panels with a 10% allowance.

Fascia is simpler: one linear foot per foot of eave, plus the waste, bought in the longest lengths that suit your runs to keep joints down.

Do not block the airflow

Vented soffit only works if air can actually get past the insulation into the attic. Baffles at every rafter bay keep a channel open above the insulation — without them, blown-in insulation seals the intake and the roof cooks.

Intake at the soffit and exhaust at the ridge work as a pair. Plenty of ridge vent with no soffit intake moves very little air.

Net free area is not panel area

A “fully vented” panel is mostly solid: the holes give roughly 9 square inches of free area per square foot of panel, and some products give far less.

That is why the check above uses the free area rather than the panel area. Manufacturers publish the figure for each product — use theirs if you have it.

How this is worked out

No black box — here is the arithmetic the calculator runs, so you can check it against your own numbers.

  1. soffit area = eave run × width ÷ 12 Width is entered in inches.
  2. panels = area × (1 + extra) ÷ (panel length × panel width ÷ 12) A 12 ft × 12 in panel covers 12 sq ft, however it is cut.
  3. channel = eave run × 2 ÷ 12 ft lengths One channel at the wall and one behind the fascia.
  4. free area = soffit area × net free area per sq ft Only the perforations count as ventilation.
  5. needed = attic area ÷ 300 × 144 ÷ 2 The 1-in-300 rule, split half low and half high.

Common questions

How much soffit do I need?

Eave run in feet times soffit width in feet. 120 ft of eave at 16 in wide is 160 sq ft.

How many soffit panels in a box?

Vinyl soffit is usually sold by the square (100 sq ft) or in boxes of ten to twelve 12 ft panels. Ask how the price is quoted — by the piece or by the square.

How much ventilation do I need in a soffit?

Half of 1 sq ft of net free area per 300 sq ft of attic. A 1,500 sq ft attic needs about 360 sq in of soffit intake and the same at the ridge.

What is the difference between J-channel and F-channel?

J-channel is a slot that panels drop into; F-channel has a flange that fastens flat to the wall and is quicker where the soffit meets the house. Both work — many jobs use F at the wall and J behind the fascia.

Should soffit be vented or solid?

Vented on any roof with an unconditioned attic. Solid panels are for porch ceilings and for houses with a sealed, insulated roof deck.

Do I need baffles?

Yes, wherever insulation reaches the eaves. Without a baffle holding the channel open, the vent is decorative.

Can I put new soffit over old?

It is done, but it hides rot. Pull the old material, check the rafter tails and fascia for water damage, then fit the new.

Where these figures come from

Every figure below is traceable. How we check the numbers explains how they are verified, and the coverage reference collects them all in one place.

  • International Residential Code, attic ventilation — the 1/300 and 1/150 ventilation ratios and the balanced split
  • Vinyl Siding Institute installation manual — soffit and channel installation practice
  • Manufacturer product data for vented soffit — typical net free area per square foot

Think a number here is wrong? Tell us — we check every report, correct the page, and date the change. Last checked September 27, 2026.