The question has no answer without a document
Every sign has exactly one physical size and at least three defensible square footages. The bounding rectangle drawn around everything. The sum of the boxes around each element. The face of the cabinet or panel the whole thing sits on. Each of those is written into real ordinances and real lease criteria packages, and each produces a different number from the same drawing.
Run the defaults, which are an ordinary storefront layout: a 20 by 20 logo mark at the left, a 66 by 14 wordmark beside it, a 40 by 5 tagline under the wordmark, all on a 100 by 24 panel. The bounding rectangle is 92 by 20, which is 12.78 sq ft. The three elements added together come to 10.58 sq ft. The panel face is 16.67. Same sign, and the largest figure is 57 percent above the smallest.
What each method actually charges you for
The bounding rectangle charges you for the air between the elements. Move the tagline two inches further from the wordmark and the rectangle grows; the sum of the elements does not move at all. Under one method a more open layout costs square footage and under the other it is free, which is a genuinely strange thing to have riding on a typographic decision.
The cabinet method charges you for the background. At the defaults the panel is 3.89 sq ft bigger than the bounding rectangle, all of it blank. Where a document measures cabinets that way, a smaller panel or letters mounted straight to the wall changes the measured area without changing a thing anybody reads from the street.
The sum-of-elements method is the most generous to a busy layout and the most work to apply, because somebody has to agree with you about what counts as one element. A wordmark is one element to a fabricator and eight letters to a clerk with a different reading.
The second face multiplies whatever the method gave you
A double faced sign is one sign with two identical faces. Some documents count one face. Some count both. Some count both unless the faces are back to back within a stated separation. That is a straight doubling, and the important thing about it is that it does not compete with the method choice but multiplies it. Whichever of the three figures your document lands you on, the face convention is applied on top, so a generous method and a both-faces count compound. How far apart the extremes end up depends on the layout: with a small graphic inside a large cabinet, the method choice alone moves the number much further than the doubling does.
The page has a selector for it and no opinion about which to pick, because there is nothing to have an opinion about — it is written down for your property and somebody can hand it to you.
Why this page refuses to tell you if it fits
You can type the allowance from your permit set or your lease into the last field and the page will subtract. That is all it does: subtraction on two numbers you supplied. It will not tell you whether the sign is permitted, because permission depends on the definition, the allowance, how the frontage was measured, what else is already on the building, and the judgement of the person reviewing it. Sign codes are local, they are amended, and two properties across the street from one another are routinely under different ones.
The useful move is the boring one: get the definition and the allowance in writing from the permit office or the landlord before the artwork is finished, then come back here and apply their sentence. A layout revised at the drawing stage costs an afternoon. A layout revised after fabrication costs the letters.
Nearby
Placing the elements on the wall, with coordinates and the drift that chained measurements introduce, is on the dimensional letter mounting pattern calculator. How tall the letters need to be for the distance people read them from — a different constraint that often fights the area limit — is on the sign letter height calculator.
Questions people ask
How is sign area measured?
It depends on the document that governs the property, and the common methods genuinely disagree. A bounding rectangle around the whole layout, the sum of the individual elements, and the face of the cabinet the graphics sit on are all in real use. On the default layout here they come out at 12.78, 10.58 and 16.67 sq ft for the same sign. Get the definition from the permit office or the landlord.
Does the background panel count as sign area?
Under some definitions yes and under others no, which is exactly why this page reports it as a separate figure rather than folding it in. Where the cabinet face is what gets measured, a panel larger than the graphics on it is charged in full — 3.89 sq ft of blank background at the default sizes.
Is a double sided sign counted once or twice?
All three conventions exist: one face, both faces, and both faces unless they are back to back within a stated separation. It is a straight doubling, and it is applied on top of whichever measurement method your document names rather than instead of it, so the two choices compound. Which convention applies is written down for your property, and the selector on the form applies whichever one you are told to use.
Does spacing the letters further apart increase the sign area?
Under a bounding rectangle method, yes — the rectangle grows to enclose the air. Under a sum-of-elements method, no, because the elements have not changed size. That is a real consequence of a typographic decision and it is worth knowing which method applies before the layout is finalised rather than after.
Can this calculator tell me if my sign is within the allowance?
No. It will subtract the allowance you type in from each measured figure, which is arithmetic on two numbers you supplied. Whether a sign is permitted depends on the local definition, how frontage was measured, what else is on the building and the reviewer applying it. That determination belongs to the authority that issues the permit and to nobody else.