Where the two formulas come from
Draw one stall as a parallelogram. Its long sides run at the bay angle, its ends are square to those sides. Now slide a second one alongside it. The two are separated by the stall width measured square to the stall, but you are measuring their spacing along the row, and that spacing is the stall width divided by the sine of the angle. At 90 degrees the sine is 1 and the module equals the stall width. At 45 degrees the sine is 0.707 and a 9 ft stall claims 12.73 ft of frontage.
The depth works the other way and needs both trig functions, which is where people go wrong. Project the parallelogram onto the direction square to the row. The long side contributes stall depth times the sine. The short side contributes stall width times the cosine. Add them: for a 9 by 18 stall at 60 degrees that is 15.59 plus 4.50, or 20.09 ft of bay. At 45 degrees it is 12.73 plus 6.36, or 19.09 ft. At 90 degrees the cosine is zero and the bay is simply the stall depth, 18 ft.
Drop the cosine term and you get 15.59 ft for the 60 degree bay, which is more than four feet shallow and will happily convince you that angled parking fits an extra row it does not fit. Swap the sine and the cosine over and the whole comparison inverts. Both mistakes are easy to make and neither one looks wrong on the page.
Why the answer is not simply 90 degrees
Per square foot of paved rectangle, head-in parking usually does win, and the reason is that the module width penalty of an angle grows faster than the bay depth saving. But two things interfere. The first is the aisle: an angled bay is one-way and the aisle serving it is narrower, and that saving is real and is subtracted from every module across the depth. The second is that rows come in whole numbers. A lot 130 ft deep divided by a 60 ft head-in module gives two modules and 10 ft left over, which is worth nothing. The same depth divided by a 58.2 ft angled module gives two modules and 13.6 ft, which is also worth nothing. Change the depth to 145 and one of them picks up a row and the other does not.
That discretisation is why a general rule is worth so little here and why this page runs every angle you name rather than telling you which one to use. The interesting output is often not the winner but the spread: if four angles land within a stall or two of each other, the decision belongs to circulation, snow, and how comfortable drivers are backing into a 24 ft aisle, none of which is arithmetic.
What the count is not
It is a ceiling. It assumes the rectangle is a rectangle, that the rows can run in the direction you said, that drainage does not object, and that nothing structural is in the way. It contains no accessible stalls at their own dimensions, no loading zone, no cart corral, no fire lane and no snow storage. Every one of those comes out of the number rather than being added to it. The obstruction allowance field exists so you can put your own honest haircut on the grid, and a figure you have measured on a comparable lot beats anything a calculator would suggest.
Overhang, and why it is set to zero
Some plans allow the front of a vehicle to hang past a wheel stop over a walk or planting strip, and where that is allowed the paved bay can be shortened by that amount. Whether it is allowed at all, and by how much, is a decision on your plan and not a property of parking. It starts at zero here for that reason. If you take the credit, take it only where a walk or a strip actually exists to hang over, and remember that a walk being partly occupied by a bumper is a walk that is narrower for anyone using it.
Questions people ask
How much frontage does an angled parking stall take?
Stall width divided by the sine of the angle. A 9 ft stall takes 9 ft at 90 degrees, 9.32 ft at 75, 10.39 ft at 60, 12.73 ft at 45 and 18 ft at 30. That is the penalty side of angling, and it is why shallow angles need long rows to make sense.
How deep is a 60 degree parking bay?
For the stall size you enter, it is stall depth times the sine of 60 plus stall width times the cosine of 60. With a 9 by 18 stall that is 15.59 plus 4.50, so 20.09 ft. The second term is small but it is not optional, and leaving it out is the single most common error in this arithmetic.
Does 90 degree parking always fit more cars?
Usually per square foot, but not always in a given rectangle. Angled bays come with a narrower one-way aisle, and rows only exist in whole numbers, so a lot whose depth happens to land just short of another head-in module can fit more cars at 60 or 75 degrees. Enter your actual depth and the angles you would consider and let the counts settle it.
Why does the page not include accessible stalls?
Because their number and their dimensions are set by law and by your permitting authority and vary by jurisdiction, and because their position is decided by the route to the entrance rather than by packing. They come out of this count, not on top of it. Get the requirement from the authority reviewing your plan and apply it to the total.
What does the obstruction allowance actually cover?
Everything that makes a real lot lose stalls a grid does not: transformer and generator pads, light pole bases sitting inside a stall, an island the drainage needed, corners that are not square, and the row that ends three feet short of the property line. Five percent is a placeholder. If you have laid out a comparable lot, measure the real loss there and use that instead.