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You do not need to remove the sail from the airframe. You might want an assistant, for this process. You will need an asphalt/vinyl tiled floor (maybe 20 feet by 20 feet, minimum), and a measuring tape (10 feet long). See attached image.

Measure one of the floor tiles. In the USA, most commercial asphalt/vinyl floor tiles are one foot (12") square, but they may be nine inches square, or other sizes, in some areas. If you can work with square inches, fine; otherwise, it may be easier to find a floor with 12" tiles, first.

Sail laid out on tiles

Lay out the glider with one wing fully open, and the sail spread flat on the floor, from the leading edge to the keel. If the glider has curved tips, install the tip wand, but not the ribs. Align the leading edge exactly on the seam between two rows of tiles. Shift the sail left and right along this seam line, to align the tail of the sail (at the trailing edge of the sail) with any one seam, between two columns of tiles (Point Y).

Imagine a red line, down the middle of each column of tiles, from the leading edge to the trailing edge. We are going to measure the length of all of the (imaginary) red lines, and add them all together.

For the first red line, stretch the tape measure from the leading edge to the trailing edge (or to the keel pocket), and record the length of this imaginary line. Move to the next red line, and repeat this process, until you have measured all of the red lines (in this case, all 17 of them). Add all of these lengths together.

The total length of red lines here will be about ~82.5 feet long. Using 12" tiles, the sail area for one wing then is ~82.5 square feet. Now, double that number, to account for the other wing (the total sail area is ~165 sq.ft.).

(If the floor tiles are 9" then there will be 22 imaginary red lines. Add up that total length of red lines, in inches this time, and multiply by 9. That gives you the total square inches of one side of the glider. Double that number, to account for the other half of the glider. Divide by 144, to get the square feet of the wing.)

You can guesstimate about a tenth of a square foot more, for the extreme wingtip, beyond the last column of 12 inch tiles, here.

Please record the sail area of the glider in a safe and visible place on the glider, for future reference. Dye-marker pens are safe on aluminum; do NOT use ordinary pencils or ball-point pens.

Do not be surprised to find the manufacturer has advertised a slightly different number, such as 160 or 170 square feet, for the glider. Some manufacturers will advertise a "projected" sail size, not the actual square feet of the sail, but that is a more complex computation than this direct process. This same process can also be used with Metric sizes of floor tiles and gliders.