The WASP beret consists of two parts, the Beret Top Piece and Beret Side Flap. The dimensions for each must be calculated as explained below. Please note that my calculations are given in centimeters, which must be converted to inches if not using the metric system.

A. Beret Top Piece: 

First, you must construct the Beret Top Piece. Use the formula below. 
Formula to calculate Beret Top Piece diameter: 

2  x  Pi  x  Radius = Perimeter   (Pi ~ = 3.1416 ; Radius = diameter / 2 )

For example, if your head size measurement (measure the circumference around your head where you want the beret to fit) is 54 cm (21 ½  inches), the Beret Top Piece should have a diameter of 24.5 cm (9 6/8 inches). A Beret Top Piece with a 24.5 cm diameter has an outer perimeter of 77 cm (this length is important for the Beret Side Flap pattern in Part B). 

In case you have a 24.5 cm diameter Beret Top Piece, you have to calculate the perimeter (green line in the drawing) like this: 
2  x  3.1416  x  12.25 = 77 cm (30 3/8 inches) 

Also, remember that the radius of your Beret Top Piece should be proportional to your head measurements. You can calculate your Beret Top Piece radius like this: 

12.25  x  (your head size measurement)   /   54  =  Radius of the circle formed by your Beret Top Piece.

In case you have a head size measurement of 60 cm ( 23 5/8 inches), you must calculate the top piece radius like this: 
12.25 x  60  /  54  =  Radius of 13.61 cm ( 5 3/8 inches) 

Now, you should be able to make a pattern for the Beret Top Piece suitable for your own head size by drawing a circle on paper with the calculated radius. 

B. Beret Side Flap:

Secondly, you must construct the Beret Side Flap, which is in the pattern of a semi-circle. Use the formula below. To calculate your width for the beret side piece, do it like this:

7.5  x  (your head size measurement)   /   54  =  width of your side piece 

The outer dimension of the Beret Side Flap (green line in the drawing) is based on the same measurements used for the Beret Top Piece determined in Part A.  The inner dimension of the Beret Side Flap (red line in drawing) is based on your head size measurement.

The width of the Beret Side Flap (yellow line h) is 7.5 cm (3 inches) for a beret that will fit a 54 cm (21 ½  inches) head size measurement. 

In case you have a head size measurement of 60 cm ( 23 5/8 inches), you have to calculate the width of the Beret Side Flap like this: 
7.5  x   60  /  54  =  width of 8.3 cm ( ~ 3 2/8 inches)

Once you have determined these dimensions, construct the pattern for the Beret Side Flap.  Please make certain that extra material is available for the seams around both pieces (seam allowances). All measurements given here are without seam allowances! 

The front seam of the Beret Side Flap (where it joins together with the Beret Top Piece to form the complete beret) must be a double-welted seam that overlaps (similar to jeans). 

The seam allowance (about ½ inch) of the lower portion of the Beret Side Flap must be turned back, both to form the double-welted seam at the front and to stiffen up the lower brim of the beret. 

This will be a bit difficult and it is recommended that cloth glue be used first, until final adjustment can be made correctly.

Picture Source: U.S. Army Airforces Regulation 40-9, 14 February 1944,  Exhibit 3, Page 7
Beret with insignia of a WASP Trainee
It is always best to make a dummy beret from cheap fabric first and see how it fits. ....... 

Once the Beret Top Piece is joined with the Beret Side Flap, you can complete your beret with a dark silk lining and by sewing a thin (one-inch) leather sweatband inside the lower brim of the completed beret.

Good Luck, Ladies! 

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