One way is as follows:

1. Convert the wingspan of the full size airplane to inches.

2. Divide the number from step 1 by the wingspan ( in inches ) of your model. This number is the scale factor at which the model was built.

3. Make this number a fraction by placing it below 1.

EX. Full size P-47 with 40 foot wingspan. Model has a 92 inch wingspan.

40X12= 480inches; 480/92= 5.29

Scale factor is 1/5.29. You can round off to 1/5 scale model pilot. Hope this helps!