Calculate display PPI (pixels per inch) given resolution, aspect ratio and diagonal size.

display_ppi.py

# Calculate display PPI (pixels per inch) given resolution, aspect ratio and diagonal size.
# Luke McCarthy 2013-08-02

import math

def display_ppi(resolution, aspect_ratio, size_inches):
    res_h, res_v = resolution
    aspect_h, aspect_v = aspect_ratio
    theta = math.atan(float(aspect_v) / aspect_h)
    width = math.cos(theta) * size_inches
    height = math.sin(theta) * size_inches
    return (res_h / width, res_v / height)

if __name__ == '__main__':
    resolution = (1920, 1080)
    aspect_ratio = (16, 9)
    monitor_sizes = [5, 7, 10, 18.5, 21.5, 24, 27, 32, 40, 50]

    for size in monitor_sizes:
        print '%.1f" @ %dx%d -> %.f PPI' % (
            size, resolution[0], resolution[1], display_ppi(resolution, aspect_ratio, size)[0])