Step 1: Capture screen or window by pressing PrtScn or Alt+PrtScn

Step 2: Paste in this page by pressing Ctrl+V

Step 3: Click on 2 pixels to get length

Step 1: Capture screen or window by pressing ⌘ Command+⇧ Shift+3 or ⌘ Command+⇧ Shift+4

Step 2: Paste in this page by pressing ⌘ Command+V

Step 3: Click on 2 pixels to get length

Step 1: Capture screen by pressing Home ▢+Hold / Power buttons

Step 2: Paste in this page by long tap and pressing Paste button.

Step 3: Click on 2 pixels to get length

Step 1: Capture screen by pressing Volume up + Power buttons

Step 2: Paste in this page by pressing Paste button.

Step 3: Click on 2 pixels to get length