# How to obtain a subject's measurement from image [duplicate]

I am trying measure real object height from an image. I'm using the below formula

``````real height = (object px height * pixel size * distance)/(focal length)
``````

my camera is Canon_EOS_550D, my values are below

sensor size=22.3 x 14.9 mm focal length=30mm distance to object =1000mm pixel height =1201 I applied values in this equation but I cannot measure object, can you please help me

• To calculate the pixel size you also need the image pixel height. – Guffa Jun 7 '14 at 9:43
• thanks for your reply ,my object pixel height =802 i converted to mm =212.2 mm then( 16mm(sensor size aprox)*212.2(mm)*1000(mm,distance)/30mm(focal length) can you please advice me about my mistake – Arun Jun 7 '14 at 9:58
• Check this question photo.stackexchange.com/questions/23414/… – Hugo Jun 7 '14 at 10:02
• Thanks for reply, one doubt i checked sensor size of Canon_EOS_550D find out 22.3 x 14.9 mm,how can i use this value in equation,eg 22.3*14.9 or? – Arun Jun 7 '14 at 10:12
• below equation also i tried but i did not get actual measurement (2*distance*(sensor size / focal length)) / number of pixels (2*1000(mm)*(22.3*14.9(mm))/30mm))/802 can you please help me – Arun Jun 7 '14 at 10:49

## 1 Answer

The correct formula is this:

"Object Height [units]" = "Distance to Object [units]" X "Object Height [pixels]"/"Image Height [pixels]" X "Sensor Height [mm]"/"Focal Length [mm]"

In your case, you are missing the image height in pixels: Object Height mm = 1000mm X 1201px/IMAGEpx X 14.9mm/30mm