Calculate Size on Photo

Calculator for the true size of objects in a photo.
It is possible to calculate the size of something visible in a photograph. However, this requires knowing a few values. The relationship between the actual size, the size in the photo, and the distance from which the photo was taken is obvious. Other values ​​needed for the calculation include the overall size of the photo in the same direction (width or height), the camera's sensor size, and the focal length used in the shot. The sensor size can be found in the product description, while the focal length for digital photos can be found in the EXIF ​​data of the image file. The sensor size of full-frame cameras is 36 millimeters, while smartphones have sensor widths ranging from approximately 4.8 millimeterss to 10.2 millimeterss. If two different focal lengths are specified, the physical focal length is the relevant one, not the 35mm equivalent.

Since the size of sensors in smartphones is often specified as an optical format in the form 1/x ″ (not a true inch diameter), here is a converter that approximates the actual sensor size in millimeterss for calculation purposes:

Sensor size 1 / ″

 

The following calculators will only provide correct results if the photos have not been cropped. Results are rounded to one decimal place. Please enter all but one value to calculate this.

Calculation from length

Object size in photo:		millimeters, mmcentimeters, cminches, in, ″
Photo size:		millimeters, mmcentimeters, cminches, in, ″
Sensor size:		millimeters, mmcentimeters, cminches, in, ″
Focal length:		millimeters, mmcentimeters, cminches, in, ″
Distance to object:		millimeters, mmcentimeters, cmdezimeters, dmmeters, mkilometers, kminches, in, ″feet, ft, ′yards, ydmiles, mi
Real object size:		millimeters, mmcentimeters, cmdezimeters, dmmeters, mkilometers, kminches, in, ″feet, ft, ′yards, ydmiles, mi

The formula for the calculation is:
Real object size = Object size in photo * (Distance to object - Focal length) * Sensor size / (Focal length * Photo size)

All quantities ​​in the formula are calculated in millimeters.

Example: A two-centimeter object in an 18-centimeter photograph, taken with a camera with a full-frame sensor (36 mm) and a 100-millimeter focal length, was photographed from a distance of 5 meters. Then in reality, the object is just under 20 centimeters in size.

Calculation from pixels

Object pixels:
Photo pixels:
Sensor size:		millimeters, mmcentimeters, cminches, in, ″
Focal length:		millimeters, mmcentimeters, cminches, in, ″
Distance to object:		millimeters, mmcentimeters, cmdezimeters, dmmeters, mkilometers, kminches, in, ″feet, ft, ′yards, ydmiles, mi
Real object size:		millimeters, mmcentimeters, cmdezimeters, dmmeters, mkilometers, kminches, in, ″feet, ft, ′yards, ydmiles, mi

The formula for the calculation is:
Real object size = Object pixels * (Distance to object - Focal length) * Sensor size / (Focal length * Photo pixels)

Pixels have no unit. All other quantities in the formula are calculated in millimeters.

Example: An object has a width of 500 pixels in a 6000-pixel wide photograph. The sensor size is 1/2.5 inch, which corresponds to 7.2 millimeters. With a focal length of 6 millimeters and a distance of 10 meters, the object is actually almost exactly one meter wide.

Last updated on 02/14/2026. Author: Jürgen Kummer

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