How to Measure Someone's Size on a Photo

Measuring someone's size in a photo can be a useful tool for scientific research or to estimate how tall someone was years ago. Because a photo is entirely self-contained and can magnify objects, the only true way to measure something is to locate a point of reference. A point of reference in this context is simply something you already know the size of. By measuring the difference in scale of that object, you can successfully compute someone's size.

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A point of reference will guide you to other calculations.

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Locate your point of reference. If possible, use a point that is the same distance from the camera as the subject you're trying to measure.

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Measure the size of the object in the physical photograph with a ruler or tape measure. The units you use are up to your preference.

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Divide the known height of the object of reference by the physical size of it on the photo. Write this number down; this the scale multiplier.

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Measure the physical size of the subject with the unknown size.

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Multiply the physical size of the subject by the scale multiplier to produce the actual height.