# How to calculate area in a binary image?

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Asmalina Mohamed Saat on 17 Sep 2020
Commented: Image Analyst on 13 Oct 2020
I have two questions to ask:
1) How to calculate area at the white region in binary image?
2) How to convert the answer of area unit that get from matlab into cm^2 or mm^2 (SI UNIT)?
here I attach the original image and binary image (this image is marine growth on the fiberglass plate) Deepak Meena on 22 Sep 2020
Hi Asmalina,
1) To find the area in pixel size we just need to find the number of white pixel which would be
area = bwarea(bw);%bw is your binary threshold image
The unit would be pixel
2) To find the area in real world units , you need to find out how big one pixel is in appropraite units, if you working with DICOM image refer to this answer.
Another way is we can find the percentage of white area so if you have the plate area , you can find out the actual white area then.
to find out the percentage area:
percentageWhite = nnz(bw) / numel(bw);
Asmalina Mohamed Saat on 1 Oct 2020
Edited: Asmalina Mohamed Saat on 1 Oct 2020
i have another questions;
1)can I used this way to get the distance or area in real world units (SI unit)?
calibrationFactor = length of the image in cm/length of the image in pixel;
distanceInCm = distanceInPixels * calibrationFactor;
areaInSquareCm = areaInPixels * calibrationFactor ^ 2
2) what is distanceInPixels, is it same value with length of the image in pixel?
3) is it areaInPixels is the value i get after i used this formula
(area = bwarea(bw);%bw is your binary threshold image)?

Image Analyst on 22 Sep 2020
See my attached spatial calibration demo.
Image Analyst on 13 Oct 2020
The number of white pixels is, of course, unitless. It's just simply a count, like 235 or 14983 or whatever. It can have an area in pixels or square cm but the count is obviously just a number and has no units at all.

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