Convert pixels in lat-lon to kilometer square

How can I change the pixels (in lat longs) of some monthly .nc files to kilometere square for every month. Spatial resolution of the data is 0.5 x 0.625.

[KSSV]

Read about deg2km

[Joydeb Saha]

ncFiles = dir('E:\COT Work\New\COTdata\cot2\*.nc');
N = length(ncFiles) ; 
Q = zeros(N,1) ; 
for i = 1:N
    ncFile = fullfile(ncFiles(i).folder,ncFiles(i).name); 
    data_Wwind=netcdf(ncFile); 
    cot=data_Wwind{'M2TMNXRAD_5_12_4_TAUHGH'}(:);
    th3 = cot > 3;
    B = double(th3);
    Q(i)=sum(B(:) == 1);
end

I writing the code here. This counts the COT values which are above 3. Then it is counts the total number of pixels which are above 3 for for every data file. Basically, I want the cot values which are above pixel 3, and convert those pixel (sum up) to area (in km square). Therefore Q shall give the monthly area values of COT which are above threshold 3. May be a little modification in the present code is needed.