How to tell BB size on my bmx bike?

I recently bought a mirraco bmx bike from a friend, and it has sealed bearings in it. They are really broken and I need a new pair, but I'm unsure of the size. Does anyone know how to tell what size bb I need?
 
it all depends there is a few diffrent types of bb theres mid,us ect.
mid only comes in 19mm and 21mm and it most likley a mid bb
 
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