Why cells(1,1) = 500 * 100 causes overflow but 50000*100 doesn't?

Consider:

Sub add()
    'This works:
    Cells(1, 1) = CLng(500) * 100
    'as does this:
    Cells(2, 2) = 50000 * 100
End Sub

Evidently VBA was picking a default type of Integer for the first expression because that type is large enough to hold the literals on the right hand side. 50000 is too big for an Integer so it interprets it as a Long. CLng explicitly triggers a promotion to Long.

The maximim value for a Integer is 32767 and since 50000 is more it is cast as a Long type. Thus the results fits in Long just fine. But in the first case everything is done with Integer types and it overflows.

(Integer=500) * (Integer=100) = (Integer=50000)  'Overflow
(Long=50000) * (Integer=100) = (Long=5000000)    'Ok

This is because of how VBA evaluates mathematical expressions. The return type of expression will be the type of first operand in the expression or its nearest numeric type equivalent. However, the final result may not fit in that return type and throw overflow error.

When you do 500*100 , return type is integer. While when you do 50000*100 the return type of this expression is Long.

To avoid overflow error you can do an explicit cast to let it know your intentions

CLng(500) * 100