It is not permissible to estimate anything other than fruits
Question

                Is it permissible to estimate trade goods when it is not possible or is too difficult for the businessman to count them accurately?.
Praise be to Allah.
    What is meant by estimating is when experts look at the date 
    palms – for example – after it is clear that the unripe dates are in good 
    condition, and they see how many dates they are likely to produce, and the 
    zakaah is given on that basis. This is a kind of speculation and conjecture 
    without precisely counting the wealth. The reason why this is prescribed in 
    the case of fruits is that the owner of the fruit may need to eat some of it 
    or give it to others or sell it when it is still unripe, before it ripens. 
    In that case it is not possible to define zakaah precisely, so Islam allows 
    estimating it so as to avoid hardship and difficulty. 

    There is no report that sharee’ah allows estimating with 
    regard to any kind of wealth other than fruits, because fruits are the type 
    of wealth which it is difficult or impossible to count precisely. But with 
    other types of wealth that are subject to zakaah – such as trade goods – it 
    is possible to count them precisely. 

    Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) was 
    asked about estimating trade goods if it is impossible or too difficult for 
    the businessman to count them. He replied: 

    It is not permissible to estimate them, because estimating 
    was mentioned only with regard to fruits, and some scholars also added 
    crops. As for other types of wealth, it is not possible to estimate them 
    because they are of many types. But the person should try hard as hard as he 
    can and be cautious. If he counts this item as being one hundred and twenty, 
    he should pay zakaah on one hundred and twenty, so that he will have done 
    his duty. End quote. 

    Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen 
    (18/232).
And Allaah knows best.