It is not permissible to estimate anything other than fruits

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.