godan verb (-ru), intransitive verb
pile up; accumulate; be deposited
Intransitive use: something gathers into a pile, layer, or accumulated amount. Very common with 雪が積もる, ほこりが積もる, and also abstract things such as 不満が積もる.
昨夜から雪がかなり積もった。
A lot of snow has piled up since last night.
机の上にほこりが積もっている
Dust has accumulated on the desk.
小さな不満が積もって、大きな問題になった。
Small frustrations piled up and became a big problem.
godan verb (-ru), transitive verb
estimate; reckon
Transitive use meaning to estimate an amount or value. In modern everyday Japanese, 見積もる is usually the clearer and more common verb; 積もる alone can sound somewhat limited or old-fashioned in this sense.
See also: 見積もる
費用を十万円と積もって計画を立てた。
We made the plan estimating the cost at 100,000 yen.