per (...), each (...) when with acounter
一個当たり５円で作ることができた。 We were able to make these for 5 yen per 1 article
一食当たりのカロリーは500kcalです。 Per 1 serving, it has 500 calories.
この車は一台当たり１トンあります。 Per 1 vehicle, this car has 1 ton (of weight)
Not to be confused with With (...) attached to (ごと)に or それぞれ