容積を計算するためには縦と横と深さをかければよい。

。

To calculate the volume, just multiply length, width, and depth.

Literal

Volume [object-を] calculate [for-ためには], length [and-と] width [and-と] depth [object-を] multiply-[if-ば] good.

~ためには ('in order to / for the purpose of') combines ~ために with は for topic emphasis — 'as for how to do X.' ~ばよい ('it's enough to do X / just do X') is a fixed pattern giving minimal sufficient instructions. 掛ける ('multiply,' in this mathematical sense).