Difference between 延ばす and 伸ばす

Question

As far as I can tell, they both mean something like stretch or extend. When would you use each one?

Answer 1

Try converting のばす into Kanji and your IME should have a pop up that tells you the difference.

Mine (Google IME) says:

伸ばす
1.長くする。「爪を伸ばす」「路線を伸ばす」
2.まっすぐにする。「背筋を伸ばす」「しわを伸ばす」
3.勢いなどを増やす。「勢力を増やす」

延ばす
1.時間を遅らせる。延期する。「締め切りを延ばす」「出発を延ばす」
2.〈「伸ばす」とも〉既存のものに付け加えて長くする。「路線を延ばす」

So...

伸ばす for
1. Making something longer. Ex. growing your nails, extending a route.
2. Making something straight. Ex. straightening your back or smoothing a wrinkle.
3. Increasing things like power/momentum. Ex. extending one's influence or power.

延ばす for
1. Making the time of something later. Postponing something. Ex. extending a deadline or departure time.
2. Adding to something already existing such as a route to make it longer (伸ばす also OK for this).

Answer 2

延ばす is mainly used for extending "time or duration"

伸ばす is mainly used for extending "something tangible"