In Undertale , Sans’s bone attacks are generally not about one fixed “damage per bone” number, because his attacks often hit repeatedly and his real threat comes from rapid chip damage plus KR, not just the bones themselves. A Reddit discussion also notes that regular bones do normal damage, half-broken bones do about 2x, and spiked bones can be instant-kill in that specific fan context, but that is not the same as a canonical main-game damage chart.

What that means in practice

  • Normal bones: low individual damage, but they can add up fast.
  • Repeated hits: Sans’s attacks can stack damage very quickly over a turn.
  • KR effect: in the Genocide fight, many hits keep draining HP even after the immediate attack.

Simple answer

If you mean “how much damage does one bone do” , there isn’t a single official universal number given in the source I found. If you mean Sans’s bone attacks overall , they can be deadly because they land many hits and are paired with KR, which makes the damage snowball.

TL;DR

Bone attacks in Undertale usually do small per-hit damage , but Sans becomes dangerous because he hits many times and KR keeps draining you afterward.