Shearwaters are much larger.
Other storm petrels are very similar. Black, Ashy, and Least storm-petrels lack the white rump. Band-rumped and Leach's storm petrels are larger with different flight styles and their feet don't protrude beyond the end of their tails. Leach's has a forked tail and a divided or absent rump patch. Band-rumped has a thinner rump patch.