Sometimes, you’ll get a match on AncestryDNA, 23andme, FTNDA or another commercial DNA test and not know who this person is, or how they fit in to your tree. Using DNA Painter you can eliminate the relationships that aren’t possible and narrow your list down to a few possibilities and find out what relationship you have based on DNA shared.
Firstly just enter in the cM shared or percentage between you and the person you’ve matched with, then it should automatically highlight the possible relationships.
The tool can be found here: https://dnapainter.com/tools/sharedcm/1000
You can also find out your relationship based on DNA shared manually by visiting this post: https://familytreetips.co.uk/2017/01/24/dna-shared-between-two-people-checklist/