Online Relationship Calculator for Autosomal DNA

Have you taken a DNA test? Have you looked at the results and wondered what they mean?  What is a centimorgan? Why do you and relatives share them? Is a DNA match your great aunt or your evil twin brother? The calculator chart at the end of this article will help you narrow down the possibilities. Is a DNA match your 2nd cousin, or your 4th cousin twice removed? What does twice removed even mean? The calculator chart will help you visualize the relationship. Simply type in the shared centimorgans (cM) of a DNA match, and the calculator will show you how you're likely related. Here's a picture of the chart with no relationships highlighted. To use this solely as a reference chart, you can download it as a pdf at this link. But to use this chart as a dynamically-highlighted calculator, keep reading...

The calculator is based on the Shared cM Project data, version 3.0 dated Aug 2017. This data is a compilation of data from volunteers who input their shared centimorgans indicated by their DNA test for known relationships. For example, if you and your 1st cousin get DNA tests, the tests will show common shared DNA in centimorgans. The higher the centimorgans, the more closely related to you they are. Here's an example of the DNA relationship chart highlighting likely relationships for 900 centimorgans of shared DNA:

When you take a DNA test like the ones at AncestryDNA.com, 23andMe.com, FamilyTreeDNA.com, or MyHeritage.com, it is common for you to see a list of DNA matches resulting from your test. The DNA matches are other individuals who have taken a DNA test and that have been found to have matches indicating they are a relative. These results are usually ranked by the amount of shared DNA, measured in centimorgans. The higher the shared centimorgans, the closer the relationship.

Here's a hint: If you buy your DNA test from AncestryDNA.com, FamilyTreeDNA.com, or MyHeritage.com, you can also establish accounts at the other two sites and upload your DNA results for free! There is good reason to do so, because you will find different matches on different sites, as some other people only have their DNA results on one of the sites. Follow this link for a full treatment on which company's DNA tests you can transfer to other company's websites.

The cM Relationship Calculator

The calculator is a Google Sheets document. You can use it right on this webpage (it is embedded at the end of this post), or access it directly from this link, where you can save a copy for your own use (at least that is what Google implies, but the unintuitive Google Sheets combination of Protect Sheet settings and Sharing settings and Make Public settings leaves me wondering if Google Sheets can handle this. These settings are all individual and don't make reference to one another, leaving me wondering if Google just threw this app together over lunch but didn't really think it through. I mean, why do you need separate Sharing and Make Public settings? Aren't they essentially all sharing options?) Here's an example of the chart highlighting likely relationships that share 122 centimorgans:

Here's how the calculator works: You input your shared cM in the yellow cell near the top, and the chart will highlight all the possible relationships.

Note that this calculator only focuses on full-sibling relationships. Though the cM values exist for half-sibling relationships, the values change when an additional half-sibling relationship occurs in your grandparent or somewhere else in the line between you and the DNA matched relative.

The Online Calculator: 

Note: I don't know if Google will allow you to enter shared cM directly into this embedded chart. Google Docs Publishing and Sharing is all very confusing, as the help information is very sparsely written and doesn't address my purpose of sharing the spreadsheet with only a single cell being editable so you can enter your cM values, but not allowing any edits whatsoever to the spreadsheet (such as adding rows). So in the near term, I have set the sharing settings as view only. If anyone has any hints on how to accomplish the sharing of a Google Sheets document where users can edit nothing but designated cells, please leave them in comments. My preliminary tests showed that other users could do all kinds of unwanted things, such as adding rows, adding sheets, editing content, and so forth. In the meantime, you can access the Google Sheets doc from this link (which is hopefully fully protected, I honestly don't know, I tried).