The Are Your Parents Related? (AYPR) tool at GEDmatch compares your DNA against itself, checking for runs of homozygosity (ROH). These are significantly long segments (>= 7 cMs) of DNA that match on both (paternal and maternal) chromosome copies. You can also get total cMs from ROHs using David Pike's tool.
cM = centiMorgan; 1C1R = 1st cousin, once removed.
Please don't multiply your cM value by four before entering it into the predictor. That's just a trick to be able to use a traditional predictor, but it's much better to use a predictor that's designed for ROH scenarios (which is what you're doing if you're on this page).
Averages and ranges for the same relationships can be found here.
The methodology for this tool is the same as for other relationship prediction tools at this site. For more information about the science of AYPR predictions, please click here. For information about why this automatic tool is better than more difficult methods, or to see how you could get even more out of the AYPR tool, click here.
The probabilities on this page assume no endogamy, however the effects of endogamy will likely be slight compared to some of the highest results possible from the AYPR tool. These probabilities are also unweighted by proportions in the population, although population weights would likely be a great idea for parents who are distant cousins to each other.
These probabilities are only calculated as far back as 6C. The huge advantage of this tool, other than the accuracy of the data and the inclusion of relationship types for which we'll never have good empirical data, is that it treats certain relationship types differently even when the averages are the same, including by separating out some paternal and maternal relationship types. Any of the probabilities shown above are only relative to the other relationships listed, therefore they’re only meaningful in comparison to the other relationships.
There is also a tool that gives possible relationships, without probabilities, at Borland Genetics. It has the additional feature of suggesting whether or not the ROH likely came from endogamy.
This site is home to the most advanced relationship predictors available: