🩸Blood Type Calculator

About this tool

Choose each parent's ABO type and Rh factor. The eight blood types light up if they're possible for a child of those parents and are crossed out if they aren't. Open the details to see the exact probabilities for each combination of genotypes the parents could carry.

How blood type is inherited

Your ABO blood type depends on one gene with three common alleles. A and B are co-dominant and O is recessive, so type A blood can come from AA or AO, type B from BB or BO, type AB only from AB, and type O only from OO. Each parent passes one allele to the child at random. The Rh factor is a separate gene: positive (D) is dominant, so Rh-positive people are DD or Dd and Rh-negative people are dd. Because you usually can't tell someone's hidden allele from their blood type, the calculator considers every genotype each parent could have and shows all the types their child could have. Some combinations are surprising: an A+ parent and a B− parent can have a child of any of the eight blood types, because each might carry O and the A+ parent might carry d. Two O− parents can only have O− children. Two AB parents can't have an O child. Blood type matters medically for transfusions and in pregnancy, where an Rh-negative mother carrying an Rh-positive baby needs monitoring. Honest limits: rare variants such as the Bombay phenotype and cis-AB break these rules, and lab or recording errors happen, so an unexpected blood type is never proof of anything about parentage; only a DNA test can establish that. Not medical advice. The Punnett square calculator shows the underlying genetics.

Frequently asked questions