Blood Type Calculator

How blood type is inherited

Blood type comes from a single gene on chromosome 9, and that gene has three common versions: A, B and O. A and B are dominant over O, and A and B are codominant with each other, which is why a person can carry both and show type AB. Every person inherits two alleles, one from each parent, so six combinations sit behind the four blood types you see on a lab report.

The letters describe antigens on the surface of red blood cells. Type A cells carry the A antigen, type B cells carry the B antigen, type AB cells carry both, and type O cells carry neither. The same gene also sets which antibodies float in the plasma: type A plasma has anti-B, type B plasma has anti-A, type O plasma has both, and type AB plasma has neither. That pairing is why a mismatched transfusion is dangerous, and it is the reason blood banks type every unit twice.

The Rh factor is a separate gene, RHD, with a positive and a negative version. Rh positive is dominant, so a person who tests positive can be DD or Dd, and the two look identical on a lab report even though they pass the trait on differently. The MedlinePlus blood typing page walks through the antisera used in the test.

Because a lab reports the visible type and not the alleles behind it, any calculator has to make an assumption for a parent who is type A, type B or Rh positive. This page states every assumption it makes, and the dropdowns let you replace the assumption when you know more.

What the calculator assumes

Each parent passes one allele at random. The table below shows how the six genotypes map to the four visible types.

For a parent with type A the default is AO, one A allele and one O allele. For type B the default is BO. Those are the common cases in the United States, where the O allele is the most frequent of the three. If you know a parent is AA or BB, switch the allele dropdown and the odds change, sometimes a lot: two AO parents have a 25% chance of a type O child, while one AA parent drops that to zero.

For a parent who tests Rh positive the default is Dd, one positive and one negative allele. That is the cautious choice, because it keeps an Rh negative child on the table. A positive parent who is known to be DD cannot have an Rh negative child, and the note under the results says so when the assumption is in play.

Nothing here replaces a lab test. The calculator answers a probability question, and a lab answers the factual one.

Worked example: an A parent and a B parent

Take a mother with type A (AO) and a father with type B (BO). Each passes one allele at random, so there are four equally likely pairs: A with B, A with O, O with B, and O with O.

Now add the Rh factor. Both parents are positive and both are assumed Dd, so each passes d half the time and the chance of an Rh negative child is 25%. Multiply the two systems together and each of the eight full types lands at 18.8% for the positive versions and 6.3% for the negative versions, which is what the calculator shows for this pair.

Change one detail and the answer moves. If the father is BB instead of BO, no type O child is possible, and the split becomes 50% AB and 50% B. If the mother is AA instead of AO, the type O child disappears from her side too, and every child is A, B or AB.

A second example: a type AB parent and a type O parent. The AB parent passes A or B, and the O parent always passes O, so the children are type A at 50% and type B at 50%. With both parents Rh positive and assumed Dd, the four full types come out at 37.5% for A positive and B positive, and 12.5% for A negative and B negative.

Combinations that cannot happen

Some results are ruled out completely, and that is often the reason people run the numbers in the first place.

Two type O parents can only have type O children, because neither has an A or B allele to pass on. A type AB parent and a type O parent can have A or B children but never AB or O. Two AB parents can have A, B or AB children, and never O.

Two type A parents are the case people ask about most. If both are AO, one child in four is type O. If either parent is AA, no type O child is possible. The same logic runs for two type B parents.

This is also where blood type gets used in paternity questions. A type O mother and a type AB father cannot produce a type O child, so a reported type O would mean the test or the parentage needs a second look. A type that fits proves nothing on its own, which is why courts moved to DNA testing decades ago.

The Rh factor matters more than ABO in pregnancy

ABO mismatch between mother and baby is common and almost always mild. Rh mismatch is the one that gets watched. An Rh negative mother carrying an Rh positive baby can make antibodies against the baby's red cells, and those antibodies cause anemia in the baby during a later pregnancy.

Rh immune globulin is offered at about 28 weeks and again within 72 hours of delivery when the baby is Rh positive. The shot stops the mother's immune system from making antibodies, and it has cut hemolytic disease of the newborn in Rh negative pregnancies to a rare event in countries where it is routine.

About 15% of people in the United States are Rh negative, so this comes up in a large share of pregnancies. If the father is Rh positive and his genotype is unknown, the chance the baby is Rh negative is 25% under the Dd assumption. A father known to be DD makes an Rh negative baby impossible, though the standard monitoring still runs unless a clinician says otherwise.

How common each blood type is in the United States

The American Red Cross blood types page publishes the shares below for the US population. The numbers are rounded, and they shift by ethnic group: type O runs higher among African American and Hispanic donors, which is one reason donor drives ask for a mix of donors.

O positive is the most common type at about 37%, and it is also in steady demand because it is the type most patients can take. O negative, the universal red cell donor, is about 7% of the population and carries the highest emergency demand. AB negative is the rarest of the eight at roughly 1%, which makes it the hardest to stock.

What this calculator cannot tell you

It cannot prove who a child's father is. A blood type can rule a man out when the child's type is impossible for that pairing, and that is the whole of what it can do. Modern parentage testing reads DNA markers, not blood groups, and a legal test goes through an accredited lab with a chain of custody.

It also cannot see subgroups. A2, weak D and the Bombay phenotype all read as ordinary types on a simple test but behave differently in transfusion and in pregnancy, and more than 300 other antigens sit outside the ABO and Rh systems. A hospital that types blood for transfusion screens for the ones that matter for that patient.

Rare types are a separate world. Some combinations appear in a handful of people per million, and donor registries exist to find them. If someone in your family carries a rare type, a standard calculator will not flag it.

And this is not medical advice. If a pregnancy is involved, or a transfusion is being planned, the numbers that matter come from a lab report and the clinician reading it. Use this page to understand the pattern, not to replace the test.