Blood type calculator
Predict a baby's possible blood types from both parents, or find what blood types one parent could be given the child's blood type and the other parent's.
Interested in inheritance patterns?
Build a full family pedigree and check which inheritance patterns are consistent with your data.
Open the pedigree toolHow the blood type calculator works
The blood type calculator takes both parents' ABO and Rh blood types and shows every possible blood type their child could have, along with the probability for each outcome.
ABO blood type genetics
Blood type follows Mendelian inheritance with a wrinkle. The ABO system has three alleles: A, B, and O. A and B are codominant, meaning neither masks the other. Inherit one of each and you get type AB blood, with both antigens on your red blood cells. O is recessive to both A and B, so a person with type A blood could have genotype AO or AA. The same goes for type B: BB or BO. Type O always means two O alleles.
Codominance is what makes blood type different from a standard dominant-or-recessive trait. A type A parent who is genotype AO has a 50% chance of passing on the O allele to each child. Two type A parents who are both AO can produce a type O child if each contributes their O allele. The same logic applies to type B parents carrying a BO genotype.
Rh factor inheritance
Rh factor is a separate gene with two alleles. Rh-positive (D) is dominant over Rh-negative (d). An Rh-positive person might be homozygous DD or heterozygous Dd. Two Rh-positive parents who both carry the Dd genotype have a 25% chance of an Rh-negative (dd) child with each pregnancy. Two Rh-negative parents, both dd, will always have Rh-negative children because neither carries a D allele.
The blood type calculator combines both systems. It works through every genotype pairing the parents could produce and tallies the resulting phenotype probabilities. For example, a type A (AO) parent crossed with a type B (BO) parent can produce any ABO type, A, B, AB, or O, each at 25%.
What the results tell you
Type O red blood cells lack both A and B surface antigens, which is why O is called the universal donor: a recipient's immune system has nothing to react against. Hospitals still crossmatch before transfusing, but type O causes the fewest immediate immune reactions. Type AB is the rarest ABO type in most populations (roughly 4% in the US) because it requires an A allele from one parent and a B allele from the other. Two type O parents can only pass on O alleles, so under standard Mendelian rules they cannot have a child with type A or B blood.
The probabilities shown apply to each pregnancy independently. A 25% chance of type O means each child independently has that probability; it does not mean exactly one in four children will be type O.
Limitations
The Bombay phenotype is a rare exception the calculator does not model. A mutation in the H gene prevents A and B antigens from reaching the red blood cell surface. Someone who genetically carries A or B alleles can test as type O because those antigens never appear. This condition is very uncommon, but it means standard ABO predictions will not hold in those cases.
Frequently asked questions
What blood type will my baby have?
Enter both parents' blood types (ABO and Rh) into the calculator. It will list every possible blood type for the child along with the probability of each. The result shows all genotypes the parents could produce, not a single guaranteed type.
Can two O parents have an A child?
Under standard Mendelian inheritance, no. Two type O parents each carry two O alleles, so they can only pass on O. A child who tests as type A would need at least one A allele from a parent. The known exception is the Bombay phenotype, where a parent genetically carries A or B alleles but tests as type O due to an H gene mutation.
Can two Rh-positive parents have an Rh-negative child?
Yes. If both parents are heterozygous (Dd), each has a 50% chance of passing on the d allele. There is a 25% chance the child inherits dd and is Rh-negative. If either parent is homozygous DD, all children will be Rh-positive.
What is the rarest blood type?
AB-negative is the rarest blood type in most populations, occurring in roughly 1% of people in the US. AB is already the least common ABO type (about 4%), and Rh-negative is less common than Rh-positive, so AB-negative combines two lower-frequency traits.
What is the difference between ABO and Rh blood types?
ABO and Rh are two separate blood group systems controlled by different genes. ABO determines whether your red blood cells carry A antigens, B antigens, both, or neither. Rh determines whether they carry the D antigen. Your full blood type includes both, such as "A-positive" or "O-negative."