Blood type calculator
Pick both parents’ blood types to see every blood type their baby could have, with the odds. It runs in your browser, instantly.
- O positive0% – 25%Typical ≈ 18.1%
- O negative0% – 6.3%Typical ≈ 1.6%
- A positive0% – 50%Typical ≈ 24.9%
- A negative0% – 12.5%Typical ≈ 2.2%
- B positive0% – 50%Typical ≈ 20.5%
- B negative0% – 12.5%Typical ≈ 1.8%
- AB positive18.8% – 100%Typical ≈ 28.3%
- AB negative0% – 25%Typical ≈ 2.5%
typical estimate highest possible, depending on hidden genes
How the blood type calculator works
Every person carries two copies of the ABO gene, one from each parent. There are three common versions (alleles): A, B and O. A and B are co-dominant, so someone with one of each is type AB. O is recessive: you are type O only if both copies are O. That is why two type A parents can have a type O child if both quietly carry an O.
The Rh factor works the same way with one dominant allele (D, Rh positive) and one recessive (d, Rh negative). An Rh-positive parent may be DD or Dd; you cannot tell from the blood type alone.
The calculator runs every combination of genotypes each parent could have and shows the lowest and highest chance for each child type. Want to see the four boxes behind a single cross? Try the blood type Punnett square, or scan the full parent–child blood type chart.
When the answer matters
- Pregnancy: if the mother is Rh negative and the baby could be Rh positive, she will usually be offered anti-D (RhoGAM). See the Rh factor guide.
- Curiosity and family trees: a “surprising” child blood type is almost always explained by hidden recessive alleles.
- Donation: once you know a type, the blood type compatibility chart shows who can give to whom.
Frequently asked questions
Can two O parents have an A or B child?
Under normal ABO inheritance, no. Two type O parents each carry two O alleles, so every child is type O. Rare exceptions exist (the cis-AB allele, Bombay phenotype, lab error or a very weak A or B subgroup), which is why blood type alone is never used to prove or disprove parentage.
Can two A+ parents have an O- baby?
Yes. If both parents carry a hidden O allele (genotype AO) and a hidden Rh-negative allele (Dd), each child has a 1 in 4 chance of being O and a 1 in 4 chance of being Rh negative, so 1 in 16 (6.25%) of O negative.
Why does the calculator show a range instead of one percentage?
Your blood type (phenotype) does not reveal which alleles you carry. Someone who is type A could be AA or AO; someone Rh positive could be DD or Dd. The range covers every genotype you could have. If you know your genotype, for example because one of your parents is type O, choose it under "I know my genotype" to get exact odds.
What does "typical" mean in the results?
It weights each possible genotype by how common it is, using approximate allele frequencies for a European-ancestry population (Hardy–Weinberg). It is a sensible best guess when you know nothing else, but real odds depend on your family.
Can blood type prove who the father is?
Blood type can sometimes show a man could not be the biological father under normal inheritance, but it cannot prove that he is. Many people share each blood type. Parentage is established with DNA testing.
Does the child get its blood type from the mother or the father?
From both. A child receives one ABO allele and one Rh allele from each parent. A and B are co-dominant, O is recessive, and Rh positive (D) is dominant over Rh negative (d).