How does the Blood Type Calculator work?

Each parent passes one ABO allele (A, B, or O) and one Rh allele (+ or −) to their child. The calculator works through every possible inherited combination and tallies how often each resulting blood type appears. The output is the set of possible child blood types with the probability of each.

Can two O parents have a child with type A or B?

Almost never. Type O parents normally carry only the O allele, so they can only have type O children. If a type A or B child is born to two parents reported as type O, one of the reported blood types may be wrong. Or one parent may have a very rare type, such as the Bombay phenotype, which tests as O but can still pass on an A or B gene. Specialist testing can check this.

Can two Rh-negative parents have an Rh-positive child?

Almost never. The Rh-positive allele is dominant — for a child to be Rh-positive, at least one parent must carry the + allele. Two truly Rh− parents have Rh− children. But some people carry a rare, weak form of the Rh gene and can test Rh− on some tests. If results don't fit, a specialist can check.

Why does an A parent and an O parent produce both A and O kids?

Because a type A parent can carry either two A alleles (genotype AA) or one A and one O (genotype AO). Without prior testing, we assume an equal chance of each. An AO parent crossed with an OO parent gives a 50/50 chance of A versus O children.

Is this calculator a paternity test?

No. The Blood Type Calculator is a genetics estimator, not a paternity tool. The American Red Cross says ABO blood typing can't prove or disprove who a parent is. Rare gene variants and testing problems cause exceptions. For paternity questions, only DNA testing through an accredited lab provides reliable answers.

What about the rare Bombay phenotype?

Bombay phenotype (hh) is an extremely rare blood type in which someone can carry A or B genes but can't make the A or B markers on their red cells, so they test as O. Our calculator assumes standard ABO inheritance and does not model Bombay. If you have an unusual family pattern, a specialist haematology test can resolve it.

Blood Type Calculator

See every possible blood type your child could have based on both parents’ ABO group and Rh factor. The Blood Type Calculator works through the genetics for you and shows the probability of each outcome.

Parent 1 blood type

Parent 2 blood type

Possible child blood types

AB+53%
A+18%
B+18%
O+5.9%
AB-3.5%
A-1.2%
B-1.2%
O-0.39%

Probabilities assume a parent of phenotype A, B or Rh+ has an equal chance of being homozygous or heterozygous. Real outcomes can shift if a parent’s exact genotype is known from prior tests. Rare gene variants and lab errors mean a type shown here as impossible can still happen, so blood type can’t prove or disprove who a parent is.

How blood type inheritance works

You inherit two blood-type signals from your parents: one for the ABO group (A, B, AB, or O) and one for the Rh factor (positive or negative). You get one copy of each from each parent, and together they decide your blood type.

The ABO system has three alleles: A, B, and O. A and B are codominant — if you inherit both, you express both (type AB). O is recessive — it only shows up when you inherit two copies. The four phenotypes are:

Type A = genotype AA or AO
Type B = genotype BB or BO
Type AB = genotype AB
Type O = genotype OO

The Rh factor is simpler: Rh+ is dominant, Rh- is recessive. Rh+ × Rh+ usually gives Rh+ kids (with a 25% chance of Rh- if both parents secretly carry the negative allele). Rh- × Rh- almost always gives Rh- kids.

Why blood types matter beyond curiosity

Pregnancy: if you are Rh-negative and your baby is Rh-positive, you may need a RhoGAM injection to prevent immune complications in future pregnancies. Knowing both parents’ Rh status helps your provider plan.
Blood donation and transfusions: type O- is the universal donor; type AB+ is the universal recipient. Knowing your type is useful if you donate or need a transfusion.
Family genetics: blood type can’t prove or disprove who a biological parent is. Rare gene variants and lab errors cause exceptions, so only a DNA test can answer that.

A quick example

Say one parent is A+ and the other is B-. The A parent could be genotype AA or AO; the B parent could be BB or BO. Crossing both possibilities:

ABO outcomes: if both parents carry a hidden O (AO and BO), A, B, AB and O are each 25%. If neither does (AA and BB), every child is AB.
Rh outcomes: 50% Rh+ and 50% Rh- if the A+ parent carries a hidden Rh- gene. If not, every child is Rh+.

The Blood Type Calculator above does this for any combination of parents. It can’t know which hidden genes each parent carries, so it averages the options and lists every possible child blood type (up to eight), ranked by how likely each one is.

Frequently asked questions

How does the Blood Type Calculator work?: Each parent passes one ABO allele (A, B, or O) and one Rh allele (+ or −) to their child. The calculator works through every possible inherited combination and tallies how often each resulting blood type appears. The output is the set of possible child blood types with the probability of each.
Can two O parents have a child with type A or B?: Almost never. Type O parents normally carry only the O allele, so they can only have type O children. If a type A or B child is born to two parents reported as type O, one of the reported blood types may be wrong. Or one parent may have a very rare type, such as the Bombay phenotype, which tests as O but can still pass on an A or B gene. Specialist testing can check this.
Can two Rh-negative parents have an Rh-positive child?: Almost never. The Rh-positive allele is dominant — for a child to be Rh-positive, at least one parent must carry the + allele. Two truly Rh− parents have Rh− children. But some people carry a rare, weak form of the Rh gene and can test Rh− on some tests. If results don't fit, a specialist can check.
Why does an A parent and an O parent produce both A and O kids?: Because a type A parent can carry either two A alleles (genotype AA) or one A and one O (genotype AO). Without prior testing, we assume an equal chance of each. An AO parent crossed with an OO parent gives a 50/50 chance of A versus O children.
Is this calculator a paternity test?: No. The Blood Type Calculator is a genetics estimator, not a paternity tool. The American Red Cross says ABO blood typing can't prove or disprove who a parent is. Rare gene variants and testing problems cause exceptions. For paternity questions, only DNA testing through an accredited lab provides reliable answers.
What about the rare Bombay phenotype?: Bombay phenotype (hh) is an extremely rare blood type in which someone can carry A or B genes but can't make the A or B markers on their red cells, so they test as O. Our calculator assumes standard ABO inheritance and does not model Bombay. If you have an unusual family pattern, a specialist haematology test can resolve it.

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Sources

Checked against the following public guidance on October 2, 2026. Advice can differ between countries; your doctor, nurse or midwife can apply it to you.