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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+42%
  • AB-14%
  • A+14%
  • B+14%
  • A-4.7%
  • B-4.7%
  • O+4.7%
  • O-1.6%

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.

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). One signal comes 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- 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 rules can sometimes rule out a biological parent in a specific case — but never confirm one. Always pair this with proper testing if it matters.

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: roughly 25% A, 25% B, 25% AB, 25% O (when each parent could carry an O allele)
  • Rh outcomes: 50% Rh+, 50% Rh-

The Blood Type Calculator above does this for any combination of parents and shows the eight possible child blood types 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?
No. Type O parents only carry the O allele, so they can only produce type O children. If a type A or B child appears in a family with two reported type O parents, the reported blood types may be inaccurate, or there is a rare cis-AB or Bombay-phenotype edge case that requires specialist testing.
Can two Rh-negative parents have an Rh-positive child?
No. The Rh-positive allele is dominant — for a child to be Rh-positive, at least one parent must carry the + allele. Two Rh− parents always produce Rh− children.
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. Blood-type combinations can rule out paternity in some cases but never confirm it on their own. 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 genetically carries A or B alleles but cannot express them, 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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