acalculator

What blood type can our child have?

Pick each parent’s ABO blood type and Rh factor. The blood type calculator lists the blood types your child can have, the ones that are not possible, and the chance of each ABO type and of Rh positive or negative.

Your numbers

Possible blood types
A+, A−, B+, B−, AB+, AB−, O+, O−

The child can have blood type A+, A−, B+, B−, AB+, AB−, O+, O−.

Not possible
None
Chance of type A
0% to 50%
Chance of type B
0% to 50%
Chance of type AB
25% to 100%
Chance of type O
0% to 25%
Chance of Rh positive
75% to 100%
Chance of Rh negative
0% to 25%

Possible blood types: A+, A−, B+, B−, AB+, AB−, O+, O−. The child can have blood type A+, A−, B+, B−, AB+, AB−, O+, O−.

How to calculate

Gives the blood types a child can and cannot have from the parents’ ABO and Rh blood types, with the chance of each ABO type and of Rh positive or negative.

Example with the default inputs (Parent 1 blood type A, Parent 1 Rh factor Rh positive (+), Parent 2 blood type B, Parent 2 Rh factor Rh positive (+)): The child can have blood type A+, A−, B+, B−, AB+, AB−, O+, O−.

Method: Each parent passes one ABO allele and one Rh allele, each with chance 1/2; A and B are codominant over O, and RhD positive (D) is dominant over RhD negative (d).

  • A parent typed only by blood type may carry either genotype (A: AA or AO; Rh+: DD or Dd), so each chance is shown as the lowest to the highest over those genotypes.
  • The ABO and Rh genes are inherited independently.
  • Rare variants (such as the Bombay phenotype, cis-AB and weak D) are not counted; a blood test is the only way to know a child’s type.

Machine-readable copies: Markdown, JSON.

Worked examples

Each example is checked against the calculator on every build.

  1. Parent 1 blood type A, Parent 1 Rh factor Rh positive (+), Parent 2 blood type B, Parent 2 Rh factor Rh positive (+) gives Possible blood types A+, A−, B+, B−, AB+, AB−, O+, O−, Not possible None, Chance of type A 0% to 50%, Chance of type AB 25% to 100%, Chance of type O 0% to 25%, Chance of Rh negative 0% to 25%.Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03)
  2. Parent 1 blood type A (genes AO), Parent 1 Rh factor Rh + (genes Dd), Parent 2 blood type B (genes BO), Parent 2 Rh factor Rh + (genes Dd) gives Chance of type A 25%, Chance of type B 25%, Chance of type AB 25%, Chance of type O 25%, Chance of Rh positive 75%, Chance of Rh negative 25%.Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03)
  3. Parent 1 blood type O, Parent 1 Rh factor Rh negative (−), Parent 2 blood type O, Parent 2 Rh factor Rh negative (−) gives Possible blood types O−, Not possible A+, A−, B+, B−, AB+, AB−, O+, Chance of type O 100%, Chance of Rh negative 100%.Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03)
  4. Parent 1 blood type AB, Parent 1 Rh factor Rh positive (+), Parent 2 blood type O, Parent 2 Rh factor Rh negative (−) gives Possible blood types A+, A−, B+, B−, Chance of type A 50%, Chance of type B 50%, Chance of type AB 0%, Chance of type O 0%, Chance of Rh negative 0% to 50%.Source: OpenStax, Anatomy and Physiology 2e, section 28.7 Patterns of Inheritance (blood type A is IᴬIᴬ or Iᴬi, B is IᴮIᴮ or Iᴮi, AB is IᴬIᴮ, O is ii; Iᴬ and Iᴮ are codominant, i is recessive), CC BY 4.0, https://openstax.org/books/anatomy-and-physiology-2e/pages/28-7-patterns-of-inheritance (retrieved 2026-10-03); Flegel WA, The genetics of the Rhesus blood group system, Blood Transfusion 2007;5(2):50–57 (the D-negative phenotype is caused by a deletion of the RHD gene on both chromosomes; a father heterozygous for the deletion gives a 50% chance of a D-negative fetus), https://pmc.ncbi.nlm.nih.gov/articles/PMC2535884/ (retrieved 2026-10-03)

How it works

Genes. ABO has three alleles: A, B and O. A and B are codominant and both are dominant over O. The genotypes give the blood types AA or AO → A, BB or BO → B, AB → AB, OO → O. Rh has two: D (the RhD gene present) is dominant over d (deleted). DD or Dd → Rh positive (+); dd → Rh negative (−).

One cross. Each parent passes one of their two alleles of each gene, each with chance 1/2. So the four pairs (an allele of parent 1 with an allele of parent 2) each have chance 1/4, and the chance of a child type is 1/4 × the number of pairs that give it. The ABO and Rh genes are inherited independently.

Parents typed by blood type only. A parent picked as A may be AA or AO; B may be BB or BO; Rh positive may be DD or Dd. O, AB and Rh negative have one genotype each, and the options with genes (for example “A (genes AO)”) fix one. The calculator works out the chances for every pair of genotypes the parents could have and shows, for each child type, the lowest to the highest chance (one number when they are equal).

  • Possible blood types: every combination of an ABO type and an Rh type whose highest chance is above 0, in the order A+, A−, B+, B−, AB+, AB−, O+, O−.
  • Not possible: the rest, in the same order, or “None”.
  • Chances: written x% or x% to y%, with x and y multiples of 25.

Rules. Rare variants (Bombay phenotype, cis-AB, weak and partial D) are not counted.

Worked examples by hand

A+ and B+ parents, genes not known. ABO: AA × BB gives only AB (100%); AA × BO gives AB and AO, half each; AO × BB gives AB and BO, half each; AO × BO gives AB, AO, BO and OO, a quarter each. So A is 0% to 50%, B 0% to 50%, AB 25% to 100%, O 0% to 25%. Rh: DD × anything gives only +; Dd × Dd gives dd a quarter of the time, so Rh negative is 0% to 25%. Every type can happen.

AO, Dd and BO, Dd. Each ABO type is 25%. Rh: DD, Dd, dD, dd, so 75% positive and 25% negative.

O− and O−. OO × OO gives only OO, and dd × dd only dd: the only possible type is O−.

AB+ and O−. AB × OO gives AO or BO, half each: A 50%, B 50%, AB and O 0%. Rh: DD × dd gives Dd (all +); Dd × dd gives Dd or dd, half each; so Rh negative is 0% to 50%. Possible: A+, A−, B+, B−.

Other questions people ask

How is blood type inherited?

Each parent passes one of their two ABO alleles and one of their two Rh alleles. For ABO, A and B are codominant and both are dominant over O, so AO is type A and AB is type AB. For Rh, the RhD gene is dominant: one copy makes you Rh positive.

Why does the calculator show a range like 0% to 25%?

A person with type A can have the genes AA or AO, and blood tests show only the type. With AA × BB parents every child is AB; with AO × BO each type has a 25% chance. If you know a parent’s genes, pick them (for example A (genes AO)) and the chance becomes one number.

Can two A parents have an O child?

Yes, if both carry O (genes AO). Each passes O with a 1 in 2 chance, so a child has a 1 in 4 chance of OO, type O. Two O parents can only have O children.

Can two Rh positive parents have an Rh negative child?

Yes, if both are Dd: each passes the d allele with a 1 in 2 chance, so 25% of their children are Rh negative. Two Rh negative parents (dd) have only Rh negative children.

Can a blood type prove who a child’s parent is?

No. It can only show that a match is possible. Rare variants such as the Bombay phenotype and cis-AB break the usual rules, and a DNA test is the right tool for parentage.

How do I know a parent’s genes?

A parent with type O or AB has only one possible genotype (OO or AB). For A or B, a parent whose own parent or child is type O must carry O. A genetic test also shows it.