Blood Type Calculator

Example: A+ and O+ parents: child A+ 54%, O+ 38%, never B or AB

Which blood types can your baby have, and how likely is each one? Choose both biological parents' blood types to see every possible ABO and RhD result with its chance, the Punnett square behind it, and the types that are ruled out. Switch modes to work out which types the other parent could have.

Last reviewed by SparkCalc editorial team · September 2026

Runs in your browser No signup

Find

Mother's blood type

Father's blood type

Chance of each blood type

ABO group split

Most Likely Type

All Possible Types

Ruled Out

ABO Group Chances

RhD Factor

Share Link:

Did this answer your question?

Related Calculators

You might also find these calculators helpful: estimate your baby's due date, work out the likely conception date, predict your child's adult height, and check growth percentiles.

How to use this calculator

Keep the mode on Child's blood type to predict a baby from both biological parents.
Choose the mother's and father's blood types, including the RhD sign.
Read the most likely type, every possible type with its chance, and the types that are ruled out.
Open Punnett Square and Hidden Genes to see which carried genes make each result possible.
Switch to Other parent's type to see which blood types could explain a child, given one known parent.

How We Calculate This

ABO follows three alleles: A and B are codominant and O is recessive. RhD positive is dominant over RhD negative, and the two systems are inherited independently. Each parent's phenotype is expanded into possible genotypes weighted by Hardy-Weinberg frequencies. Allele frequencies come from Stanford Blood Center US blood type shares using Bernstein's estimates (O allele = square root of the type O share; A allele = square root of A plus O shares, minus the O allele; likewise for B), and the RhD negative allele is the square root of the RhD negative share. Every genotype pairing contributes its four equally likely Punnett outcomes. The reverse mode multiplies each candidate type's population share by the chance it produces the entered child, then normalises.

Methodology last reviewed: September 2026. How SparkCalc works

Sources: Dean L. Blood Groups and Red Cell Antigens: The ABO blood group (NCBI Bookshelf) · Dean L. Blood Groups and Red Cell Antigens: The Rh blood group (NCBI Bookshelf) · Dean L. Blood Groups and Red Cell Antigens: The Hh blood group and Bombay phenotype (NCBI Bookshelf) · Stanford Blood Center: Blood Types (US population shares) · Stanford Blood Center: Blood Type Compatibility Chart · Yoshida A et al. Genetic mechanism of cis-AB inheritance (American Journal of Human Genetics) · ACOG: The Rh Factor: How It Can Affect Your Pregnancy

Why the most common answer is often a hidden O

In the US, about 44% of people are type O, so the O gene is by far the most common ABO gene. That means most people who test as type A or B carry a hidden O. Using Stanford Blood Center shares, roughly 83% of type A people are AO rather than AA, which is why A and O parents have about a 42% chance of an O child rather than zero.

How the RhD factor combines with ABO

RhD is inherited separately from ABO, so the calculator works out each system and multiplies the chances. About 15% of the US population is RhD negative, which puts the RhD negative gene in roughly 39% of genes and makes about 56% of RhD positive people carriers. Two RhD positive parents therefore have about an 8% chance of an RhD negative child.

Reading the reverse mode

Given one parent and the child, the reverse mode lists every type the other biological parent could have, with chances weighted by how common each type is. A result of no standard combination means the entered types break the usual rules. That points to a recording error or a rare variant and is not evidence about parentage on its own.

Possible child ABO blood types by parent types

Standard ABO inheritance, before RhD. Chances depend on hidden genes and are shown in the calculator.

Possible child ABO blood types by parent types
Parents	Possible child types	Not possible
O and O	O	A, B, AB
O and A	O, A	B, AB
O and B	O, B	A, AB
O and AB	A, B	O, AB
A and A	A, O	B, AB
A and B	A, B, AB, O	none
A and AB	A, B, AB	O
B and B	B, O	A, AB
B and AB	A, B, AB	O
AB and AB	A, B, AB	O

RhD: two RhD negative parents have an RhD negative child. Any parent pair with at least one RhD positive parent can have either.

Key terms

Phenotype: The blood type a test reports, such as A+ or O-.
Genotype: The pair of genes behind the phenotype. Type A can be AA or AO, and RhD positive can be DD or Dd.
Codominant: Both genes show. A person with one A and one B gene is type AB.
Recessive: A gene that only shows with two copies. Type O and RhD negative each need a copy from both parents.
Punnett square: A grid pairing each gene one parent can pass on with each gene from the other parent. Each cell is one of four equally likely outcomes.

Frequently Asked Questions

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

Under standard ABO inheritance, no. Type O people carry two O genes, so two O parents can only pass on O. Very rare exceptions exist, such as the Bombay phenotype, where a person tests as O while carrying A or B genes, so an unexpected result calls for retesting rather than conclusions.

How can two A+ parents have an O- baby?

Type A and RhD positive are both dominant, so each parent can carry a hidden O gene and a hidden RhD negative gene. If both pass on O and both pass on RhD negative, the child is O-. The calculator estimates how often A+ people carry these hidden genes from US blood type frequencies.

Why are the chances estimates rather than exact odds?

A blood type test shows the phenotype, such as A, not whether the person carries AA or AO. The calculator weights the possible genotypes with Hardy-Weinberg frequencies derived from US blood type shares. Ancestry changes those frequencies, and a genotype test or a known family history gives exact odds.

Does an RhD negative mother need to worry about her baby?

An RhD negative mother carrying an RhD positive baby can make antibodies that affect a later pregnancy. Prenatal care checks blood type early and may offer Rh immune globulin to prevent this. The calculator shows the chance the baby is RhD positive; your care team decides what testing is needed.

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

No. Blood type can show that a parent combination is unlikely under standard rules, but many unrelated people share each type, and rare variants such as cis-AB break the usual rules. DNA testing is the reliable way to establish parentage.

Embed this calculator

Add this free calculator to your own website. Copy the code below and paste it into your page’s HTML. It is responsive and resizes to fit.

This calculator gives educational estimates based on standard inheritance rules. It cannot determine anyone's actual blood type: only a blood test can. Rare variants such as cis-AB, the Bombay phenotype, weak D and chimerism, as well as recording errors, can produce results outside these rules. Consult a healthcare professional for medical, pregnancy or transfusion questions.

References