Simple mode
The child’s possible types from the parents’ blood types alone, with no percentage.
Child blood type calculator
List the ABO and Rh blood types a child can have from the parents’ groups, or get theoretical percentages when their genotypes are known.
Possible blood types for your child
A+
A-
B+
B-
AB+
AB-
O+
O-
A known blood type does not always identify a genotype. Reliable percentages cannot be calculated from the parents' blood types alone.
Method
Each parent passes on one ABO allele and one RhD variant. The model combines four ABO outcomes and four RhD outcomes, assuming independent Mendelian inheritance.
Formula
P(ABO, RhD) = P(ABO) × P(RhD)Limitations
- A blood type does not always reveal its underlying genotype.
- Rare variants, weak or partial D and exceptional inheritance patterns are not modelled.
- Do not use this tool for parentage, transfusion decisions or pregnancy risk assessment.
Was this tool useful?
How does this tool work?
A child’s blood type depends on the genes each parent passes on. From the mother’s and father’s ABO and Rh groups, this calculator shows which blood types the child can have; if you know the genotypes, it also gives the theoretical probability of each. It is an educational tool: it does not replace a blood test and cannot establish parentage.
The ABO system relies on three alleles: A and B are codominant, O is recessive. A group A person can therefore be AA or AO, a group B person BB or BO, while AB and O each have a single genotype. For Rh, the D allele (positive) dominates d: an Rh+ parent can be DD or Dd, an Rh− parent is always dd.
In simple mode, the tool starts from the parents’ blood types alone, tries every compatible genotype and shows each type the child could have. Since we do not know which genotypes are real, no percentage is shown.
In advanced mode, you enter both parents’ ABO and RhD genotypes. Each parent passes on one of their two alleles with a one-in-two chance, and the two systems are assumed independent: the probability of a type is the ABO probability times the Rh probability. Rare variants (Bombay phenotype, cis-AB, weak or partial RhD) are not modelled.
How to use this tool
Choose the mode
Simple mode if you only know the blood types, advanced mode if you know the genotypes (for example AO, Dd).
Enter the parents
Enter the mother’s and father’s blood type or genotypes.
Read the result
The table marks each type as possible or not expected; in advanced mode, it shows the theoretical percentage of each.
Good to know
Advanced mode
The theoretical probability of each type when the parents’ ABO and RhD genotypes are known.
ABO and Rh
Two systems inherited independently, following Mendel’s laws, in a simplified model.
Probability of a blood type
Each parent passes on one ABO allele and one RhD allele, each with a one-in-two chance; the two systems are assumed independent.
ABO × RhD
P(type) = P(ABO) × P(Rh)
Example: AO × BO and Dd × Dd give P(A+) = 25% × 75% = 18.75%.
Simplified model: the Bombay phenotype, cis-AB, weak or partial RhD and other rare variants are not taken into account. Do not use for parentage, transfusion or pregnancy care.
Use cases
Understanding inheritance
See why two group A parents can have a group O child, or why an AB parent never has an O child.
Preparing a question for a doctor
Know which types are expected before an appointment, to interpret a lab result with the professional.
Illustrating a genetics lesson
Show ABO and Rh crosses in class and the difference between genotype and phenotype.
Examples
Two O negative parents
An O- parent is necessarily OO and dd, so can only pass on O and d.
Input
Simple mode; mother: O-; father: O-Result
Only possible type: O-An AB parent and an O negative parent
The child gets A or B from the mother and O from the father: A or B, never AB or O. Rh depends on whether the mother is DD or Dd.
Input
Simple mode; mother: AB+; father: O-Result
Possible types: A+, A-, B+, B-Known genotypes AO × BO
Each ABO group has a 25% chance and Rh positive 75%: 25% × 75% = 18.75%.
Input
Advanced mode; mother: AO, Dd; father: BO, DdResult
A+, B+, AB+, O+: 18.75% each
A-, B-, AB-, O-: 6.25% eachTips and best practices
- An observed blood type does not always reveal the genotype: a group A parent can be AA or AO.
- A “not expected” result in the simplified model is not proof: rare variants and typing errors exist.
- Do not use this calculation to establish parentage, decide on a transfusion or assess Rh incompatibility during pregnancy.
Sources and references
- American Red Cross, Blood Types Explained (opens in a new tab)Page in English
- Australian Red Cross Lifeblood, ABO blood group system (opens in a new tab)Page in English
Frequently asked questions
What does this calculator show?
The ABO and Rh types a child can have based on the parents’ types and, if the genotypes are known, the theoretical probability of each type.
Why does simple mode show no percentages?
Groups A, B and Rh+ each match two possible genotypes. Without knowing which ones the parents carry, we can say which types are possible, but not how likely they are.
Can two group A parents have a group O child?
Yes, if both are AO: each passes on O with a one-in-two chance, giving a 25% chance of an OO child in the model.
Can an AB parent have a group O child?
Not in the standard model: an AB parent always passes on A or B. Rare exceptions such as cis-AB exist but are not modelled.
Can two Rh+ parents have an Rh− child?
Yes, if both are Dd: the child then has a 25% chance of being dd, that is Rh−.
Can I use this result for a medical decision?
No. A child’s blood type is determined by a blood test, and any decision (transfusion, pregnancy care) is a matter for a health professional.
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