Blood Type Calculator
Predict a child's possible blood types from the parents. Enter each parent's ABO group (A, B, AB, or O) and Rh factor (+ or −) to see every ABO/Rh combination the child could inherit, and which are ruled out.
Example: with Parent 1 ABO O · Parent 1 Rh Rh positive (+) · Parent 2 ABO A · Parent 2 Rh Rh positive (+) → Possible child types: O+, O-, A+, A-.
- Possible ABOABO: O or A
- Possible RhRh: + or -
- Ruled outNot possible: B+, B-, AB+, AB-
Computed by the calculator below using its default values. Change any input to see your own numbers.
ABO follows simple inheritance: A and B are codominant, O is recessive. Each parent passes one allele, and a parent showing A or B may secretly carry O.
How blood type is inherited
Everyone carries two ABO alleles, one from each parent. A and B are codominant, so inheriting both gives type AB, while O is recessive and only shows when both alleles are O. The catch is that a type A parent could be AA or AO and a type B parent could be BB or BO — so either can quietly pass an O. That is why two type A parents can still have a type O child, and why the tool lists every outcome the pairing allows rather than a single answer.
The Rh factor works separately: positive is dominant. Two Rh-positive parents can both carry a hidden negative and have an Rh-negative child, but two Rh-negative parents can only have a negative child. Combining the ABO and Rh possibilities gives the full list.
How it’s calculated
Each parent contributes one ABO allele: A → {A,O}, B → {B,O}, AB → {A,B}, O → {O}. Child phenotype: A+B → AB, A only → A, B only → B, OO → O. Rh: a positive parent may carry d, so positive parents give {+,−}; two negatives give only −. The tool crosses all allele combinations.
Standard textbook ABO/Rh inheritance; it ignores rare exceptions such as the Bombay phenotype, cis-AB, and weak D. It shows what is possible, not which type a child will have, and is not a paternity test.
ABO inheritance examples
| Parents (ABO) | Possible child | Not possible |
|---|---|---|
| O × O | O | A, B, AB |
| O × A | A, O | B, AB |
| O × AB | A, B | O, AB |
| A × B | A, B, AB, O | — |
| A × A | A, O | B, AB |
| AB × AB | A, B, AB | O |
Standard ABO Mendelian inheritance (A/B codominant, O recessive).
Common mistakes
- Assuming two type A parents cannot have a type O child — they can if both carry O.
- Thinking two Rh-positive parents can't have an Rh-negative child; a hidden negative allele allows it.
- Using this as a paternity test — many men share a compatible type, so it can only rule out, never confirm.
- Forgetting rare types (Bombay, weak D) that break the usual pattern.
Frequently asked questions
Can two O parents have a non-O child?
No. Type O parents carry only O alleles, so under standard inheritance every child is type O. Rare exceptions like the Bombay phenotype are the only caveat.
How does the calculator decide possible types?
It lists the alleles each parent can pass (for example A → A or O), crosses them for ABO, and combines with Rh, where positive parents may carry a hidden negative.
Can this prove paternity?
No. Blood type can sometimes exclude a parent but never confirm one, because many people share the same type. Only DNA testing establishes paternity.
Should I worry about Rh incompatibility in pregnancy?
An Rh-negative mother carrying an Rh-positive baby can need preventive treatment, but that is a clinical decision. Discuss Rh status and any results with your OB or a genetic counselor.