Use this Blood Type Calculator to estimate a child’s possible ABO and Rh blood types from the mother’s and father’s blood types. It is useful for educational genetics questions, family curiosity, biology homework, and understanding how common blood-group inheritance patterns work.

Important Note: This calculator shows possible child ABO and Rh outcomes using simplified phenotype-based inheritance rules. Exact probabilities may differ when hidden parental genotypes are unknown. It cannot prove paternity, confirm a child’s actual blood type, determine transfusion compatibility, or assess pregnancy-related Rh risk.

Reviewed by: AjaxCalculators Editorial Team
Last updated: May 3, 2026
Method source: ABO and Rh inheritance patterns using phenotype-level parent blood types and standard Mendelian teaching rules
Editorial standards: AjaxCalculators Editorial Policy

What This Blood Type Calculator Calculates

This calculator estimates the possible blood types a child may have based on the parents’ reported blood types.

It shows possible outcomes for the common ABO groups:

• Type A
• Type B
• Type AB
• Type O

It also considers the common Rh label:

• Rh positive, shown with a plus sign such as A+
• Rh negative, shown with a minus sign such as O−

The result is designed as an educational inheritance estimate. It is not a laboratory blood-typing service, transfusion compatibility tool, prenatal risk assessment, or parentage test.

What ABO and Rh Mean

Most everyday blood-type labels combine two systems:

• ABO system: the A, B, AB, or O part of blood type
• Rh system: the positive or negative part, usually referring to RhD status

For example, A+ means the person has type A in the ABO system and is Rh positive. O− means the person has type O in the ABO system and is Rh negative.

Blood Type Part	Examples	What It Describes
ABO	A, B, AB, O	Main ABO blood group
Rh	Positive, negative	Common RhD positive or RhD negative label
Full type	A+, O−, AB+, B−	ABO group combined with Rh label

How ABO Blood Type Is Inherited

The ABO system is commonly taught using three alleles:

• A
• B
• O

A child receives one ABO allele from each parent. The A and B alleles are codominant, while the O allele is recessive. That creates six common ABO genotypes but only four common ABO phenotypes.

Genotype	Visible Blood Type	Simple Meaning
AA	A	Two A alleles
AO	A	A allele visible, O allele hidden
BB	B	Two B alleles
BO	B	B allele visible, O allele hidden
AB	AB	A and B both visible
OO	O	Two O alleles

This is why a parent with type A may be genetically AA or AO, and a parent with type B may be genetically BB or BO. The visible blood type does not always reveal the full genotype.

How Rh Blood Type Is Commonly Estimated

The Rh part of blood type is often simplified in educational calculators as a dominant/recessive inheritance pattern:

• Rh positive if the RhD factor is present
• Rh negative if the RhD factor is absent

In this simplified model, Rh positive is treated as dominant over Rh negative.

Simplified Rh Genotype	Visible Rh Type	Can Pass On
++	Rh positive	Positive allele only
+−	Rh positive	Positive or negative allele
−−	Rh negative	Negative allele only

This is a useful teaching model, but real Rh genetics can be more complex than a simple plus/minus chart. Laboratory testing is required for real blood-type decisions.

Phenotype vs Genotype

A major limitation of blood-type prediction is the difference between phenotype and genotype.

Term	Meaning	Blood-Type Example
Phenotype	The visible or reported blood type	A, B, AB, O, A+, O−
Genotype	The underlying allele combination	AA, AO, BB, BO, AB, OO

Most people know their phenotype, not their genotype. Because hidden alleles can affect child outcomes, this calculator should be understood as a possible-outcome estimator, not a guaranteed probability engine.

ABO Parent Combination Summary

The table below shows common possible ABO outcomes using standard teaching rules. Exact probabilities can depend on hidden genotype when a parent is type A or type B.

Parent ABO Types	Possible Child ABO Types	Important Note
O × O	O	Both parents usually pass only O alleles
O × A	A or O	O is possible if the A parent is AO
O × B	B or O	O is possible if the B parent is BO
O × AB	A or B	AB parent passes A or B, O parent passes O
A × A	A or O	O is possible if both A parents are AO
A × B	A, B, AB, or O	All four can be possible if parents are AO and BO
A × AB	A, B, or AB	O is not expected under standard ABO assumptions
B × B	B or O	O is possible if both B parents are BO
B × AB	A, B, or AB	O is not expected under standard ABO assumptions
AB × AB	A, B, or AB	O is not expected under standard ABO assumptions

How the Calculator Combines ABO and Rh

The calculator estimates ABO outcomes and Rh outcomes separately, then combines them into full blood-type possibilities.

For example, if the ABO possibilities are A or O, and the Rh possibilities are positive or negative, the combined full blood types may include:

• A+
• A−
• O+
• O−

The final output should be interpreted as possible blood types under simplified inheritance assumptions.

Worked Example A: A+ Parent and O− Parent

Suppose one parent is A+ and the other parent is O−.

Step 1: Think about the ABO part

The parent with type O is usually OO and can pass only an O allele.

The parent with type A may be:

• AA, passing only A
• AO, passing A or O

Step 2: Work out possible ABO outcomes

• If the A parent is AA, the child would be type A.
• If the A parent is AO, the child could be type A or type O.

Step 3: Think about the Rh part

The Rh-negative parent is simplified as passing a negative Rh allele. The Rh-positive parent may carry either one or two positive-associated alleles under the simplified teaching model.

Step 4: Interpret the result

The possible full blood types may include A or O with positive or negative Rh outcomes, depending on hidden parental genotype. Exact probabilities cannot be guaranteed from phenotype alone.

Worked Example B: O− Parent and O− Parent

Suppose both parents are O−.

Step 1: ABO inheritance

Type O parents are usually OO, so each parent passes an O allele.

ABO result: The child is expected to be type O under standard ABO rules.

Step 2: Rh inheritance

Rh-negative parents are simplified as −−, so each parent passes a negative allele.

Rh result: The child is expected to be Rh negative under the simplified model.

Result: The expected child blood type is O− under standard simplified assumptions.

Worked Example C: AB Parent and O Parent

Suppose one parent is AB and the other parent is O.

Step 1: ABO alleles

• The AB parent can pass A or B.
• The O parent can pass only O.

Step 2: Possible child genotypes

• AO, which appears as type A
• BO, which appears as type B

Result: Under standard ABO inheritance, the child may be type A or type B. Type AB and type O are not expected from this ABO pairing under the simplified model.

Worked Example D: A Parent and B Parent

Suppose one parent is type A and the other parent is type B.

The type A parent may be AA or AO. The type B parent may be BB or BO.

Hidden Genotype Pair	Possible Child ABO Types
AA × BB	AB only
AA × BO	A or AB
AO × BB	B or AB
AO × BO	A, B, AB, or O

Result: A × B parents can produce different possible outcomes depending on hidden genotypes. This is one of the clearest examples of why phenotype-only calculators cannot always provide exact probabilities.

Rh Factor and Pregnancy

Rh status can matter during pregnancy when a pregnant person is Rh negative and the fetus is Rh positive. This situation is commonly called Rh incompatibility and should be handled through prenatal care, blood testing, and clinician guidance.

This calculator can estimate possible Rh outcomes under a simplified inheritance model, but it cannot assess pregnancy risk, antibody status, sensitization, or treatment needs.

If pregnancy is involved, follow prenatal testing guidance from a qualified healthcare professional. Do not use an online blood-type calculator as a substitute for Rh testing or prenatal care.

Why This Calculator Cannot Prove Paternity

Blood type inheritance can sometimes show that a claimed parent-child combination is unlikely under standard assumptions, but it cannot prove a biological relationship.

There are several reasons:

• many parents have hidden ABO or Rh genotypes
• blood-type records can be mistaken or incomplete
• rare blood-group variants can change expected patterns
• blood type uses only a small part of genetic information
• DNA testing is the appropriate method for parentage questions

For legal, family, or relationship questions, use qualified DNA-based testing rather than blood-type prediction.

Why This Calculator Is Not a Transfusion Tool

Blood transfusion compatibility is more complex than predicting inherited ABO and Rh possibilities. Transfusion decisions may require:

• ABO typing
• RhD typing
• antibody screen
• crossmatch testing
• patient history
• clinical urgency
• laboratory blood bank procedures

Never use this calculator to choose blood for transfusion, donation, pregnancy care, surgery, emergency care, or newborn care.

Rare Exceptions and Limitations

Most educational blood-type calculators use simplified inheritance rules. Real blood-group biology can include exceptions and complexities.

Issue	Why It Matters
Bombay phenotype	Can make ABO inheritance appear unusual under simple A/B/O rules
Weak D or partial D	Can complicate Rh-positive/Rh-negative interpretation
cis-AB and rare ABO variants	Can create uncommon inheritance patterns
Recent transfusion	Can affect blood typing results in some contexts
Stem-cell or bone-marrow transplant	Can change blood-cell production and blood-type findings
Lab or record error	Incorrect parent blood type entry can make the prediction wrong

These are not everyday situations for most users, but they are important reasons not to treat a simple calculator as a final medical or legal answer.

How to Use This Blood Type Calculator

1. Select the mother’s full blood type, such as A+, O−, AB+, or B−.
2. Select the father’s full blood type.
3. Review the possible ABO outcomes.
4. Review the possible Rh outcomes.
5. Read the combined possible child blood types.
6. Treat the result as an educational inheritance estimate, not a laboratory fact.

How to Interpret the Result

The result shows which child blood types are possible under standard ABO and simplified Rh inheritance teaching rules.

If several outcomes appear, that does not mean the calculator is broken. It often means more than one genotype combination could fit the parents’ visible blood types.

Result Type	Meaning	Important Limitation
Possible child blood type	The type can occur under the calculator’s inheritance assumptions	Not a guarantee that the child will have that type
Not listed as possible	The type is not expected under simplified standard rules	Rare variants, lab errors, or nonstandard cases may need professional review
Multiple outcomes	Hidden genotype can allow more than one result	Exact probability may be unknown from phenotype alone

If you need to know someone’s real blood type, a lab blood test is the correct method. If you need relationship testing, DNA testing is the correct method.

When This Calculator Is Useful

This calculator is useful for general education and simple inheritance exploration.

• Learn how ABO and Rh inheritance works
• See possible child blood-type outcomes from parents
• Understand why phenotype does not always reveal genotype
• Use blood-group examples for biology or genetics study
• Explore common family blood-type patterns for curiosity
• Understand why type A can hide an O allele
• Understand why type B can hide an O allele
• Review basic Mendelian inheritance with Punnett-square logic

When You Need More Than This Calculator

Use professional testing or medical guidance when the result affects real decisions.

• You need a confirmed blood type for medical records
• You need transfusion compatibility information
• You are pregnant and Rh status may matter
• A newborn blood type or Rh incompatibility concern is involved
• You need paternity or maternity testing
• You have unusual or conflicting blood-type results
• There has been a recent transfusion or transplant
• You need legal, immigration, custody, or relationship evidence

Common Mistakes to Avoid

• Treating possible outcomes as guaranteed: the calculator shows possibilities, not certainty.
• Ignoring hidden genotype: type A may be AA or AO, and type B may be BB or BO.
• Using blood type to prove paternity: blood type cannot confirm biological parentage.
• Using this as a transfusion guide: transfusion compatibility requires laboratory testing and medical procedures.
• Assuming Rh is always simple: RhD inheritance and testing can include additional complexity.
• Ignoring rare blood-group variants: unusual variants can produce unexpected patterns.
• Entering unconfirmed blood types: incorrect parent blood type entries produce unreliable results.
• Using the result for pregnancy care: Rh concerns during pregnancy require prenatal testing and clinician guidance.

Assumptions and Important Notes

• This calculator works from parent phenotypes, such as A+, O−, or AB+, not confirmed full genotypes.
• A parent with type A may genetically be AA or AO.
• A parent with type B may genetically be BB or BO.
• Because hidden genotype matters, exact child probabilities can vary by the parents’ unseen genetic combinations.
• The Rh system is simplified as Rh positive dominant over Rh negative for educational use.
• Real blood-group genetics may include rare variants and exceptions.
• This calculator is not a transfusion compatibility tool.
• This calculator is not sufficient for paternity or maternity determination.
• Laboratory blood typing is required to know a person’s actual blood type.

Practical Uses of a Blood Type Calculator

• Estimate possible child blood types from parent blood types
• Study ABO inheritance
• Study simplified Rh inheritance
• Practice Punnett-square reasoning
• Understand phenotype and genotype differences
• Explore family blood-type curiosity
• Support biology homework examples
• Learn why DNA testing is needed for parentage certainty

References

1. NCBI Bookshelf: The ABO Blood Group
2. NCBI Medical Genetics Summaries: ABO Blood Group
3. NHS: Blood Groups
4. NHS Blood Donation: Blood Types
5. MedlinePlus: Rh Incompatibility
6. American Red Cross: Blood Types Explained
7. American Red Cross: DNA Relationship Testing Services

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Frequently Asked Questions

What does this Blood Type Calculator do?

It estimates possible child ABO and Rh blood types from the mother’s and father’s reported blood types.

Can this calculator tell my child’s exact blood type?

No. It shows possible outcomes under simplified inheritance rules. A laboratory blood test is needed to know the actual blood type.

What are the four ABO blood types?

The four common ABO blood types are A, B, AB, and O.

What does Rh positive or Rh negative mean?

In common blood-type labels, Rh positive means the RhD factor is present, while Rh negative means it is absent.

How is ABO blood type inherited?

A child receives one ABO allele from each parent. A and B are codominant, while O is recessive.

What genotype gives type A blood?

Type A can come from AA or AO genotype.

What genotype gives type B blood?

Type B can come from BB or BO genotype.

What genotype gives type AB blood?

Type AB comes from one A allele and one B allele.

What genotype gives type O blood?

Type O usually comes from two O alleles, written as OO.

Can two O parents have an A, B, or AB child?

Under standard ABO inheritance assumptions, two type O parents are expected to have type O children. Rare variants, errors, or unusual cases require professional review.

Can an AB parent and an O parent have an O child?

Under standard ABO inheritance, an AB parent and an O parent are expected to have type A or type B children, not type O.

Can A and B parents have an O child?

Yes, if the A parent is AO and the B parent is BO, a child can inherit O from both parents and have type O blood.

Why does hidden genotype matter?

Because a visible type A parent can be AA or AO, and a visible type B parent can be BB or BO. Those hidden alleles change possible child outcomes.

Can this calculator give exact probabilities?

It can show possible outcomes, but exact probabilities may require knowing the parents’ underlying genotypes, not just visible blood types.

Can this calculator prove paternity?

No. Blood type can sometimes exclude a relationship under standard assumptions, but it cannot prove paternity. DNA testing is required for parentage certainty.

Can this calculator be used for transfusion compatibility?

No. Transfusion decisions require laboratory testing, antibody screening, crossmatching, and professional blood-bank procedures.

Can this calculator assess Rh risk in pregnancy?

No. Rh concerns during pregnancy require prenatal blood testing and clinician guidance.

What if the calculator result conflicts with a real lab result?

Use the laboratory result and ask a qualified healthcare professional or lab professional. Rare variants, incorrect records, or special medical history can make simple predictions misleading.

Can rare blood-type variants affect results?

Yes. Rare variants such as Bombay phenotype, weak or partial D, cis-AB, and other blood-group complexities can affect simple inheritance expectations.

Is this calculator medical advice?

No. It is for education and general genetics learning only. Use qualified laboratory testing and medical guidance for real decisions.

Disclaimer: This Blood Type Calculator provides educational possible-outcome estimates for a child’s ABO and Rh blood type from the mother’s and father’s reported blood types. Results depend on phenotype-level parent blood types, hidden parental genotypes, simplified Mendelian inheritance assumptions, RhD inheritance assumptions, and whether the entered blood types are correct laboratory-confirmed results.

A parent with type A may genetically be AA or AO, and a parent with type B may be BB or BO, so exact probabilities may differ when genotypes are unknown. Rh-positive status is commonly simplified as dominant over Rh-negative in educational calculators, but real Rh blood-group genetics and laboratory interpretation can include additional complexity.

This calculator is not a paternity test, blood test, transfusion-compatibility tool, prenatal Rh-risk assessment, or medical advice. Use qualified laboratory testing and professional medical guidance for real health, pregnancy, parentage, or transfusion decisions.