Health calculator

Free blood type calculator

See every blood type your child could have in two seconds. Pick both parents' blood types — A, B, AB, or O, each Rh-positive or Rh-negative — and the calculator returns the full set of possible child blood types (ABO and Rh), works through the inheritance, and shows whether two parents can have only one type or many, updated live, as you select.

On this page14 sections

InputsLive

Mother's blood type (ABO)

Mother's Rh factor

Father's blood type (ABO)

Father's Rh factor

How the prediction works

Each parent passes one ABO allele and one Rh allele. Because an A or B type can hide a recessive O allele — and an Rh+ type a recessive Rh− allele — the calculator takes every allele each parent could passand lists all the resulting child types. It shows what's possible, not exact odds.

Result

Possible child blood types (ABO)

A, B, AB, O

A+ × B+ · Rh: + or − · 8 full types

Possible ABOA, B, AB, O

Possible Rh+ or −

Full types8

Possible full blood types

Blood type	ABO	Rh
A+	A	Rh+
A−	A	Rh−
B+	B	Rh+
B−	B	Rh−
AB+	AB	Rh+
AB−	AB	Rh−
O+	O	Rh+
O−	O	Rh−

This predicts what's genetically possible — it is not a paternity test. Why not?

Results are estimates. Consult a professional.

Definition

What blood type can my child have?

Your child's blood type is set the moment they inherit one ABO allele and one Rh allele from each parent. Because some blood types hide a second, recessive allele, most parent pairs can produce several different child blood types rather than one. This blood type calculator takes both parents' types — A, B, AB, or O, each as Rh-positive or Rh-negative — and returns the full set of blood types their child could have.

It reports possible types, not exact odds. Whether a specific outcome is likely depends on each parent's hidden genotype (for example, whether an A parent is AA or AO), which a blood type alone cannot tell you. The robust, defensible answer — and the one this tool gives — is the complete set of types that are biologically possible. Select both parents' types above to see it instantly.

This tool predicts what is genetically possible for a child. It is an educational genetics tool, not a paternity test or a medical diagnosis — see the paternity section below for why blood type can rule a parent out but never confirm one.

The genetics

How ABO blood type inheritance works

Everyone carries two ABO alleles — one from each parent — drawn from three possibilities: A, B, and O. A and B are co-dominant (if you have both, you are type AB), and both dominate O, which is recessive. That is why the blood type you can see (your phenotype) can hide more than one underlying genotype.

Blood type	Possible genotypes	Alleles it can pass on
A	AA or AO	A or O
B	BB or BO	B or O
AB	AB	A or B
O	OO	O only

An A or B parent may secretly carry a recessive O allele, so they can pass on either allele to a child.

One of the alternative versions of a gene. For ABO the alleles are A, B, and O; you carry two.

The actual allele pair you carry, e.g. AO. It can't always be read off your blood type.

The blood type that shows up on a test — A, B, AB, or O — which is what the genotype produces.

An allele (like O) that is only expressed when paired with itself. AO still tests as type A.

Each parent passes one of their two alleles at random. The child's possible blood types are every combination of one allele from the mother with one from the father — so an AO mother and a BO father can produce AB, A, B, or O children. The calculator works through all of these combinations for you.

The chart

Blood type chart: parents and possible child types

This is the ABO inheritance chart the calculator above uses. It lists every possible child blood type for each combination of parent types (order doesn't matter — A × O is the same as O × A). Rh factor is inherited separately and is covered in the next section.

Parents (ABO)	Possible child blood types
O × O	O
O × A	A, O
O × B	B, O
O × AB	A, B
A × A	A, O
A × B	A, B, AB, O
A × AB	A, B, AB
B × B	B, O
B × AB	A, B, AB
AB × AB	A, B, AB

Source: standard Mendelian inheritance of the ABO system (American Red Cross; Stanford 'Ask a Geneticist').

Note the standout row: A × B parents can have a child of any ABO type — A, B, AB, or O. It surprises people, but it happens whenever both parents carry a hidden O allele (genotypes AO and BO), letting an O × O combination reach the child.

The +/− part

How the Rh factor (+/−) is inherited

The plus or minus after your blood type is the Rh factor, a separate gene inherited independently of ABO. It has two alleles: Rh-positive (often written D), which is dominant, and Rh-negative (d), which is recessive. You are Rh-negative only if you carry two negative alleles.

Rh-negative × Rh-negative — the child is always Rh-negative. Two negative parents have no positive allele to pass on.
Rh-positive × Rh-negative — the child can be positive or negative, because a positive parent may secretly carry a recessive negative allele (genotype +−).
Rh-positive × Rh-positive — usually positive, but can be negative if both parents are carriers (+−), giving a 1-in-4 chance of a negative child each pregnancy.

Because ABO and Rh are independent, the calculator works them out separately and then combines them. That is why a single parent pair can produce up to eight full blood types — for example, A+ and B+ parents who are all carriers could have a child of A+, A−, B+, B−, AB+, AB−, O+, or O−.

Rh-negative blood is most common in people of European descent (about 15%) and rarer in other populations. Rh incompatibility between an Rh-negative mother and Rh-positive baby is managed in pregnancy with Rh immunoglobulin (RhoGAM).

Worked example

A worked example using the blood type calculator

Example: an A+ mother and a B+ father

Sara is A+ and her partner Daniel is B+. They want to know what blood types their baby could have. Here is exactly how the calculator works it out — ABO first, then Rh, then combined.

Step 1 — List the alleles each parent can pass

Type A can be AA or AO, so Sara can pass an A or an O. Type B can be BB or BO, so Daniel can pass a B or an O.

Step 2 — Combine every ABO pairing

Pair each of Sara's possible alleles with each of Daniel's: A+B → AB, A+O → A, O+B → B, O+O → O. All four ABO types are possible.

Step 3 — Work out the Rh factor

Both parents are Rh-positive, but each could be a carrier (+−). So the child can be Rh-positive or Rh-negative. The Rh result is + or −.

Step 4 — Combine ABO with Rh

A, B, AB, or O — each as + or −

Multiplying the four possible ABO types by the two possible Rh types gives eight possible full blood types: A+, A−, B+, B−, AB+, AB−, O+, O−. The calculator shows this set instantly.

This is the widest possible outcome. An A × B pairing with carrier parents is the one combination that can yield any blood type at all — which is exactly why blood type cannot confirm a parent. The next sections cover the two questions people ask most: the two-O-parents case, and what blood type can and cannot say about paternity.

Common question

Can two type O parents have a non-O child?

For ABO, the answer is no. Type O is genotype OO, so each O parent can only ever pass an O allele — the child must be O. Two O parents cannot have an A, B, or AB child. If a child of two O parents tests as A, B, or AB, the explanation is not inheritance but something else: a non-biological parent, a lab mix-up, or one of the rare exceptions below.

The Rh factor is different. Two O-positive parents absolutely can have an O-negative child — in fact most O-negative children have positive parents — because each parent can carry a hidden recessive negative allele. So 'two O parents → only O' applies to the ABO letter, not the +/− sign.

Rare real exceptions exist. The Bombay phenotype can make someone who genetically carries A or B test as type O, and chimerism or recent transfusions can confuse a result. They are uncommon, but they are why a single blood-type surprise is a reason to test, not to conclude.

Important

Can blood type prove paternity?

No. Blood type can exclude a possible parent but can never prove one. The logic only runs one way: if a man could not have passed an allele the child carries, he is ruled out — but millions of unrelated people share any given blood type, so a 'match' proves nothing.

Exclusion (valid): two O parents with an AB child — impossible, so an alleged O father is excluded.
Exclusion (valid): an AB parent can never have an O child, because AB cannot pass an O allele.
'Match' (proves nothing): a child who is A+ has an A+ alleged father — consistent, but so are countless other A and O men.

For an actual answer, only a DNA paternity test can confirm a biological relationship. Treat this calculator as a genetics lesson — useful for understanding what is possible, never as legal or medical proof.

Accuracy

How accurate is this blood type calculator?

The set of possible types is exact. The calculator enumerates every allele combination both parents could pass and reports the full, deduped set of child blood types — the same set a geneticist would derive from a Punnett square. For any pair of parent types you select, it returns every type that is genetically possible and none that aren't.

What it deliberately does not give is exact probabilities. Whether a particular outcome is 25%, 50%, or 100% depends on each parent's hidden genotype — for instance, an A parent who is AA can never have an O child, while an AO parent can. Because a blood type alone cannot reveal carrier status, the only honest, universally correct answer is the set of possibilities, which is what this tool reports.

Methodology

Data sources and methodology

The rules come from standard Mendelian inheritance of the ABO and Rh blood group systems. ABO uses three alleles (A and B co-dominant, O recessive); Rh uses a dominant positive allele and a recessive negative one, inherited independently of ABO. The calculator takes the union over each parent's possible genotypes and reports the resulting set of child phenotypes.

American Red Cross — Blood Types (ABO and Rh).Stanford at The Tech Interactive — 'Ask a Geneticist': all possible blood types for any combination of parents.

Questions

Frequently asked questions about the free blood type calculator

A blood type calculator is a free online tool that helps you predict a child's possible blood types (ABO and Rh) from both parents' blood types. Each parent passes one ABO allele and one Rh allele; the calculator lists every blood type a child could inherit — reporting possibilities, not exact odds. It runs entirely in your browser with instant results and no sign-up.

It depends on both parents' types. Each parent passes one ABO allele and one Rh allele at random, so most parent pairs can have children of several blood types. For example, an A parent and a B parent can have a child of any ABO type — A, B, AB, or O — because each may carry a hidden O allele. The calculator lists every type that's genetically possible for your specific combination.

Not for the ABO letter. Type O is genotype OO, so two O parents can only pass O alleles — every child is type O. They can't have an A, B, or AB child. The Rh factor is different: two O-positive parents can have an O-negative child if both carry a hidden negative allele.

No. Blood type can rule a parent out but can never prove one. If a man couldn't have passed an allele the child carries, he's excluded — but because millions of people share any blood type, a 'match' proves nothing. Only a DNA test can confirm a biological relationship.

The Rh-positive allele is dominant and the Rh-negative allele is recessive, inherited independently of ABO. Two Rh-negative parents always have an Rh-negative child. A positive parent may secretly carry a negative allele, so positive × negative — and even positive × positive — can produce a negative child.

No. Type AB can only pass an A or a B allele, never an O, so an AB parent cannot have an O child. Likewise, an O parent (OO) and an AB parent (AB) can only have A or B children — never AB or O.

No — it reports the possible types, not the odds. The chance of any one outcome depends on whether a parent carries a hidden recessive allele (for example, whether an A parent is AA or AO), which a blood type alone can't reveal. The full set of possibilities is the only universally correct answer.

About

About this blood type calculator

This blood type calculator runs entirely in your browser. The types you select never leave your device — nothing is sent to a server, logged, or shared. It applies standard ABO and Rh inheritance, taking the union over each parent's possible genotypes to list every blood type a child could have, updating instantly as you change a selection.

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