Blood Type Calculator
Blood Type Calculator
Predict child blood types from parent genetics using ABO and Rh factor inheritance
Select Parent Blood Types
Choose both parents' ABO type (O, A, B, or AB) and Rh factor (+ or -) to see all possible child blood types with probability percentages.
Blood Type Population Frequency (US)
38%
of US population
7%
of US population
36%
of US population
6%
of US population
9%
of US population
2%
of US population
3%
of US population
1%
of US population
Frequencies vary by ethnicity and region. Data based on American Red Cross and Stanford Blood Center statistics.
Important
This calculator is provided for general information only and is not medical advice, diagnosis, or treatment. Results are estimates based on population averages and cannot account for your individual circumstances. Speak to a qualified healthcare professional before acting on anything you read here.
Predict child blood types from parent genetics using ABO and Rh factor inheritance
Welcome to our free Blood Type Calculator, a comprehensive genetics tool that predicts the possible blood types of a child based on the ABO and Rh factor blood types of both parents. Whether you are curious about your future child's blood type, planning for a pregnancy, or simply learning about human genetics, this calculator gives you accurate probability percentages alongside educational visualizations including Punnett squares, compatibility charts, and population frequency data. Blood type is one of the most fundamental characteristics of human biology, yet few people fully understand how it is inherited. The ABO blood group system, discovered by Karl Landsteiner in 1901, classifies blood into four types — A, B, AB, and O — based on which antigens are present on the surface of red blood cells. The Rh factor, named after the Rhesus monkey in which it was first discovered, adds a positive or negative designation, resulting in eight major blood types: O+, O-, A+, A-, B+, B-, AB+, and AB-. Understanding blood type inheritance matters for several important reasons. First, blood type compatibility is critical for safe blood transfusions and organ transplants. Receiving incompatible blood can cause a life-threatening immune reaction called hemolytic transfusion reaction. Second, Rh factor incompatibility between a Rh-negative mother and a Rh-positive fetus can cause a condition called Hemolytic Disease of the Fetus and Newborn (HDFN), which is preventable with Rh immunoglobulin (Rhogam) injections when identified early. Third, blood type can provide partial information in paternity determination, though DNA testing remains far more definitive. The ABO blood type system is controlled by a single gene with three alleles: I^A (A allele), I^B (B allele), and i (O allele). The A and B alleles are codominant, meaning both express their antigens when present together, producing blood type AB. The O allele is recessive, expressing only when both copies are O (genotype OO). This is why someone with blood type A may have either the genotype AA or AO — you cannot tell from the phenotype alone without genetic testing. Similarly, type B individuals may be BB or BO, while type AB is always AB and type O is always OO. The Rh factor follows a simpler dominant-recessive pattern. The D allele (dominant) produces the Rh antigen; the d allele (recessive) does not. Rh-positive individuals may be either DD or Dd genotype, while Rh-negative individuals are always dd. This means two Rh-positive parents who both carry one d allele (Dd × Dd) have a 25% chance of producing an Rh-negative child, which often surprises people who expect matching Rh factors to always produce the same Rh factor in offspring. Our calculator uses a complete probabilistic model that accounts for all possible parental genotypes. Because type A parents could be AA or AO, and type B parents could be BB or BO, the calculator averages over all genotype combinations to give you the true statistical probability of each child blood type. The visual Punnett squares show the most informative genotype combination (the heterozygous version when applicable) to help you understand the inheritance mechanism. Beyond blood type prediction, this calculator includes a full blood transfusion compatibility reference chart showing which blood types can donate to or receive from one another. The chart highlights O-negative as the universal red cell donor and AB-positive as the universal recipient, along with AB plasma being universally compatible. This information is valuable for anyone interested in blood donation or facing a medical situation requiring transfusion. The reverse lookup mode is particularly useful for situations where you know a child's blood type and one parent's blood type and wish to determine the possible blood types of the other parent. This can be used for paternity exclusion purposes: blood type alone can exclude paternity (for example, an O-type child cannot have an AB-type father, since AB parents cannot pass the O allele), but cannot confirm paternity due to the large number of people sharing compatible blood types. All calculations run entirely in your browser. No personal health data is stored or transmitted. Results are based on classic Mendelian genetics and standard medical blood type tables, but are theoretical estimates. Rare exceptions including the Bombay blood group, chimerism, and cis-AB alleles may produce results that differ from genetic predictions. Always consult a healthcare professional for medical decisions related to blood type.
Understanding Blood Type Genetics
Blood type is determined by genes inherited from both parents. The ABO system and Rh factor together create the eight major blood types recognized in medicine today.
The ABO Blood Group System
The ABO system classifies blood by the presence or absence of A and B antigens on red blood cell surfaces. Type A has A antigens, type B has B antigens, type AB has both, and type O has neither. These types are controlled by three alleles of the ABO gene: I^A, I^B, and i. Since A and B are codominant, a person with one of each becomes AB. The O allele (i) is recessive, so type O requires two O alleles. Because type A can be either AA or AO, and type B can be BB or BO, two parents of the same blood type may produce children with different blood types — for instance, two type A parents (each AO) can have a type O child.
Rh Factor Inheritance
The Rh factor is determined by the presence or absence of the D antigen on red blood cells. The D allele is dominant over d. Rh-positive individuals carry at least one D allele (DD or Dd), while Rh-negative individuals are dd. When two Rh-positive parents are each heterozygous (Dd), there is a 25% probability their child will be Rh-negative. This surprises many families who assume matching Rh signs guarantee the same Rh in children. A critical medical concern arises when a Rh-negative mother carries a Rh-positive fetus: maternal antibodies can develop and cross the placenta in subsequent pregnancies, potentially causing hemolytic disease in the newborn. Rh immunoglobulin injections prevent this complication.
Blood Type Compatibility for Transfusion
Blood type compatibility is essential for safe transfusions. The ABO and Rh systems determine whether donated red blood cells will be attacked by the recipient's immune system. O-negative is the universal red blood cell donor because it lacks all ABO antigens and the Rh antigen, making it safe for almost any recipient in emergencies. AB-positive is the universal recipient, able to receive any blood type. For plasma donations, the compatibility is reversed: AB plasma contains no ABO antibodies and can be given to anyone. Mismatched transfusions cause hemolytic reactions ranging from fever and chills to life-threatening kidney failure, which is why blood typing and cross-matching before transfusion is a strict medical standard.
Genetic Exceptions and Limitations
Standard blood type inheritance prediction is based on classic Mendelian genetics, but rare exceptions exist. The Bombay blood group (Oh phenotype) occurs in approximately 1 in 10,000 people in India and 1 in 1,000,000 globally: affected individuals have an hh genotype that prevents ABO antigens from being expressed, so they appear as type O regardless of their actual ABO genotype. Chimerism — when a person has two distinct DNA populations from absorbing a twin early in development — can produce unexpected blood type results. The cis-AB allele is a rare variant where a single chromosome carries both A and B transferase activity, allowing one parent to pass both A and B to a single child. These exceptions mean blood type prediction is probabilistic and theoretical, not absolute.
Blood Type Inheritance Formulas
ABO Allele Inheritance
Child ABO genotype = one allele from Parent 1 (Iᴬ, Iᴮ, or i) + one allele from Parent 2 (Iᴬ, Iᴮ, or i)
Each parent contributes one of their two ABO alleles to the child. The A (Iᴬ) and B (Iᴮ) alleles are codominant; the O (i) allele is recessive. This produces six possible genotypes (AA, AO, BB, BO, AB, OO) mapping to four phenotypes (A, B, AB, O).
Rh Factor Inheritance
Child Rh genotype = one allele from Parent 1 (D or d) + one allele from Parent 2 (D or d)
The D allele (Rh-positive) is dominant over the d allele (Rh-negative). Rh-positive parents may be DD or Dd; Rh-negative parents are always dd. Two Dd parents have a 25% chance of a dd (Rh-negative) child.
Punnett Square Probability
P(phenotype) = (number of squares producing phenotype) ÷ (total squares, typically 4)
A Punnett square cross produces 4 equally likely genotype outcomes. The probability of each child phenotype equals the count of squares yielding that phenotype divided by 4. For combined ABO × Rh, multiply ABO and Rh probabilities independently.
Combined Blood Type Probability
P(ABO type AND Rh type) = P(ABO type) × P(Rh type)
Because the ABO gene (chromosome 9) and Rh gene (chromosome 1) are inherited independently, the probability of a specific full blood type (e.g., A+) is the product of the ABO probability and the Rh probability.
Blood Type Reference Tables
Blood Type Transfusion Compatibility
Red blood cell donation compatibility matrix showing which blood types can safely donate to and receive from each other.
| Blood Type | Can Donate RBC To | Can Receive RBC From |
|---|---|---|
| O− | All types (universal donor) | O− |
| O+ | O+, A+, B+, AB+ | O+, O− |
| A− | A−, A+, AB−, AB+ | A−, O− |
| A+ | A+, AB+ | A+, A−, O+, O− |
| B− | B−, B+, AB−, AB+ | B−, O− |
| B+ | B+, AB+ | B+, B−, O+, O− |
| AB− | AB−, AB+ | AB−, A−, B−, O− |
| AB+ | AB+ only | All types (universal recipient) |
Blood Type Frequency by Ethnicity (US)
Approximate blood type distribution among major ethnic groups in the United States, based on American Red Cross data.
| Blood Type | Caucasian | African American | Hispanic | Asian |
|---|---|---|---|---|
| O+ | 37% | 47% | 53% | 39% |
| O− | 8% | 4% | 4% | 1% |
| A+ | 33% | 24% | 29% | 27% |
| A− | 7% | 2% | 2% | 0.5% |
| B+ | 9% | 18% | 9% | 25% |
| B− | 2% | 1% | 1% | 0.4% |
| AB+ | 3% | 4% | 2% | 7% |
| AB− | 1% | 0.3% | 0.2% | 0.1% |
Worked Examples
Predict Child Blood Type from A+ Father and O− Mother
Father is blood type A+ (possible genotypes: AO or AA for ABO, Dd or DD for Rh). Mother is blood type O− (genotype: OO for ABO, dd for Rh).
ABO cross: Father could be AA or AO. If AA × OO → all children are AO (type A). If AO × OO → 50% AO (type A) and 50% OO (type O). Averaging over both possibilities: ~75% type A, ~25% type O.
Rh cross: Father could be DD or Dd. If DD × dd → all children are Dd (Rh+). If Dd × dd → 50% Dd (Rh+) and 50% dd (Rh−). Averaging: ~75% Rh+, ~25% Rh−.
Combine independently: A+ = 75% × 75% = 56.25%; A− = 75% × 25% = 18.75%; O+ = 25% × 75% = 18.75%; O− = 25% × 25% = 6.25%.
Note: Mother is Rh-negative — if the child is Rh-positive, Rh incompatibility risk exists. Consult an obstetrician about Rhogam.
Possible child blood types: A+ (56.25%), A− (18.75%), O+ (18.75%), O− (6.25%). No type B or AB children are possible from this pairing.
Universal Donor and Universal Recipient Explained
A trauma patient arrives at the ER needing an emergency blood transfusion before their blood type can be determined.
O− red blood cells lack A antigens, B antigens, and the Rh (D) antigen.
Because the recipient's immune system has no foreign antigens to react against, O− blood can be transfused to any ABO/Rh blood type without causing a hemolytic reaction.
Conversely, AB+ individuals can receive red blood cells from any blood type because they have both A and B antigens (so they don't produce anti-A or anti-B antibodies) and the Rh antigen.
For plasma transfusions, compatibility reverses: AB plasma is the universal donor because it lacks both anti-A and anti-B antibodies.
O− is the universal red blood cell donor (safe for all recipients); AB+ is the universal red blood cell recipient (can receive from all donors). Only ~7% of the US population is O−, making it a critically needed donation type.
How to Use the Blood Type Calculator
Select Both Parents' Blood Types
In the Parent to Child mode, click the ABO type button (O, A, B, or AB) for each parent, then select their Rh factor (Rh+ or Rh-). The buttons are color-coded — blue for O, red for A, amber for B, and purple for AB — matching standard blood type color conventions. Results update automatically as you select.
Read the Probability Chart and Punnett Squares
The donut chart shows all possible child blood types with their percentage probabilities. The ABO and Rh bar charts break down the chances separately. The Punnett squares show the underlying allele inheritance at the genetic level, helping you understand why certain combinations are more or less likely. For example, if both parents are type A (AO genotype), the Punnett square shows a 25% chance of type O offspring.
Check Blood Type Details and Compatibility
Each possible child blood type card shows the exact probability, US population frequency, rarity classification, and transfusion compatibility — which blood types the child could donate to and receive from. Switch to the Compatibility Chart tab to see the complete 8-type transfusion matrix with universal donor (O-) and universal recipient (AB+) highlighted.
Use Reverse Mode for Paternity Exclusion
Switch to Child + Parent mode to enter a known child blood type and one known parent's blood type. The calculator will show all blood types the second parent could be (genetically compatible) and which types are definitively excluded. Blood type can rule out paternity but cannot confirm it — only DNA testing provides definitive paternity results.
Frequently Asked Questions
Can two type O parents have a child with a different blood type?
No. Type O is the only completely unambiguous ABO genotype — it is always OO (two recessive alleles). When both parents are OO, every child will also receive one O allele from each parent, making all children type O. This is why O×O is the only parent combination that produces only one possible child blood type for the ABO system. However, the Rh factor is still variable: if both type O parents are Rh-positive but carry the recessive d allele (Dd genotype), they have a 25% chance of producing an Rh-negative child, so the full blood type could range from O+ to O-.
Can two Rh-positive parents have an Rh-negative child?
Yes, this is surprisingly common and one of the most frequent sources of confusion about blood type inheritance. Rh-positive parents may be either DD (homozygous) or Dd (heterozygous) genotype. If both parents carry the recessive d allele (Dd×Dd), each parent passes d to the child 50% of the time. The probability that the child receives d from both parents — and therefore becomes Rh-negative (dd) — is 25%. This scenario occurs in many families and does not indicate any error in parentage. Without DNA testing, there is no way to determine whether an Rh-positive person is DD or Dd simply by looking at their blood type.
What is the Rh incompatibility pregnancy warning about?
Rh incompatibility becomes clinically significant when a Rh-negative mother carries a Rh-positive fetus. During delivery (or sometimes during pregnancy), small amounts of fetal blood can enter the mother's bloodstream. The mother's immune system recognizes Rh-positive antigens as foreign and produces anti-D antibodies. In a first pregnancy, this rarely causes problems because antibody levels are low. However, in subsequent pregnancies with an Rh-positive fetus, the mother's memory immune cells rapidly produce large amounts of anti-D antibodies that cross the placenta and attack fetal red blood cells, causing Hemolytic Disease of the Fetus and Newborn (HDFN). Modern medicine prevents this with Rh immunoglobulin (Rhogam) injections at 28 weeks and after delivery. All Rh-negative mothers should discuss this with their obstetrician.
Can blood type testing confirm or deny paternity?
Blood type can exclude paternity but cannot confirm it. Exclusion works because certain blood type combinations are genetically impossible. For example, an AB-type child cannot have a type O father, since O parents can only pass the O allele, and AB requires both A and B alleles. Similarly, an O-type child cannot have an AB-type father, since AB parents can only pass A or B alleles, never O. However, when a blood type is genetically compatible with paternity, it does not mean the tested man is the biological father — millions of other men share compatible blood types. DNA paternity testing, which compares specific genetic markers, is the only method that provides greater than 99.9% certainty.
Why is O-negative called the universal donor?
O-negative red blood cells lack both ABO antigens (A and B) and the Rh antigen. Because the immune system only attacks foreign antigens it does not recognize, O-negative blood can be given to recipients of any blood type without triggering an immune reaction against the donor cells. This makes O-negative the go-to blood type in emergency situations where there is no time to type and cross-match the patient's blood. However, the universal donor status applies only to red blood cell transfusions. For plasma, the compatibility is reversed: AB plasma is universal because it contains neither anti-A nor anti-B antibodies. O-negative blood is in constant high demand and is often in short supply because only about 7% of the US population carries this type.
What are rare exceptions where blood type inheritance does not follow normal patterns?
Three rare exceptions can produce blood types that appear to violate standard Mendelian inheritance. The Bombay blood group (Oh phenotype) affects about 1 in 10,000 people in India and 1 in 1,000,000 globally. These individuals have an hh genotype that prevents the formation of the H antigen needed to build ABO antigens, so they test as type O regardless of their true ABO genotype. Chimerism occurs when a person carries two distinct DNA populations (typically from absorbing a fraternal twin embryo early in development), potentially resulting in different blood types in different tissues. The cis-AB allele is an extremely rare genetic variant where a single chromosome encodes both A and B transferase activities, allowing one parent to pass both blood type antigens to a single child. These exceptions are exceedingly rare but explain why some family blood type combinations appear impossible under normal genetic rules.
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