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.
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.