Blood Type Calculator — Predict Your Child's Blood Type
Free blood type calculator. Enter both parents' blood types (ABO + Rh) to see possible blood types for their children with probabilities. Based on Mendelian genetics.
How Blood Type Inheritance Works
Blood type is determined by genetics — specifically, by the ABO gene on chromosome 9 and the Rh gene on chromosome 1. Each person inherits one allele from each parent, and the combination determines their blood type.
The ABO system has three alleles: A, B, and O. A and B are codominant (both expressed when present together), while O is recessive (only expressed when two O alleles are inherited). This creates six possible genotypes and four phenotypes:
| Genotype | Blood Type |
|---|---|
| AA or AO | Type A |
| BB or BO | Type B |
| AB | Type AB |
| OO | Type O |
The Rh factor (positive or negative) is determined by a separate gene. Rh-positive (D allele) is dominant over Rh-negative (d allele). A person is Rh-positive if they have at least one D allele (DD or Dd) and Rh-negative only if they have two d alleles (dd).
This means a person who is Type A+ could have the genotype AA/DD, AA/Dd, AO/DD, or AO/Dd — and each genotype produces different probabilities for their children's blood types.
Blood Type Compatibility Chart
Here are all possible blood type outcomes for each parent combination:
| Parent 1 | Parent 2 | Possible Child Types | Not Possible |
|---|---|---|---|
| O | O | O | A, B, AB |
| O | A | O, A | B, AB |
| O | B | O, B | A, AB |
| O | AB | A, B | O, AB |
| A | A | A, O | B, AB |
| A | B | A, B, AB, O | None |
| A | AB | A, B, AB | O |
| B | B | B, O | A, AB |
| B | AB | A, B, AB | O |
| AB | AB | A, B, AB | O |
Key insight: Two Type O parents can only have Type O children. A Type AB parent can never have a Type O child. Two Type A parents can have a Type O child (if both carry the recessive O allele). These rules have been used in paternity disputes, though DNA testing is far more reliable.
Blood Type Distribution Worldwide
Blood type distribution varies significantly by ethnicity and region:
| Blood Type | Global Average | US | Japan | India |
|---|---|---|---|---|
| O+ | 38% | 37% | 30% | 36% |
| A+ | 27% | 36% | 40% | 23% |
| B+ | 22% | 9% | 20% | 31% |
| AB+ | 5% | 3% | 10% | 7% |
| O− | 6% | 7% | — | 2% |
| A− | 2% | 6% | — | 1% |
| B− | 1% | 2% | — | 1% |
| AB− | <1% | 1% | — | <1% |
Type O is the most common globally, while AB− is the rarest (~0.6% of the world population). The Rh-negative factor is most common in people of European descent (~15%) and very rare in East Asian and African populations (<1%).
The Basque people of Spain and France have the highest rate of Rh-negative blood (~35%), which has led to various theories about the origin and evolutionary advantage of the Rh-negative trait.
Blood Transfusion Compatibility
Blood type is critical for safe transfusions. Receiving incompatible blood can trigger a life-threatening immune reaction:
Red blood cell compatibility:
- O− is the universal donor — can give red blood cells to anyone
- AB+ is the universal recipient — can receive from anyone
- Type A can receive from A and O
- Type B can receive from B and O
- Rh− can only receive Rh− blood; Rh+ can receive both
Plasma compatibility is the reverse: AB is the universal plasma donor, and O is the universal plasma recipient. This is because plasma contains antibodies against the blood types you don't have.
In emergencies when blood type is unknown, hospitals use O− red blood cells and AB plasma. This is why blood banks are always in particular need of O− and AB donors.
Rh Factor and Pregnancy
Rh incompatibility between mother and fetus can cause serious complications. When an Rh-negative mother carries an Rh-positive baby, her immune system may produce antibodies against the baby's Rh-positive blood cells — a condition called Rh sensitization or hemolytic disease of the newborn (HDN).
This typically doesn't affect the first pregnancy, but in subsequent pregnancies with Rh-positive babies, the mother's antibodies can cross the placenta and attack the fetus's red blood cells, causing anemia, jaundice, and in severe cases, brain damage or stillbirth.
Prevention: RhoGAM (Rh immunoglobulin) injections given at 28 weeks and within 72 hours of delivery prevent the mother from forming antibodies. This treatment has made Rh disease largely preventable in developed countries. All pregnant women are tested for Rh factor at their first prenatal visit.
Rh incompatibility only occurs when the mother is Rh− and the father is Rh+. If both parents are Rh−, all children will be Rh−, and there's no risk. If the mother is Rh+, there's no risk regardless of the father's type.
Blood Type and Health
Research has found some statistical associations between blood type and disease risk, though the effects are generally small:
- Type O: Lower risk of heart disease and blood clots. Higher risk of stomach ulcers (H. pylori attaches more easily to Type O cells). May have slightly lower risk of certain cancers.
- Type A: Slightly higher risk of stomach cancer and cardiovascular disease. Some studies suggest higher susceptibility to certain infections.
- Type B: Associated with slightly higher risk of ovarian cancer and type 2 diabetes in some populations.
- Type AB: Higher risk of cognitive decline and stroke in some studies. Lower risk of some types of stomach ulcers.
Important context: These are statistical correlations found in population studies, not deterministic predictions. The risk differences are small (typically 5–20% relative increase), and lifestyle factors (diet, exercise, smoking) have far greater impact on health outcomes than blood type. "Blood type diets" have no scientific support.
Tips for Using This Calculator
- If you don't know your genotype: The calculator assumes equal probability of homozygous and heterozygous genotypes. For example, if you're Type A, it considers both AA and AO equally likely. In reality, AO is more common than AA in most populations.
- Rh-positive parents: If both parents are Rh+, there's still a chance of an Rh− child (if both parents carry the recessive d allele). The calculator accounts for this.
- For paternity questions: Blood type can exclude a potential father (e.g., two Type O parents cannot have a Type AB child) but cannot confirm paternity. DNA testing is far more accurate for paternity determination.
- Pregnancy planning: If you're Rh− and your partner is Rh+, mention this to your OB early — they'll plan RhoGAM injections to prevent Rh sensitization.
Can two parents with Type O blood have a child with a different blood type?
No. Two Type O parents (both genotype OO) can only have Type O children. Each parent can only contribute an O allele, and OO = Type O. This is one of the most reliable rules in blood type inheritance and was historically used as evidence in paternity cases — if the child is not Type O and both alleged parents are Type O, at least one is not the biological parent.
Can a Type AB parent have a Type O child?
No (with extremely rare exceptions). A Type AB parent has one A allele and one B allele, so they must contribute either A or B to every child. Since Type O requires two O alleles (one from each parent), and an AB parent cannot contribute an O allele, Type O children are impossible. The ultra-rare exception is the "Bombay phenotype" (hh genotype) where a person appears to be Type O but genetically carries A or B alleles that aren't expressed.
Is blood type linked to personality?
In Japan and South Korea, blood type personality theory is a popular cultural belief (similar to zodiac signs in the West). Type A is said to be organized and anxious, Type B creative and selfish, Type O confident and insensitive, Type AB rational and indecisive. However, multiple large-scale scientific studies have found absolutely no correlation between blood type and personality traits. It's a fun cultural phenomenon with no scientific basis.
What is the rarest blood type?
AB− is the rarest common blood type, found in less than 1% of the global population. However, there are much rarer blood types beyond ABO/Rh — the "golden blood" (Rh-null, missing all Rh antigens) has been found in fewer than 50 people worldwide. There are over 300 known blood group antigens across 36 blood group systems, and extremely rare combinations can make finding compatible blood nearly impossible.
Why do blood banks always need Type O negative?
O− red blood cells can be given to anyone regardless of their blood type, making it the universal emergency blood type. When accident victims arrive at the ER and there's no time to test their blood type, O− is used. This creates constant demand. Only about 7% of the population is O−, so supply is always tight. O+ (37% of population) is the most commonly needed type overall, simply because it's the most common blood type.
Can blood type change during a person's lifetime?
Under normal circumstances, no — blood type is genetically determined and permanent. However, there are rare exceptions: bone marrow transplants can change a recipient's blood type to match the donor's (since bone marrow produces blood cells). Some diseases (leukemia, infections) can temporarily alter the expression of blood type antigens. And a very small number of people are "chimeras" — carrying two different blood types due to absorbing a twin's cells in utero.
How is blood type tested?
Blood typing uses a simple process called agglutination testing. A blood sample is mixed with anti-A and anti-B antibodies. If the blood clumps (agglutinates) with anti-A, you're Type A. With anti-B, you're Type B. With both, you're AB. With neither, you're Type O. Rh testing adds anti-D antibody. The entire process takes minutes. Forward typing (testing red cells) is confirmed with reverse typing (testing serum) for accuracy. Home blood typing kits are available but less reliable than lab testing.
Do siblings always have the same blood type?
No. Siblings can have different blood types because each child receives a random combination of alleles from each parent. For example, if both parents are Type A with genotype AO, each child has a 75% chance of being Type A (AA or AO) and a 25% chance of being Type O (OO). In a family of four children, it's entirely possible for some to be Type A and others Type O. Only identical twins are guaranteed to share the same blood type.
What is the Bombay blood type?
The Bombay phenotype (Oh) is an extremely rare blood type where a person appears to be Type O but genetically carries A and/or B alleles. It's caused by a mutation in the H gene (FUT1) that prevents the H antigen from forming — and the H antigen is the precursor needed for A and B antigens. These individuals can only receive blood from other Bombay phenotype donors. It occurs in about 1 in 10,000 people in India (where it was first discovered in 1952) and is much rarer elsewhere.
Does blood type affect COVID-19 risk?
Early studies during the pandemic suggested Type O might have slightly lower risk of COVID-19 infection, while Type A might have slightly higher risk. A large 2020 study in the New England Journal of Medicine found a statistically significant but small association. However, the effect size was modest (10-20% relative risk difference), and subsequent studies produced mixed results. Vaccination status, age, and comorbidities are far more important risk factors than blood type.
Blood Donation and Blood Banking
Understanding blood types is crucial for blood donation — the right match saves lives, while a mismatch can be fatal. Here's what you need to know about donating and receiving blood:
Who can donate: Most healthy adults aged 17+ (16 with parental consent in some states) weighing at least 110 lbs can donate whole blood every 56 days. Platelet donation can be done every 7 days (up to 24 times per year), and plasma every 28 days. There are temporary deferrals for recent travel to malaria-endemic areas, certain medications, tattoos (varies by state), and some medical conditions.
The donation process: A whole blood donation takes about 10 minutes of actual collection time (45-60 minutes total including screening and rest). About 1 pint (473 mL) is collected — roughly 10% of your blood volume. Your body replaces the fluid within 24 hours and the red blood cells within 4-6 weeks.
Blood components: Donated whole blood is typically separated into components: red blood cells (stored up to 42 days at 1-6°C), platelets (stored up to 5 days at 20-24°C with agitation), and plasma (frozen and stored up to 1 year). One donation can help up to three patients. Directed donation (donating for a specific person) is possible but not always recommended because it doesn't improve safety and may complicate inventory management.
Greatest needs: Type O-negative is always in critically short supply because it's the universal emergency type but only 7% of people have it. Type O-positive is the most commonly transfused type. Platelet donors of all types are always needed because platelets have a very short shelf life (5 days). During summer and holidays, blood banks often face severe shortages.
Artificial blood: Scientists have been working on synthetic blood substitutes for decades, but none have been approved for general use. The challenges include replicating oxygen-carrying capacity, avoiding immune reactions, and achieving adequate shelf life. Some hemoglobin-based oxygen carriers (HBOCs) have shown promise in clinical trials. Until synthetic blood is viable, donation remains the only source.
Genetics of Blood Type: Beyond ABO
While ABO and Rh are the most clinically important blood group systems, human blood actually has over 300 antigens across 43 recognized blood group systems. Here are some notable ones beyond ABO/Rh:
Kell system: The K antigen can cause severe hemolytic reactions. Kell-negative patients who receive Kell-positive blood may develop antibodies. Kell incompatibility between mother and fetus can cause hemolytic disease of the newborn, similar to Rh incompatibility but rarer. About 9% of Caucasians are Kell-positive.
Duffy system: The Duffy antigen serves as a receptor for the Plasmodium vivax malaria parasite. People who lack the Duffy antigen (Duffy-negative, common in people of African descent — up to 68% in West Africa) are naturally resistant to P. vivax malaria. This is a striking example of natural selection shaping blood group distribution.
MNS system: Contains the M, N, S, and s antigens. Anti-M and anti-N antibodies are usually naturally occurring and rarely cause transfusion reactions, but anti-S can cause hemolytic disease.
Rh system in detail: The Rh blood group is actually far more complex than just "positive or negative." It includes over 50 antigens, with D, C, c, E, and e being the most clinically significant. "Rh-positive" and "Rh-negative" refer only to the D antigen. Patients who need frequent transfusions (e.g., sickle cell disease) require extended matching beyond ABO/Rh to include C, E, and Kell antigens to prevent alloimmunization.
This complexity is why blood banking is a specialized field and why computer cross-matching and antibody screening are performed before every transfusion — even when the ABO and Rh types match perfectly.