Blood Type Compatibility Calculator
Who Can I Donate To?
Check transfusion compatibility, pregnancy Rh risk, and blood type inheritance for all 8 blood types (A+, A-, B+, B-, AB+, AB-, O+, O-). Instant results with WhatsApp share and free PDF report.
Green = compatible for red blood cell transfusion. Red = incompatible. Plasma and platelet compatibility follows different rules.
The 8 Blood Types Explained
Knowing your blood type is one of the most practically important pieces of medical information you can have. In an emergency situation, hospitals may need to transfuse blood before testing is complete -- O- blood is given in these situations precisely because it is safe for everyone. Beyond emergencies, blood type matters for planned surgeries, pregnancy planning, and for potential blood donation. Use this calculator alongside our Blood Pressure Checker and Blood Sugar Checker for a complete cardiovascular health picture.
The ABO Blood Group System
The ABO blood group system is determined by the presence or absence of A and B antigens on the surface of red blood cells, and the corresponding antibodies in the plasma:
Type A
Has A antigens on red blood cells and anti-B antibodies in plasma. Can donate to A and AB recipients. Can receive from A and O donors. Found in approximately 30% of the population.
Type B
Has B antigens on red blood cells and anti-A antibodies in plasma. Can donate to B and AB recipients. Can receive from B and O donors. Found in approximately 9% of the population.
Type AB
Has both A and B antigens and no ABO antibodies. Universal recipient -- can receive from any ABO blood type. Can only donate to AB recipients. AB+ is the rarest common blood type at about 3% of the population.
Type O
Has neither A nor B antigens and has both anti-A and anti-B antibodies. Universal donor for red cells -- can donate to any ABO type. Can only receive from O donors. O is the most common blood type globally at about 44% of the population.
The antibodies in your plasma are why incompatible blood types cause transfusion reactions. If you receive blood with antigens your body does not recognize as its own, your immune system attacks the donated red blood cells, causing a potentially fatal hemolytic transfusion reaction. This is why ABO compatibility is the most critical step in blood transfusion medicine.
Rh Factor -- What It Means for You
The Rh factor (also called Rh antigen or D antigen) is a protein found on the surface of red blood cells. Rh positive (Rh+) means this protein is present; Rh negative (Rh-) means it is absent. The Rh system creates the positive/negative distinction for each ABO type, giving us the 8 blood types we recognize today.
Key facts about Rh factor:
- Rh positive is dominant -- approximately 85% of people globally are Rh positive
- Rh negative individuals do not naturally have anti-Rh antibodies, but they develop them when exposed to Rh positive blood
- Unlike ABO incompatibility, a first Rh-incompatible transfusion may not cause a severe immediate reaction, but sensitizes the recipient so that a second exposure causes a much more serious reaction
- In blood transfusion, Rh positive blood should never be given to an Rh negative recipient unless absolutely necessary
- Rh incompatibility in pregnancy is a significant clinical concern requiring specific prevention and monitoring
Monitor your cardiovascular health alongside your blood type knowledge with our Heart Rate Calculator, Cholesterol Checker, and Pulse Pressure Calculator.
Blood Transfusion Compatibility
Blood transfusion compatibility depends primarily on two factors: ABO compatibility and Rh compatibility. For red blood cell transfusions:
- O- (Universal Donor): Can donate to all 8 blood types. Used in emergencies before typing is complete. Only about 7% of people are O-
- AB+ (Universal Recipient): Can receive from all 8 blood types. Both A and B antigens present so no ABO antibodies. Rh positive so accepts Rh+ and Rh- blood
- Rh negative recipients should only receive Rh negative blood to avoid sensitization
- Rh positive recipients can receive both Rh positive and Rh negative blood safely
Note: These compatibility rules apply specifically to packed red blood cell transfusions. Plasma and platelet transfusions follow different, sometimes reversed, compatibility rules. Always confirm with a qualified healthcare provider or blood bank before any transfusion. Check your blood pressure and blood sugar regularly as part of overall blood health monitoring.
Rh Incompatibility in Pregnancy
Rh incompatibility in pregnancy occurs when an Rh negative mother carries an Rh positive baby (the baby inheriting the Rh+ gene from the father). During pregnancy and especially during delivery, small amounts of fetal blood can cross into the mother's circulation. In an Rh- mother carrying an Rh+ baby, this exposure causes the mother's immune system to develop anti-Rh antibodies.
The first pregnancy is usually unaffected because sensitization takes time. However, in a second pregnancy with an Rh+ baby, the mother's pre-formed anti-Rh antibodies can cross the placenta and attack the baby's red blood cells, causing Hemolytic Disease of the Newborn (HDN) -- potentially serious and life-threatening.
Prevention is highly effective. Rh immunoglobulin (RhIg, brand name RhoGAM) given to Rh- mothers at 28 weeks of pregnancy and within 72 hours of delivery prevents sensitization in the vast majority of cases. Rh incompatibility is a routine part of antenatal care -- all pregnant women have their blood type and Rh status tested at the first antenatal visit. Use our Pregnancy Calculator and Pregnancy Weight Gain Calculator alongside this Rh check during your pregnancy planning.
How Blood Type Is Inherited
Blood type is determined by genes inherited from both parents. The ABO gene has three alleles: I-A (produces A antigen), I-B (produces B antigen), and i (produces no antigen). I-A and I-B are codominant; both are dominant over i:
- Type A: Genotype I-A I-A or I-A i
- Type B: Genotype I-B I-B or I-B i
- Type AB: Genotype I-A I-B (receives one A allele and one B allele)
- Type O: Genotype ii (receives O allele from both parents)
The Rh factor is inherited separately. The Rh positive gene (D allele) is dominant over Rh negative (d allele). If you carry even one D allele, you are Rh positive. You must inherit the d allele from both parents to be Rh negative. Two Rh negative parents will always have Rh negative children. Two Rh positive parents can have Rh negative children if both parents carry the d allele.
Blood type inheritance is relevant not only for family planning but also for determining the biological relationship between parents and children. Use our Baby Height Predictor and Baby Weight Percentile Calculator for additional child health planning.
Blood Type Rarity and Why Donation Matters
Blood type frequencies vary significantly by ethnicity and geographic region. Approximate global frequencies:
- O+ : 38% (most common globally)
- A+ : 30%
- B+ : 9%
- AB+ : 3%
- O- : 7% (most critical for emergency medicine)
- A- : 7%
- B- : 2%
- AB- : 1% (rarest of the 8 main types)
O negative blood is in perpetual high demand because it is the universal donor used in all emergency situations, yet only 7% of the population has it. Blood banks consistently call for O- donors because stocks are depleted fastest. If you are O-, regular blood donation is one of the highest-impact health contributions you can make. Support your blood health with our Iron Intake Calculator -- iron is essential for red blood cell production and recovery after donation.
