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Blood Type Calculator
Use this free blood type calculator to find out which blood types your child could inherit. Enter both parents' blood type and Rh factor, and the calculator works out every possible child blood type with its probability, using ABO and Rh Punnett squares — the same genetics squares taught in biology class. It also shows a compatibility chart of who can donate blood to whom and who can receive from whom.
| Possible child blood type | Probability |
|---|---|
| A+ | 18.8% |
| A− | 6.3% |
| B+ | 18.8% |
| B− | 6.3% |
| AB+ | 18.8% |
| AB− | 6.3% |
| O+ | 18.8% |
| O− | 6.3% |
How the blood type calculation works
Your blood type is inherited through two separate systems: ABO and Rh. The ABO system is controlled by one gene with three possible alleles — A, B, and O — and you inherit one allele from each parent. Alleles A and B are dominant, and O is recessive, which means an A or B allele always shows up in the blood type, while an O allele only shows when paired with another O.
This calculator uses the standard textbook Punnett square: it treats a type A parent as AO, a type B parent as BO, and a type AB parent as AB — the most informative assumption when the parent's exact genotype is unknown. Each parent passes one allele at random, giving four equally likely combinations. For a type A mother and a type B father, the combinations are AB, AO, BO, and OO — child types AB, A, B, and O, each with a 25 percent chance.
A and B are dominant; O is recessive
Rh positive (D) is dominant over Rh negative (d)
The Rh factor works the same way: one dominant allele D for Rh positive, one recessive allele d for Rh negative. A Rh positive person carries DD or Dd, and a Rh negative person carries dd. This calculator treats a Rh positive parent as Dd, so two Rh positive parents have a 75 percent chance of an Rh positive child and a 25 percent chance of an Rh negative child — the classic three-to-one ratio. A Rh negative parent can only pass d, so two Rh negative parents always have Rh negative children.
The two systems combine independently: the ABO probability and the Rh probability multiply. A mother who is A positive and a father who is B positive have a 25 percent chance of each ABO type and a 75 percent chance of Rh positive, so an AB positive child has a 25 percent times 75 percent chance, or 18.75 percent, and an AB negative child has a 25 percent times 25 percent chance, or 6.25 percent.
Worked example
Take a mother who is A positive and a father who is B positive.
Step 1: The mother's alleles are A and O; the father's are B and O.
Step 2: The four allele combinations are AB, AO, BO, and OO, giving child types AB, A, B, and O — 25 percent each.
Step 3: Both parents are Rh positive, treated as Dd, so the Rh combinations are DD, Dd, Dd, and dd — 75 percent positive, 25 percent negative.
Step 4: Combine the two systems: each ABO type at 25 percent times Rh positive at 75 percent gives 18.8 percent for A+, B+, AB+, and O+; 25 percent times 25 percent gives 6.3 percent for A−, B−, AB−, and O−.
So all eight blood types are possible for this couple, with the four Rh positive types most likely at 18.8 percent each and the four Rh negative types at 6.3 percent each.
Blood type compatibility chart
Compatibility depends on antigens, the markers on red blood cells. Type A blood carries the A antigen, type B carries B, type AB carries both, and type O carries neither. The immune system attacks any antigen it does not recognize, so a recipient can only receive blood whose antigens they already carry — including Rh.
| Blood type | Can donate to | Can receive from |
|---|---|---|
| O− | Everyone | O− only |
| O+ | O+, A+, B+, AB+ | O−, O+ |
| A− | A−, A+, AB−, AB+ | O−, A− |
| A+ | A+, AB+ | O−, O+, A−, A+ |
| B− | B−, B+, AB−, AB+ | O−, B− |
| B+ | B+, AB+ | O−, O+, B−, B+ |
| AB− | AB−, AB+ | O−, A−, B−, AB− |
| AB+ | AB+ only | Everyone |
This chart is for red blood cells. Plasma compatibility runs the other way around, because plasma carries antibodies instead of antigens: type AB plasma can be given to anyone, while type O plasma can only go to type O recipients.
FAQ
Can two type O parents have a type A child?
No. Type O parents both carry two O alleles, so they can only pass O to their child, and the child must be type O. A child can only have a blood type built from the alleles the parents carry: type A needs an A allele from a parent, and type B needs a B allele. If a child's lab-tested type does not fit the parents' types, the test should be repeated — a testing error is the most likely explanation.
What blood type will my child have if I am type A and my partner is type B?
Your child could be any of the four ABO types: A, B, AB, or O. Each parent can pass either of their two alleles, so the combinations are AB, AO, BO, or OO. Under the standard Punnett square used by this calculator, each of the four types has a 25 percent chance. The Rh factor then splits each of those: if both parents are Rh positive, the child is usually Rh positive, with about a 25 percent chance of Rh negative.
What is the universal blood donor and the universal recipient?
Type O negative is the universal donor: its red blood cells carry no A, B, or Rh antigens, so they can be given to anyone. Type AB positive is the universal recipient: its red cells carry all three antigens, so it can receive from any type. For plasma the roles reverse, because plasma carries antibodies rather than antigens: AB plasma can go to anyone, while O plasma can only go to type O recipients.
Can a child's blood type be different from both parents?
Yes. Two type A parents can have a type O child, because each type A parent may carry a hidden O allele and pass it on. Two Rh positive parents can have an Rh negative child for the same reason. What cannot happen is a child having an allele neither parent carries: two type O parents cannot have a type A or B child, and two Rh negative parents cannot have an Rh positive one.
Is this blood type calculator accurate enough to use instead of a blood test?
No. This calculator gives the possible types and their probabilities from the parents' types, which is useful for curiosity and learning genetics, but it cannot tell you a specific person's blood type. Rare blood groups, weak antigens, and testing errors all limit it. A person's blood type must be confirmed by a laboratory blood test, and any transfusion decision is based on a lab test and a crossmatch, never on a calculator.