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Biology Tools · Updated October 21, 2026 · Reviewed by FormatFlow Editorial

Punnett Square Calculator

You can build a single-gene Punnett square in your browser from two parent genotypes, then read every gamete combination, genotype count and the predicted phenotype ratio.

Runs in your browserFree · No sign-up · No watermark
GENOTYPES → RATIOSRuns in your browser
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Everything runs locally in this tab. No file is uploaded while the tool loads.

This tool runs in your tab. Nothing is uploaded and no copy is stored.

How files are handled

At a glance

Works with
GENOTYPES → RATIOS
Processing
In your browser
Price
Free, no account
Updated
October 21, 2026
All Biology Tools

About Punnett Square Calculator

A Punnett square is the standard way to work out the possible genotypes of offspring from two parents. It lists the gametes each parent can make along the edges, fills every combination in the grid, and turns those combinations into genotype counts and a phenotype ratio. It is a model of segregation, not a forecast of a real family.

This calculator handles one gene with two alleles and writes the dominant allele in uppercase. Enter genotypes such as Aa × Aa, AA × aa or aa × AA, and the grid, counts, fractions and probabilities update as you type. A note under the result states the complete-dominance assumption and that the numbers are theoretical probabilities.

What this tool does

The details that matter when you use Punnett Square Calculator for real work.

  • Two-allele single-gene crosses

    Cross any two genotypes of one letter, including AA × AA, Aa × aa and Aa × Aa; mismatched letters are rejected with a clear message.

  • Grid built from gametes

    The square is drawn from the gametes each parent can pass on, with every combination labelled, so the layout matches the textbook method.

  • Genotype counts and fractions

    AA, Aa and aa are each reported as a count, a reduced fraction of the total combinations and a percentage probability.

  • Phenotype ratio

    Under complete dominance, AA and Aa are grouped as the dominant phenotype and aa as recessive, and the ratio is reduced, such as 3 : 1.

  • Any letter, any case

    bB and Bb give the same cross, and letters from A to Z work, so the calculator is not tied to a single example.

  • Assumptions stated

    The result says plainly that ratios are theoretical probabilities under complete dominance, not predictions of how many children will show a trait.

How to use Punnett Square Calculator

No account, no installation — the whole job happens in this browser tab.
  1. 01

    Enter the first parent's genotype

    Type two alleles of one letter, such as Aa, AA or aa, in the Parent 1 field.

  2. 02

    Enter the second parent

    Use the same letter for Parent 2; the uppercase allele is dominant and the lowercase one recessive.

  3. 03

    Read the Punnett square

    The row and column headers are the gametes, and each cell is the genotype of one equally likely combination.

  4. 04

    Check genotype counts and fractions

    The table lists AA, Aa and aa with counts, reduced fractions and probabilities out of the total combinations.

  5. 05

    Read the phenotype ratio

    The ratio and percentages show how often the dominant and recessive traits are expected under complete dominance.

How this tool works

Works with
GENOTYPES → RATIOS
Processing
Client-side, in your browser. Nothing is uploaded.
Requirements
Chrome, Edge, Firefox or Safari · JavaScript enabled

Limitations worth knowing

  • Models one gene with two alleles and complete dominance; codominance, incomplete dominance, sex-linked traits and multi-gene crosses are outside its scope.
  • Ratios are theoretical probabilities for one offspring and do not predict the outcome of one family, especially with small numbers of children.
  • Both parents' gametes are treated as equally likely, so lethal alleles, meiotic drive and new mutations are not modelled.

Privacy and file handling

The genotypes you type are used only in this browser tab. The square, counts and ratios are calculated with JavaScript on your device, so nothing is uploaded, stored or logged, and closing the page discards the input.

Read the full privacy policy

Punnett Square Calculator FAQ

Short, direct answers to the questions people ask before using this tool.
How do I read a Punnett square?
The gametes of one parent run along the top and the gametes of the other run down the side. Every cell combines one row gamete with one column gamete, so each cell is one equally likely genotype for an offspring. Counting the cells gives the genotype and phenotype ratios.
What is the difference between genotype and phenotype ratios?
The genotype ratio counts each genotype separately: a cross of Aa × Aa gives 1 AA : 2 Aa : 1 aa. The phenotype ratio groups genotypes that look the same under complete dominance, so AA and Aa both count as dominant and the ratio becomes 3 : 1.
What does complete dominance assume?
It assumes that one dominant allele is enough to produce the dominant trait, so the heterozygote looks the same as the homozygous dominant. Incomplete dominance, codominance and traits influenced by several genes do not follow these ratios, so a different model is needed for them.
Can I cross two different letters, such as Aa × Bb?
No. This calculator models a single gene, so both parents must use the same letter. A cross between Aa and Bb involves two genes and needs a larger dihybrid square, which is outside the scope of this tool.
Are these ratios predictions for a real family?
They are theoretical probabilities for each offspring. Each child's genotype is an independent event, so a family of four can easily differ from 3 : 1; the expected ratio only becomes apparent over many offspring.
Which parent should I enter first?
Either order gives the same ratios. Swapping the parents transposes the grid, so the cells appear in a different place but the counts, fractions and phenotype ratio are identical.
How are the fractions simplified?
Counts are divided by the greatest common divisor of the count and the number of equally likely combinations. A cross with four cells gives quarters, one with two cells gives halves, and a cross of two homozygotes fills a single cell.
Are the genotypes I type uploaded anywhere?
No. The square, counts and ratios are calculated with JavaScript in this browser tab, so the genotypes are never uploaded, stored or logged, and closing the page discards them.

Background reading

Format guides that explain the trade-offs behind Punnett Square Calculator.