Centimorgan to Percentage Converter

The Centimorgan to Percentage Converter converts Centimorgan to Percentage and helps interpret recombination frequencies in genetic linkage and mapping analyses.

What Is a Centimorgan to Percentage Converter?

A centimorgan is a unit that expresses how likely a crossover is to occur between two points on a chromosome. In family matching, testing companies sum the cM across all shared segments to describe how closely two people are related. A converter helps translate those cM values into a percentage that is easier to compare and communicate.

There are two common “percentages” you might want. The first is percent of DNA shared between two people, used in genealogy. The second is percent recombination between two genetic markers, used in linkage and mapping. The converter supports both, because both start with centimorgans but describe different things.

Formulas for Centimorgan to Percentage

There are two main formula sets. One estimates the percent of the genome that two people share. The other converts cM between two loci into a recombination percentage. Choose the one that matches your question.

• Percent DNA shared (genealogy): percent_shared = 100 × (shared_cM ÷ total_autosomal_cM)
• Haldane mapping (no interference): r = 0.5 × (1 − e^(−2d)), where d is in Morgans (cM ÷ 100); percent_recomb = 100 × r
• Kosambi mapping (with interference): r = 0.5 × tanh(2d), with d in Morgans; percent_recomb = 100 × r
• Small-distance approximation: for short intervals, percent_recomb ≈ cM (because 1 cM ≈ 1% recombination)
• Capping rule: recombination fraction cannot exceed 50% (r ≤ 0.5), so percent_recomb ≤ 50

For percent shared DNA, the “total_autosomal_cM” depends on the reference map used by a testing company, typically about 7,000–7,600 cM. For recombination, select Haldane or Kosambi based on whether you want to model crossover interference. At small distances, both mapping functions converge and 1 cM is roughly 1% recombination.

How to Use Centimorgan to Percentage (Step by Step)

Decide what percentage you need before you type a number. Are you describing how much DNA you share with a cousin, or how often two markers recombine? The steps differ slightly, and so do the assumptions.

• For percent DNA shared, gather the total shared cM from your testing site and pick a total genome length.
• For percent recombination, convert cM to Morgans and choose a mapping function (Haldane or Kosambi).
• Compute the percentage and check whether your result is within expected ranges for the scenario.
• Compare the result to known relationship ranges or mapping expectations to interpret it.

If you are not sure which method to choose, use “Percent DNA shared” for relatives and “Percent recombination” for marker distances. You can switch modes and re-run the numbers in seconds.

Inputs, Assumptions & Parameters

The converter accepts a few core inputs and uses common biological assumptions to give sensible results. Understanding these will help you choose settings and read the output correctly.

• Shared centimorgans (cM): the total cM you share with a match, or the cM distance between two markers.
• Total autosomal map length (cM): default 7,100–7,600 cM; pick a value consistent with your testing platform.
• Mode: percent DNA shared (genealogy) or percent recombination (mapping).
• Mapping function: Haldane or Kosambi for recombination; not used for shared DNA percentage.
• Include/exclude X chromosome: optional, since many match totals are autosomes only.
• Minimum segment threshold: optional filtering of tiny segments (e.g., under 6–10 cM) if you want stricter matching.

Inputs should be nonnegative. For recombination, the output will be between 0% and 50%. For percent DNA shared, typical relative matches are between 0% and about 50% (parent–child averages near 50%). Very small segments may be noise, and endogamy can inflate totals.

How to Use the Centimorgan to Percentage Converter (Steps)

Here’s a concise overview before we dive into the key points:

1. Choose your mode: “Percent DNA shared” or “Percent recombination.”
2. Enter the centimorgan value (shared cM or marker distance).
3. If using shared DNA, enter the total autosomal cM you want as a reference.
4. If using recombination, choose Haldane or Kosambi and the converter will handle cM to Morgan conversion.
5. Click Convert to see the percentage and any notes on expected ranges.
6. Review the interpretation hint against known relationship ranges or map behavior.

These points provide quick orientation—use them alongside the full explanations in this page.

Real-World Examples

You find a DNA match who shares 3,487 cM with you across many segments. You set total_autosomal_cM to 7,100 cM, matching a common reference. Percent shared = 100 × (3,487 ÷ 7,100) ≈ 49.1%. This lines up with a parent–child or full-sibling relationship and is far above typical cousin ranges. What this means: You likely share a first-degree relationship with this person.

A lab reports that two markers are 30 cM apart on a chromosome. Convert 30 cM to 0.30 Morgans. Haldane: r = 0.5 × (1 − e^(−0.6)) ≈ 22.6% recombination; Kosambi: r = 0.5 × tanh(0.6) ≈ 26.9% recombination. For mapping with interference, Kosambi might be preferred; both remain well below the 50% limit. What this means: These loci recombine in roughly one out of four to five meioses.

Limits of the Centimorgan to Percentage Approach

Both conversions simplify complex biology. They are excellent for quick estimates, but they do not cover every nuance. It helps to know where the approach can mislead you and when to seek more detail.

• Percent DNA shared depends on a chosen total cM; different companies use different reference lengths.
• Endogamy and pile-up regions can inflate shared cM, exaggerating percentage estimates for distant matches.
• Mapping functions are models; Kosambi includes interference, Haldane does not, and real data can vary by region.
• X and mitochondrial DNA follow different inheritance rules, so autosomal assumptions may not hold.
• Small segments (e.g., under 6–10 cM) are more likely to be false positives, altering totals and percentages.

Use percentages as guides, not proofs. Pair them with segment counts, longest segment sizes, and known relationship ranges. When decisions matter, confirm with additional relatives, triangulation, or laboratory validation.

Units Reference

Units keep your calculations consistent. Centimorgans measure genetic map length, while percentages describe either recombination or shared DNA. The table below clarifies what each unit means and how it is used in this context.

Read the table left to right: choose the unit that matches your data, then use the notes to convert. For genealogy, divide shared cM by a total like 7,100 cM. For mapping, convert cM to Morgans and apply a mapping function to get r, then multiply by 100 for a percentage.

Common Issues & Fixes

Most issues come from mixing contexts or using inconsistent references. A quick review of settings usually fixes them.

• Using centimeters (cm) instead of centimorgans (cM): check the unit label before entering values.
• Percent DNA shared over 50%: reduce the total genome cM or verify the match total and whether X is included.
• Implausible recombination over 50%: confirm you used a mapping function and capped r at 0.5.
• Inflated totals from many tiny segments: apply a minimum segment threshold (e.g., 6–10 cM).
• Mismatch with relationship expectations: compare against known ranges and add other evidence (e.g., longest segment).

If results still look off, try an alternate total cM reference, switch mapping functions, or isolate autosomes only. When in doubt, test with a known relationship to sanity-check your setup.

FAQ about Centimorgan to Percentage Converter

What is a centimorgan, and why does it relate to a percentage?

A centimorgan is a unit of genetic map distance. For small distances, 1 cM corresponds to about a 1% chance of recombination between two loci, which is why percentages appear in mapping.

How do I choose the “total autosomal cM” for percent DNA shared?

Use the reference your testing service implies, commonly around 7,100–7,600 cM. You can try 7,100 cM for Ancestry-like references or 7,400–7,600 cM for others, and note the small difference in output.

What is the difference between percent DNA shared and percent recombination?

Percent DNA shared is how much of the autosomal genome two people share. Percent recombination is how often two loci swap during meiosis. Both use cM, but they answer different questions.

Can percent DNA shared identify the exact relationship?

No. It narrows possibilities. Combine it with total cM, number of segments, longest segment, age, and family information to pinpoint the relationship.

Key Terms in Centimorgan to Percentage

Centimorgan (cM)

A unit of genetic distance tied to recombination probability; for small intervals, roughly equal to a 1% recombination chance.

Morgan (M)

The base unit of genetic length, equal to 100 centimorgans, used in mapping functions and genetic maps.

Recombination Fraction (r)

The probability that a crossover occurs between two loci in one meiosis; ranges from 0 to 0.5.

Percent DNA Shared

The proportion of the autosomal genome two individuals share, computed from shared cM divided by a reference total cM.

Haldane Mapping Function

A model that assumes no interference among crossovers, using r = 0.5 × (1 − e^(−2d)), where d is in Morgans.

Kosambi Mapping Function

A model that accounts for crossover interference, using r = 0.5 × tanh(2d), where d is in Morgans.

Autosome

Any non-sex chromosome; autosomal DNA is commonly used for relationship matching and shared cM totals.

Segment Threshold

A minimum cM length used to filter out small, likely spurious segments when summing shared DNA.

References

Here’s a concise overview before we dive into the key points:

• ISOGG Wiki: CentiMorgan
• 23andMe: What is a centimorgan?
• DNA Painter: The Shared cM Tool
• Learn.Genetics (Utah): Genetic Mapping
• Nature Education: Centimorgan definition

These points provide quick orientation—use them alongside the full explanations in this page.