Enter the percentage, centimorgan, and total centimorgan into the calculator to determine the missing variable.
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Centimorgan to Percentage Formula
The following formula is used to calculate the percentage of centimorgan for a given total centimorgan.
P = (c / T) * 100
Variables:
- P is the percentage
- c is the centimorgan
- T is the total centimorgan
To calculate the percentage, divide the centimorgan by the total centimorgan and multiply the result by 100.
| Percentage (%) | Centimorgan (cM) |
|---|---|
| 0.10 | 6.8 |
| 0.25 | 17.0 |
| 0.50 | 34.0 |
| 0.75 | 51.0 |
| 1 | 68.0 |
| 1.5 | 102.0 |
| 2 | 136.0 |
| 2.5 | 170.0 |
| 3 | 204.0 |
| 4 | 272.0 |
| 5 | 340.0 |
| 6.25 | 425.0 |
| 7.5 | 510.0 |
| 10 | 680.0 |
| 12.5 | 850.0 |
| 15 | 1020.0 |
| 20 | 1360.0 |
| 25 | 1700.0 |
| 30 | 2040.0 |
| 50 | 3400.0 |
| Assumes total autosomal length approx 6800 cM. Formula: cM = (Percentage/100) x Total cM; Percentage = (cM / Total cM) x 100. | |
What Is a Centimorgan?
A centimorgan (cM) is a unit of genetic linkage that quantifies the probability of a crossover event occurring between two chromosomal loci during meiosis. One centimorgan represents a 1% chance per generation that two markers on the same chromosome will be separated by a recombination event. The unit was introduced in 1913 by Alfred Henry Sturtevant and honors his mentor Thomas Hunt Morgan, whose work first established that genes are physically arranged on chromosomes in a linear order.
Centimorgans measure probability rather than physical length. On average, one centimorgan corresponds to roughly one megabase (1,000,000 base pairs) in the human genome, but this ratio varies substantially by location. Recombination hotspots driven by the PRDM9 protein compress many physical base pairs into a short centimorgan span, while pericentromeric regions near chromosome centers can stretch across tens of megabases without generating a single crossover, contributing almost nothing to centimorgan distance.
The Human Genome in Centimorgans
The total length of the human autosomal genome in centimorgans differs between males and females because recombination rates are sexually dimorphic. Females produce substantially more crossover events during oogenesis than males do during spermatogenesis. Published genetic maps place the female autosomal map at roughly 4,460 cM and the male map at roughly 2,710 cM. When transmissions from both sexes are averaged, the result is the 6,800 to 7,200 cM totals used by consumer DNA testing platforms. This sex asymmetry means that chromosomal segments inherited through female lineages are statistically more fragmented than those passing through male lineages over the same number of generations.
Each consumer DNA testing company selects a different set of SNP markers and references a different published genetic map, such as HapMap or the deCODE map derived from Icelandic family data. This is one reason why the same pair of individuals can receive slightly different cM totals depending on which platform they tested at. This calculator defaults to 6,800 cM as the denominator, but the advanced tab allows you to enter a custom total if your testing provider uses a different reference value.
Shared cM and Family Relationships
Converting centimorgans to a percentage identifies approximately how much of the genome two people share, which narrows the probable genealogical relationship. A parent and child always share approximately 50%, or roughly 3,400 cM, because all autosomal DNA in a child comes from exactly one copy of each parental chromosome pair. Beyond the parent-child level, expected sharing approximately halves with each additional generational step, though actual observed values vary because recombination is a stochastic process.
The ranges below are drawn from the Shared cM Project, which aggregated empirical data from thousands of verified relationship pairs across major testing platforms.
| Relationship | Shared cM Range | Approx. % (6800 cM base) |
|---|---|---|
| Parent / Child | ~3,400 cM (fixed ~50%) | ~50% |
| Full Sibling | 2,300 to 3,900 cM | 34 to 57% |
| Half Sibling | 1,160 to 2,450 cM | 17 to 36% |
| Grandparent / Grandchild | 1,156 to 2,311 cM | 17 to 34% |
| Aunt / Uncle / Niece / Nephew | 1,156 to 2,311 cM | 17 to 34% |
| First Cousin | 575 to 1,330 cM | 8 to 20% |
| Half First Cousin | 215 to 650 cM | 3 to 10% |
| Second Cousin | 41 to 592 cM | 0.6 to 8.7% |
| Third Cousin | 0 to 294 cM | 0 to 4.3% |
| Fourth Cousin | 0 to 139 cM | 0 to 2% |
| Ranges represent observed variation across thousands of verified relationship pairs. Many relationships share overlapping cM ranges, so DNA results should always be interpreted alongside documentary genealogical research. | ||
Why cM Values Differ Across Testing Platforms
The same biological match can produce materially different cM totals at different testing companies. Four factors account for most of the divergence. First, companies use different reference genetic maps; the HapMap-derived maps, the deCODE map built from Icelandic family trios, and other published maps each assign different centimorgan coordinates to the same physical chromosome positions. Second, the number and selection of genotyped SNPs varies by platform, affecting which shared segments are actually detectable.
Third, AncestryDNA applies a proprietary algorithm called Timber that downweights shared segments that are common across individuals of the same ancestral background, reasoning that widespread population-level sharing does not reflect recent genealogical connection. This can reduce the apparent cM total for some matches compared to raw segment analysis or results from other platforms. Fourth, each company sets its own minimum segment threshold for calling a match, and segments below that threshold are simply not reported, further reducing the observable cM total for distant relationships.
Limitations of Centimorgans for Relationship Identification
Shared cM establishes how much of the genome two people share, but it does not uniquely identify the relationship. A match of roughly 1,700 cM is consistent with a grandparent-grandchild pair, a half-sibling pair, an aunt or uncle with a niece or nephew, or a double first cousin pair, all of which produce similar expected DNA amounts. Distinguishing between these possibilities requires comparing family trees, examining which specific chromosome segments are shared, and sometimes testing additional relatives.
The randomness of recombination also means that two verified second cousins might share anywhere from 41 to 592 cM despite having the same genealogical distance. At the distant end of the relationship spectrum, beyond approximately the fourth cousin level, the cM ranges for adjacent relationship categories overlap so heavily that DNA evidence alone cannot separate them. Beyond the seventh cousin level, any detectable shared segments may reflect deep population-level ancestry common to a regional gene pool rather than a specific recent common ancestor.
Because of this, genetic genealogists treat shared cM as one data point rather than a definitive answer. The most reliable analyses combine cM totals with shared segment locations, matching third-party relatives through triangulation, and documentary evidence from vital records and family trees.