Guinea Pig Age Calculator

A 2-year-old guinea pig is about 35 in human years, a full-grown adult

There is no official chart for converting guinea pig years to human years. We could not find one in veterinary references or from animal hospitals and welfare charities, and the tables that circulate online, such as "1 month = 14, 1 year = 50" or "20 in the first year, then 12 more every year", do not say how they were made. So this calculator takes a different route. It matches three milestones in a guinea pig's life that have sources behind them with the same milestones in an American life, then joins them with straight lines. Sexual maturity in females at 66 days (AnAge database) is matched to puberty, using 12.5 years, the average age at first period reported by US women. The midpoint of the 5–6 year average lifespan, 5.5 years (VCA Animal Hospitals and the UK's RSPCA), is matched to US life expectancy at birth, 79.0 years in 2024. The longest record, 14 years 10.5 months (Guinness World Records), is matched to the human record of 122 years 164 days.

On that basis, a 2-year-old guinea pig is about 35 in human years, in the adult stage with 1 year to go before middle age begins at 3. Enter an age, a birthday or an adoption date in the calculator above to see the human age, the life stage and the time left until the 5–6 year average lifespan range. Guinea pigs should be kept in groups of two or more, so you can enter up to 4 at once, and the life bar shows where each one is right now. The life milestones under the bar walk through what has already passed and what is still ahead for the guinea pig you select.

When a human-age estimate is useful

A guinea pig's first months pass very quickly. Putting the age in human terms shows what needs attention at stages that are easy to think of as "still a baby."

• Not missing the time to separate males and females: Pups should not leave the mother before 2–3 weeks, but right after that males and females must be separated. Males start mounting females at 3–4 weeks, and females can become pregnant from 7–8 weeks (VCA). By this calculator, a 2-month-old is about 12 in human years.
• Knowing the right window for a female's first litter: A female's pelvis fuses firmly at about 8 months, so giving birth can become difficult if she has never had a litter by then (VCA). Japan's Ministry of the Environment gives 7–8 months. If you do not plan to breed, keeping males and females apart comes first.
• Watching health as age advances: Gastric dilatation-volvulus is more common after age 2, tumors usually appear from age 3 (middle age), and hyperthyroidism develops after age 3 (Merck). Age 3 is about 48 in human years.
• Comparing the guinea pigs that live together: VCA recommends keeping at least two together, and the RSPCA says a guinea pig on its own gets lonely. When the ages differ, one may be an adult and another a senior, and what to watch for differs accordingly.

How the calculation works

1. Three pairs of anchor points: (guinea pig 66 days, human 12.5 years), (guinea pig 5.5 years, human 79.0 years) and (guinea pig 14 years 10.5 months, human 122 years 164 days), plus birth at (0, 0). A month is counted as 365.25 ÷ 12 days (30.44 days), so 66 days is 2.17 months.
2. Straight lines between the points: between 66 days and 5.5 years, human age = 12.5 + (guinea pig months − 2.17) ÷ (66 − 2.17) × (79.0 − 12.5). For 2 years (24 months) that is 12.5 + 21.83 ÷ 63.83 × 66.5 = 35.2, which rounds to 35.
3. Past 5.5 years: the line from 5.5 years (79.0) to 14 years 10.5 months (122.4) is gentler, adding about 4½ human years per guinea pig year. Beyond 14 years 10.5 months the same slope is extended and the result is labeled "extended past the longevity record." The calculator goes up to 15 years 11 months.
4. Life stages: Baby (birth to 1 month), Juvenile (1 to 8 months), Adult (8 months to 3 years), Middle age (3 to 5 years), Senior (5 to 6 years) and Long-lived (6 years and up). The juvenile stage ends at 8 months, when a female's pelvis fuses (VCA), and middle age begins at 3 years, which Merck calls middle-aged. We found no veterinary standard for when a guinea pig becomes a senior, so the 5–6 year average lifespan range is used as the senior stage and the period after it as the long-lived stage.
5. Entering a birthday or adoption date: the age is completed months up to today plus the fraction of the current month that has passed. With an adoption date, the calculator subtracts the age at adoption (7 days per week, or the same date for months) to get an estimated birthday. For a February 29 birthday, March 1 counts as the birthday in non-leap years.
6. The axis of the life bar: so much happens in the first 6 months that the left 30% of the bar stretches birth to 6 months, and the other 70% covers 6 months to 8 years (or to the end of the next year once the oldest guinea pig reaches 7).

Human age by guinea pig age

The first 66 days are the straight line from birth to puberty, so 1 month is 6 and 2 months is 12. From 66 days to 5.5 years, each month adds a little over 1 human year (about 1.04). After 5.5 years, each month adds about 0.39.

How long do guinea pigs live? Sources disagree

VCA and the RSPCA both give an average of 5–6 years, and this calculator matches the midpoint, 5.5 years, to human life expectancy. The 4.03 years from the VetCompass study is a different kind of number. It is the median age at death of guinea pigs that had a veterinary record, and 73.78% of the deaths were by euthanasia. Only guinea pigs that saw a vet were counted, and sick animals are over-represented, so the figure can come out shorter than the average lifespan of all pet guinea pigs. In the same study females died later than males (4.58 years against 3.74 years), and the difference was statistically significant.

What to keep in mind at each life stage

Baby (birth to 1 month, human 0–6)

A guinea pig is born with its eyes and ears open and a full coat, and eats solid food from the start (Merck). A litter is 2–4 pups, and the pups also nurse (VCA). Do not separate them from the mother before 2–3 weeks, and separate males from females right after that (VCA). Guinea pigs cannot make vitamin C in their bodies, so growing pups need more than 30 mg/kg a day (Merck).

Juvenile (1 to 8 months, human 6–19)

AnAge gives sexual maturity at 66 days for females and 76 days for males. VCA puts sexual maturity at about 3–4 months while warning that females can become pregnant from 7–8 weeks. Do not keep one alone; keep two or more together, but watch for fighting and breeding when intact males are housed together or males and females share a cage (Merck). At about 8 months, when a female's pelvis fuses, this stage ends.

Adult (8 months to 3 years, human 19–48)

Adult weight is 900–1,200 g for males and 700–900 g for females (VCA). The teeth grow throughout life, so 75% of the diet should be grass hay such as timothy (VCA). An adult needs 10–25 mg/kg of vitamin C a day (Merck). In UK veterinary records the most common diagnosis was overgrown nails (26.55%) (VetCompass), and gastric dilatation-volvulus is more common after age 2 (Merck).

Middle age (3 to 5 years, human 48–73)

Tumors are uncommon in guinea pigs but usually appear at age 3 (middle age) or older, and hyperthyroidism usually develops after age 3 (Merck). The most common reproductive disease in females is ovarian cysts (Merck). In UK records, guinea pigs diagnosed with weight loss had a median age of 4.64 years, the oldest of the common diagnoses (VetCompass).

Senior (5 to 6 years, human 73–81)

This is the average lifespan range of 5–6 years (VCA, RSPCA). In older guinea pigs the scent gland on the back near the tail can clog with secretions and make the coat look greasy, more often in males (VCA). Older intact males are prone to impacted feces in the rectal pouch (Merck). In UK records the most common causes of death were loss of appetite (13.87%) and collapse (9.81%) (VetCompass), so see a vet right away if your guinea pig eats less.

Long-lived (6 years and up, human 81 and up)

This is past the average lifespan range. VCA says some guinea pigs live 8–10 years, AnAge gives a maximum lifespan of 12 years in captivity, and the Guinness record is 14 years 10.5 months. In UK veterinary death records the oldest age was 10 years (VetCompass).

Comparing the ages of guinea pigs that live together

Merck says guinea pigs usually do best in groups of two or three, and VCA adds that keeping a single guinea pig is against the law in some countries. Guinea pigs that live together often joined the household at different times, so their ages can be far apart. Use "Add another" in the calculator above to enter up to 4, and the life bar draws one line for each on the same axis, with the human age at the end of the bar. Select a name or a bar and that guinea pig becomes the one shown in the result above and in the life milestones below. A name is optional; if you enter one, it appears in the result sentence and in the copied result.

For example, if a 2-year-old guinea pig lives with one that is 5 years 3 months old, the first is about 35 in human years and an adult, and the second is about 76 and a senior. They share a cage, but one needs more attention to nails and gastric dilatation-volvulus, and the other to appetite and weight changes.

Common mistakes

• Trusting a conversion chart that gives no source: Omni Calculator treats 1 month as 14, 1 year as 50 and 5 years as 140, and LumenCalculator treats 1 year as 20 and then adds 12 for each later year. The same 1-year-old comes out as 20 or 50 depending on the chart, and neither cites a veterinary source. This calculator is also an estimate, but every anchor point it uses is listed above.
• Setting a different lifespan for each breed: We found no veterinary source for breed-specific lifespans, such as a shorter life for hairless breeds. That is why this calculator has no breed setting.
• Entering the adoption date as the birthday: If your guinea pig was already a few weeks old when you brought it home, the age comes out that much too young. If you don't know the birthday, choose "Adopted" and enter the age you were told at adoption in weeks or months.
• Treating the average lifespan and the median age at death as the same thing: The 4.03 years from VetCompass is the median age of guinea pigs that died with a veterinary record, so it is shorter than the 5–6 year average lifespan.
• Separating males and females too late: Males start mounting at 3–4 weeks and females can become pregnant from 7–8 weeks (VCA). A 2-month-old is about 12 in human years, but for a guinea pig it is already an age at which breeding is possible.

Limitations

• The human age is an estimate that lines up three points in life and joins them with straight lines. It does not mean that guinea pigs and people age at the same rate, and it does not match body condition or disease risk exactly.
• The human anchor points use US figures: an average age at first period of 12.5 years, reported by women aged 15–44 surveyed in 2013–2017, and life expectancy at birth of 79.0 years in 2024. Using another country's figures changes the human age a little for the same guinea pig.
• The life stages follow milestones from veterinary sources, and each guinea pig's condition and rate of aging differ.
• Values beyond the record of 14 years 10.5 months extend the slope of the last segment, so they rest on weaker ground.
• This calculator does not diagnose anything. If appetite, weight or droppings change, take your guinea pig to a vet who treats exotic animals, whatever its age. Your entries are calculated in this browser only and are never sent to a server.

Sources

• 01MSD/Merck Veterinary Manual: Anatomy, Physiology, and Behavior of Guinea PigsRevised May 2026; born with eyes and ears open and a full coat and eating solid food, teeth grow throughout life, do best in groups of two or three, impacted rectal pouch in older intact males
• 02MSD/Merck Veterinary Manual: Housing and Nutrition of Guinea PigsRevised May 2026; vitamin C 10–25 mg/kg a day for adults, 30 mg/kg or more for growing, pregnant, nursing and sick guinea pigs
• 03MSD/Merck Veterinary Manual: Noninfectious Diseases of Guinea PigsRevised May 2026; hyperthyroidism usually after age 3, gastric dilatation-volvulus more common after age 2, tumors usually at middle age (3 years) or older, ovarian cysts the most common reproductive disease in females
• 04VCA Animal Hospitals: Guinea Pigs - OwningUpdated Mar 14, 2019; average 5–6 years (8–10 also seen), sexual maturity at 3–4 months, males mount from 3–4 weeks, females can be pregnant from 7–8 weeks, no separation from the mother before 2–3 weeks and males and females split right after, pelvis fuses at about 8 months, males 900–1,200 g and females 700–900 g, keep at least two
• 05RSPCA: Guinea pigsTypically 5–6 years, lonely when kept alone and happiest in pairs
• 06O'Neill DG et al. (2024), Demography, commonly diagnosed disorders and mortality of guinea pigs under primary veterinary care in the UK in 2019: A VetCompass study, PLoS OneMedian age at death 4.03 years for the 674 of 757 deaths with a recorded age (females 4.58, males 3.74), euthanasia 73.78%, causes of death loss of appetite 13.87% and collapse 9.81%, most common diagnosis overgrown nails 26.55%
• 07AnAge (Human Ageing Genomic Resources): Cavia porcellusMaximum longevity 12 years (captive), sexual maturity at 66 days for females and 76 days for males, gestation 68 days
• 08Guinness World Records: Oldest guinea pig everSnowball of the UK, died Feb 14, 1979 at 14 years 10.5 months, average lifespan 4–8 years
• 09Japan Ministry of the Environment, 参考資料5: 動物取扱業における犬猫以外の哺乳類の飼養管理基準の細部解釈と運用指針(素案)Japanese draft guidance; the long gestation and large fetuses cause frequent toxemia and difficult births, and births are often difficult if the first breeding is not done by 7–8 months
• 10Guinness World Records — Oldest person everJeanne Louise Calment (France), 122 years 164 days, died Aug 4, 1997
• 11CDC National Center for Health Statistics — Mortality in the United States, 2024 (NCHS Data Brief No. 548)Life expectancy at birth 79.0 years in 2024 (males 76.5, females 81.4), published January 2026 — checked 2026-10-11
• 12CDC National Center for Health Statistics — Trends and Patterns in Menarche in the United States: 1995 through 2013–2017 (National Health Statistics Reports No. 146)Mean age at menarche 12.5 years and median 11.9 years among women aged 15–44, National Survey of Family Growth 2013–2017, published Sep 10, 2020 — checked 2026-10-11

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