# Dog years and cat years calculator

> Dog years and cat years calculator: a pet's age in human years from the AAHA/Zoetis size chart or the International Cat Care table, for fun comparisons.

Interactive version: https://www.calcopenly.com/everyday/dog-and-cat-age-in-human-years
Subject: Everyday calculators

A dog's age is read off the AAHA/Zoetis physiological age chart for its adult weight (up to 20 lb, 21–50 lb, 51–90 lb or over 90 lb) and a cat's off the International Cat Care table, with straight-line interpolation between rows. For dogs there is also the DNA methylation formula of Wang et al. (2020): human age = 16 ln(dog age) + 31.

On the chart every dog counts as 15 at one year and 24 at two. After that larger dogs age faster, so a 7-year-old medium dog is 47 while a 7-year-old giant dog is 56. Cats count as 15 at one year, 24 at two and 4 more for each year after, so a 10-year-old cat is 56.

Human-year figures are a way to picture life stage, not a health check, and they come from population charts rather than from your own pet. The methylation formula was fitted on 104 Labrador retrievers and ages young dogs faster than the chart does.

## Inputs

- **Pet** (options: Dog, Cat)
- **Age**
- **Age in** (options: Years, Months)
- **Adult weight** (options: Small (up to 20 lb / 9 kg), Medium (21–50 lb / 9.5–22.7 kg), Large (51–90 lb / 23–40.8 kg), Giant (over 90 lb / 40.8 kg))
- **Method** (options: Size chart (AAHA), DNA methylation: 16 ln(age) + 31)

## Results

- Age in human years (years) — main result
- By the size chart (years)
- Life stage

## Formula

$$
\text{dog (DNA methylation)} = 16\ln(\text{age}) + 31,\qquad \text{chart ages are interpolated linearly between rows}
$$

## Worked examples

### Medium dog, 7 years

- Pet: Dog
- Age: 7
- Age in: Years
- Adult weight: Medium (21–50 lb / 9.5–22.7 kg)
- Method: Size chart (AAHA)
- **Age in human years: 47 years**
- Checked against: AAHA/Zoetis 2019 chart: 7 years, 21–50 lbs = 47

### Giant dog, 10 years

- Pet: Dog
- Age: 10
- Age in: Years
- Adult weight: Giant (over 90 lb / 40.8 kg)
- Method: Size chart (AAHA)
- **Age in human years: 78 years**
- Checked against: AAHA/Zoetis 2019 chart: 10 years, >90 lbs = 78

### Small dog, 3½ years (between rows)

- Pet: Dog
- Age: 3.5
- Age in: Years
- Adult weight: Small (up to 20 lb / 9 kg)
- Method: Size chart (AAHA)
- **Age in human years: 30 years**
- Checked against: AAHA/Zoetis chart rows 28 (3 y) and 32 (4 y), midpoint

### Large dog at the last chart row

- Pet: Dog
- Age: 20
- Age in: Years
- Adult weight: Large (51–90 lb / 23–40.8 kg)
- Method: Size chart (AAHA)
- **Age in human years: 120 years**
- Checked against: AAHA/Zoetis 2019 chart: 20 years, 51–90 lbs = 120

### Dog, 7 years, DNA methylation formula

- Pet: Dog
- Age: 7
- Age in: Years
- Adult weight: Medium (21–50 lb / 9.5–22.7 kg)
- Method: DNA methylation: 16 ln(age) + 31
- **Age in human years: 62.1 years**
- **By the size chart: 47 years**
- Checked against: Wang et al. 2020: 16 ln 7 + 31, Python decimal

### Kitten, 6 months

- Pet: Cat
- Age: 6
- Age in: Months
- **Age in human years: 10 years**
- **Life stage: Kitten**
- Checked against: International Cat Care chart: 6 months = 10 years, kitten stage

## Questions

### Is one dog year really seven human years?

No. On the AAHA/Zoetis chart every dog reaches the equivalent of 15 human years at age one and 24 at age two, then ages 4 to 8 human years a year depending on size. A 10-year-old dog is 56 if it weighs 20 lb or less but 78 if it weighs over 90 lb; the seven-year rule gives 70 for both.

### How old is my cat in human years?

International Cat Care counts a cat's first two years as 24 human years (6 months is about 10 and 1 year about 15) and each year after that as 4. A 10-year-old cat is 56 and a 15-year-old is 76, the start of the super senior stage. It puts the average lifespan at around 12 to 14 years, with some cats reaching their late teens or early twenties.

### What is the DNA methylation formula for dog age?

Human age = 16 × ln(dog age) + 31, from Wang et al. (2020) in Cell Systems, who compared DNA methylation in 104 Labrador retrievers with human data. It ages puppies quickly, since a 1-year-old dog comes out at 31, then slows: a 7-year-old is 62 and a 12-year-old 71. It was fitted on one breed, so it is a rough guide for others.

### When is a cat considered senior?

International Cat Care calls cats senior from 11 to 14 years and super senior from 15 years. The earlier stages are kitten (up to 6 months), junior (7 months to 2 years), adult (3 to 6 years) and mature (7 to 10 years). An 11-year-old cat equals about 60 human years on its chart.

### What are the life stages of a dog?

The 2019 AAHA Canine Life Stage Guidelines use five: puppy, young adult, mature adult, senior and end of life. When each stage starts depends on the dog's size and breed, and the size chart shows why: a giant dog of 10 already equals 78 human years, while a small dog of the same age equals 56.

### How accurate is the dog years and cat years calculator?

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 8 worked examples whose answers come from independent sources; for example, “Medium dog, 7 years” is checked against AAHA/Zoetis 2019 chart: 7 years, 21–50 lbs = 47.

### Where does the method come from?

AAHA / Zoetis — My pet's physiological age (cat and dog age chart, 2019); International Cat Care — How to tell your cat's age in human years; Wang T. et al. (2020). Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome. Cell Systems 11(2), 176–185.

## Sources

- [AAHA / Zoetis — My pet's physiological age (cat and dog age chart, 2019)](https://www.aaha.org/wp-content/uploads/globalassets/02-guidelines/canine-life-stage-2019/canine_and_feline_age_chart_poster.pdf)
- [International Cat Care — How to tell your cat's age in human years](https://icatcare.org/advice/how-to-tell-your-cats-age-in-human-years/)
- [Wang T. et al. (2020). Quantitative translation of dog-to-human aging by conserved remodeling of the DNA methylome. Cell Systems 11(2), 176–185](https://doi.org/10.1016/j.cels.2020.06.006)

_Note: entertainment information, not professional advice._