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Dog Age Calculator

Convert your dog's age to human years using the common size-based rule: 15 for year one, 9 for year two, then 4, 5 or 6 a year.

What this tool does

Convert your dog's age to human years using the common size-based rule: 15 for year one, 9 for year two, then 4, 5 or 6 a year. You fill in these fields: Dog's age and Dog size. It shows human age equivalent and human years per dog year at this age.

Inputs

  • Dog's age (years): A number. Accepts 0 to 30. Use decimals for part years, such as 0.5 for six months.
  • Dog size: Choose one: Small (20 lb or less), Medium (21 to 50 lb), Large (over 50 lb). Starts as Medium (21 to 50 lb).

How to use

  1. Enter the dog's age in years.
  2. Choose Small (20 lb or less), Medium (21 to 50 lb), or Large (over 50 lb) for "Dog size".
  3. Read the result: human age equivalent and human years per dog year at this age. It appears once every required field has a valid value.

Worked examples

Example 1. Inputs:

  • Dog's age: 5 years
  • Dog size: Medium (21 to 50 lb)

Results:

  • Human age equivalent: 39 years
  • Human years per dog year at this age: 5

How this is worked out: Rule of thumb: 15 + 9 = 24 for the first two years, then 5 per year for a medium dog. 24 + (5 - 2) * 5 = 39.

Example 2. Inputs:

  • Dog's age: 10 years
  • Dog size: Large (over 50 lb)

Results:

  • Human age equivalent: 72 years
  • Human years per dog year at this age: 6

How this is worked out: 24 + (10 - 2) * 6 = 24 + 48 = 72.

Example 3. Inputs:

  • Dog's age: 0.5 years
  • Dog size: Small (20 lb or less)

Results:

  • Human age equivalent: 7.5 years
  • Human years per dog year at this age: 15

How this is worked out: First year counts 15 human years: 0.5 * 15 = 7.5.

Example 4. Inputs:

  • Dog's age: 1.5 years
  • Dog size: Large (over 50 lb)

Results:

  • Human age equivalent: 19.5 years
  • Human years per dog year at this age: 9

How this is worked out: 15 for year one plus half of the 9 for year two: 15 + 0.5 * 9 = 19.5.

Limitations

  • A rule-of-thumb estimate, not a veterinary or scientific measurement.
  • Three size bands only. Breed, health and weight within a band are not considered.
  • The result stops at 30 dog years of age.
  • This tool gives an estimate. It is based on a general model, not on an exact published formula, so a real figure can differ from the result.

FAQs

Q: How are dog years turned into human years?
A: The first year counts as 15 human years and the second adds 9, giving 24 at age two. After that each dog year adds 4 for a small dog, 5 for a medium dog or 6 for a large dog.
Q: Why does size matter?
A: Larger dogs generally have shorter lives, so the rule counts each of their years as more human years.
Q: Is this exact?
A: No. There is no single scientific conversion. This is a widely used rule of thumb, so treat the result as a rough guide.
Q: Can I enter a part year?
A: Yes. Use decimals, such as 0.5 for six months or 7.25 for seven years and three months.

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