A little science.
Every assumption, shared.

Sources & assumptions

Published charts and coverage

For cats, we use the FelineVMA 2025 chart: 1 month ≈ 1 human year, 3 months ≈ 4, 6 months ≈ 10, 1 year ≈ 15 and 2 years ≈ 24, then four more human years per cat year, through age 25. For dogs, we store the year-by-year values from the AAHA-hosted Zoetis 2019 chart for small, medium, large and giant adult sizes. Coverage ends at age 20 for the first three sizes and 16 for giant dogs.

Our interpolation assumptions

We use straight lines between chart points. The birth origin (0, 0) is our assumption, and interpolation before a dog’s first birthday is not a puppy-development curve specified by the source. Other veterinary references use different approximations; there is no single universal pet-age formula.

The equality equation and its limits

We solve F(p + t) = h + t on every linear segment and deduplicate shared-boundary roots. Both beings must have been born, the pet must be within chart coverage, and human age must be between 0 and 120. Coincident parallel segments return intervals rather than a made-up single date. No crossing means none within the current coverage, not “never.”

Dates, rounding and precision

Birth-date ages use the actual number of days between anniversaries. February 29 uses February 28 in non-leap years. Predictions advance both ages on one common elapsed-year axis, converting decimal years to approximate dates using 365.2425 days per year. They do not re-evaluate biological age on each future calendar day. Age-only inputs do not produce exact dates. Display values are rounded.

The progress display is not a lifespan bar

Both bars share one human-year scale whose maximum is based on the explorable timeline. A chart’s last age is a coverage boundary, never an expected lifespan or a prediction of time remaining.