Resting Energy Requirement (RER)

The foundation of every calorie result is the Resting Energy Requirement — the energy an animal needs at rest. We use the allometric formula recommended by the National Research Council:

The exponent of 0.75 reflects the fact that smaller animals burn more energy per pound than larger ones. This is why a 10 lb Chihuahua needs more calories per pound than a 100 lb Great Dane.

From RER to Daily Energy (MER)

Maintenance Energy Requirement (MER) is RER multiplied by a series of factors that reflect the individual animal:

• Life-stage factor — puppies and kittens (growth) require 2–3× RER; adults 1.2–1.6×; seniors slightly less.
• Activity factor — from sedentary (1.2×) up to working/very active (2.0×+).
• Breed metabolic factor — each breed carries a multiplier (e.g. French Bulldog 1.02×, Persian cat 0.93×) reflecting documented metabolic and brachycephalic differences.
• Health adjustments — neuter status, overweight/underweight, diabetes, and kidney disease each apply a clinically-aligned adjustment.

The "multiply by 7" rule is scientifically inaccurate. Dogs and cats age rapidly in their first two years and then more slowly, and the rate depends heavily on body size. BreedCalc uses size-banded aging curves derived from veterinary longevity research:

Larger breeds accumulate "human years" faster in later life, which is why a 5-year-old Great Dane is biologically older than a 5-year-old Chihuahua. Each breed page also references peer-reviewed work on the canine epigenetic clock.

Puppy adult-weight prediction uses breed-specific growth curves. The core principle is that puppies reach a predictable fraction of their adult weight at a given age, which varies by size category:

• Small breeds reach ~99% of adult weight by 10–12 months.
• Medium breeds by 12–16 months.
• Large & giant breeds may keep growing until 18–24 months.

We estimate adult weight from current age and weight against the breed's growth curve, then present a confidence range to reflect natural variation within a breed.

Canine gestation averages 63 days (range 58–68) from ovulation; feline gestation averages 65 days (range 58–67). From the mating date, our calculator projects a week-by-week timeline with key milestones:

• Embryo implantation — days 19–21
• Heartbeat detectable by ultrasound — day 25
• Skeletal development / sex determination — day 35
• Expected whelping/queening date — day 63 (dogs) / 65 (cats)

Estimated litter size is drawn from breed-typical averages and should be confirmed by your veterinarian via ultrasound or radiography.