Methodology & sources
Every calculator on this site shows its working, and this page shows where the working comes from. If you ever find a number here that disagrees with a calculator, that's a bug; tell us and we'll fix it.
Our approach
- Primary sources only. Formulas and thresholds come from veterinary organizations and peer-reviewed research (AAHA, WSAVA, Merck Veterinary Manual, published studies), never from other calculator sites. Every tool lists its references at the bottom of its page.
- Estimates, labeled as estimates. Biology ignores neat formulas. Where a source gives a range, our calculators use the midpoint and the page shows the range. The Pet Nutrition Alliance puts it plainly for calorie math: individual animals can vary from predicted values by as much as 50%.
- Honest credentials. We are pet people and product builders, and we cite the veterinarians rather than impersonating them. Nothing here is veterinary advice, and no page claims "vet-reviewed" status. When a named veterinary reviewer joins, you'll see them credited by name.
- Freshness. Every page carries a visible updated date, and we re-verify numbers on a rolling basis, at minimum once a year per page.
Age calculators
The dog age calculator uses the size-adjusted veterinary chart pattern: 15 human years for year one, 9 for year two, then a per-year increment by breed size (small +4, medium +5, large +6, giant +7). As a second opinion it offers the epigenetic formula from Wang et al. (2020, Cell Systems):human_age = 16 × ln(dog_age) + 31, derived from DNA methylation in Labrador Retrievers and valid from age 1 up.
The cat age calculator follows the standard feline chart used by AAHA/AAFP guidance and International Cat Care: 15 for year one, 24 by year two, then 4 per year, with a steeper curve under 12 months. Life stages follow the 2021 AAHA/AAFP Feline Life Stage Guidelines. The kitten age chart triangulates weight (Alley Cat Allies progression data), tooth eruption (Merck), and eye/ear milestones.
Pregnancy calculators
The dog pregnancy calculator uses 63 days from mating (normal spread 58 to 68, since mating date rarely equals ovulation date), with milestones per the Merck Veterinary Manual: ultrasound from day 25, relaxin from day 28, skeletal X-rays from day 45, temperature checks from day 58. The cat pregnancy calculator uses 64 days (range 60 to 67); cats are induced ovulators, so mating-based dates run tighter. All date math is computed in UTC days so daylight saving never shifts a milestone.
Growth: puppy adult-weight estimates
The puppy weight calculator divides current weight by the fraction of adult weight typical for the age and breed size, interpolated from size-specific growth curves (a medium puppy is about 50% grown at 16 weeks; a giant puppy about 34%). Anchors are consistent with published growth standards (Salt et al. 2017, PLOS ONE; WALTHAM growth charts). The likely range moves the growth fraction six percentage points each way.
Calories and food portions
The calorie calculators (cat,dog) use the standard veterinary formulas: resting energy requirementRER = 70 × (weight in kg)^0.75, multiplied by a life-stage factor per the Pet Nutrition Alliance table. Weight-loss factors apply to ideal body weight. The portion calculators (cat,dog) divide the daily calories by the food's labeled kcal per can or per cup.
| Life stage | Dog | Cat |
|---|---|---|
| Neutered adult | 1.6 | 1.2 |
| Intact adult | 1.8 | 1.4 |
| Inactive / overweight-prone | 1.3 | 1 |
| Weight loss (of ideal weight) | 1 | 0.9 |
| Growth, under 4 months | 3 | 3 |
| Growth, 4 months to grown | 2 | 2 |
| Gestation | 1.8 | 2.5 |
| Lactation | 3 | 3 |
Where the source states a range (inactive dogs 1.2 to 1.4, cat weight loss 0.8 to 1.0, growth 2 to 3, lactation 2 to 6), the calculator uses the value above and the tool page shows the range.
Primary sources
- Pet Nutrition Alliance:Calculating Calories Based on Pet Needs
- WSAVA Global Nutrition Committee:Global Nutrition Guidelines
- AAHA/AAFP:2021 Feline Life Stage Guidelines and 2019 Canine Life Stage Guidelines
- Merck Veterinary Manual:Whelping and Queening,Reproduction of Cats,Dental Development of Cats
- Wang T, et al. (2020):Quantitative Translation of Dog-to-Human Aging, Cell Systems
- Salt C, et al. (2017):Growth standard charts for dogs, PLOS ONE
- Alley Cat Allies:Kitten Progression: At-a-Glance
Corrections
Found an error, a stale number, or a source we should be using?Contact us; corrections jump the queue.