How we calculate

Every tool’s formula, where it comes from, and where we made a judgement call.

Dog age

Size-based chart (years 1–16) as published by US veterinary groups: 15 human years at one, 24 at two, then 4 (small) to about 7 (giant) a year. Between whole years we interpolate; under one year we split the first 15 years in a straight line; past 16 we continue each size’s last step. We also show human age = 16 × ln(dog age) + 31 (Wang et al., 2020).

Cat age

15 human years at one, 24 at two, then 4 a year (AAHA/AAFP). Life stages from the 2021 AAHA/AAFP Feline Life Stage Guidelines: kitten under 1, young adult 1–6, mature adult 7–10, senior over 10.

Puppy weight

Logistic growth curve, weight = A ÷ (1 + e^{−(age − x0) ÷ b}). Per size class we average the fitted x0 and b of the breeds in a published study (small: Yorkshire terrier, toy poodle; medium: fox terrier, teckel, basset hound, beagle; large: golden, Labrador, Weimaraner, German shepherd). Giant breeds weren’t studied: we use x0 = 26 weeks and b = 10, our assumption. Adult weight = current weight ÷ the curve’s share at that age; the range is ±25% under 12 weeks, ±15% to 20 weeks and ±10% after.

Dog and cat food

RER = 70 × kg^0.75. Daily calories = RER × factor (Pet Nutrition Alliance). Dogs: neutered 1.6, intact 1.8, inactive 1.2–1.4 (we use 1.3), weight loss 1.0, puppies 3.0 under 4 months and 2.0 after (the PNA gives 2–3 for growth). Cats: neutered 1.2, intact 1.4, inactive 1.0, weight loss 0.8–1.0, kittens 2.5 (PNA 2–3). Cups = calories ÷ the label’s kcal per cup. Treats ≤10% of calories.

Dog gear sizes

Crate: dog length + 4 in, rounded up to the next standard size (18, 22, 24, 30, 36, 42, 48, 54 in); height + 4 in. Collar: neck + 2 in. Harness: chest + 2 in. Bed: length + 8–12 in. These are rules of thumb from makers’ size guides.

Aquarium

Geometry: in³ ÷ 231 = US gallons; litres = gallons × 3.785; UK gallons = gallons × 0.8327; water weight = gallons × 8.34 lb. Water volume subtracts the gap below the rim and the substrate depth across the full footprint. Heater 3–5 W per gallon and filter 4–6 turnovers an hour are hobby rules of thumb.

Reptile tanks, cages, coops and litter boxes

Minimums are copied from the cited sources, unchanged, and compared dimension by dimension with yours (length and width in either order). “Gallons of space” = L × W × H ÷ 231. Litter boxes: one per cat plus one; box ≥ 1.5 × body length.

Raw dog food

Adults: 2% (sedentary), 3% (average), 4% (high) or 5% (working) of body weight a day, +1 point under 10 lb, never below 2% (Hepper, vet-reviewed). Puppies: share of current weight by age, 10% at 8–10 weeks to 3% at 56–68 weeks (Raw & Fresh feeding guide, an industry source). Optional calorie check: daily kcal from our dog food method ÷ label kcal per kg.

Pet insurance

Payout = (bill − annual deductible) × reimbursement, capped at the annual limit. Break-even bills = deductible + yearly premiums ÷ reimbursement. Average premiums: NAPHIA State of the Industry 2025, US accident and illness, year-end 2024.

Pregnancy due dates

Dogs: 63 days from mating on average, normal 57–65 (VCA). Cats: 63–65 days, range 60–69 (University of Illinois ANSC 207; we use 64 as the midpoint). Checks: palpation from about day 21, ultrasound best days 25–35, relaxin test in dogs after day 22–27, x-ray from about day 42–45 (Merck Veterinary Manual).

What we don’t build

No symptom checkers, toxicity calculators (chocolate, grapes, xylitol) or medicine doses. Those need a vet who can examine your pet; if you’re worried, call your vet or an animal poison helpline now.