How we calculate
Every formula our calculators use, with its source and its limits. Each tool page shows the same math with a worked example.
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How do we calculate calories?
The resting energy requirement (RER) is the energy used at rest. The maintenance energy requirement (MER) adjusts it for life stage and activity. Example: a 50 lb dog (22.68 kg) has an RER of about 727 kcal a day. The linear shortcut 30 × kg + 70 is only reasonable between about 2 and 45 kg, so we use the exponent formula for all weights.
| Pet | Starting factor | Range shown |
|---|---|---|
| Dog, neutered adult, typical activity | 1.4 | 1.4–1.6 |
| Dog, intact adult, typical activity | 1.6 | 1.6–1.8 |
| Dog, low activity or obesity-prone | 1.2 | 1.0–1.4 |
| Dog, active (1–3 hours of hard exercise a day) | 1.8 | 1.6–2.0 |
| Dog, working | 2.0 | 2.0–5.0 |
| Dog, senior, typical activity | 1.2 | 1.1–1.4 |
| Dog, senior, inactive | 1.0 | 1.0–1.2 |
| Dog, senior, still active | 1.5 | 1.5–1.7 |
| Dog, weight loss (at ideal weight) | 1.0 | 0.8–1.0 |
| Puppy under 4 months | 3.0 | 3.0 |
| Puppy 4 months to adult | 2.0 | 2.0 |
| Cat, neutered adult | 1.2 | ±10% |
| Cat, intact adult | 1.4 | ±10% |
| Cat, inactive or obesity-prone | 1 | ±10% |
| Cat, weight loss (at ideal weight, vet-supervised) | 0.8 | ±10% |
| Kitten | 2.5 | ±10% |
Dog ranges follow the 2021 AAHA Nutrition and Weight Management Guidelines (Box 1). For a typical neutered pet dog we start at the low end of AAHA’s 1.4–1.6, because the WSAVA calorie chart (about 1.2–1.5 × RER), the NRC figure for inactive pet dogs (95 kcal per kg0.75, about 1.36 × RER) and FEDIAF’s low-activity figure all land there. The Merck Veterinary Manual and Ohio State use 1.6 (neutered) and 1.8 (intact). Senior factors use the NRC’s “about 20% fewer calories than middle-aged adults”, checked against FEDIAF’s figures for dogs over 7. Cat factors are the Merck values (weight loss from AAHA). The neutered cat factor of 1.2 falls inside the WSAVA cat chart, so cats show a ±10% band.
Changed in September 2026: the dog calculators used to start every neutered adult at 1.6 × RER and every intact adult at 1.8. Individual needs can differ by about 30% either way (Merck; WSAVA), or up to 50% (Ohio State), so every result is a starting point to adjust with regular weigh-ins.
How do we turn calories into food?
Calories per cup, per kg and per can come from the calorie statement on US pet food labels (AAFCO). Cups are rounded to the nearest ⅛ cup. We flag values below 150 or above 700 kcal per cup as unusual for dry food. Treats should stay under about 10% of daily calories (WSAVA, AAHA).
How do we estimate adult size for puppies?
Shares are our rounded readings of the median lines of the growth standards published by Salt and colleagues (PLOS ONE, 2017) for five size groups up to 40 kg. The giant row is our own extension, as those curves stop at 40 kg. The range shown is ±15% before 16 weeks, ±12% to 26 weeks and ±8% after that.
| Size | 8 wk | 12 wk | 16 wk | 20 wk | 24 wk | 30 wk | 36 wk | 44 wk | 52 wk | 65 wk | 78 wk | 104 wk |
|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Toy | 32% | 45% | 58% | 70% | 80% | 90% | 95% | 98% | 100% | 100% | 100% | 100% |
| Small | 28% | 40% | 53% | 65% | 75% | 86% | 93% | 97% | 100% | 100% | 100% | 100% |
| Medium | 24% | 35% | 48% | 60% | 70% | 82% | 90% | 95% | 98% | 100% | 100% | 100% |
| Large | 20% | 30% | 42% | 53% | 63% | 75% | 84% | 91% | 95% | 99% | 100% | 100% |
| Extra large | 17% | 26% | 37% | 47% | 57% | 69% | 79% | 87% | 92% | 97% | 100% | 100% |
| Giant | 13% | 20% | 30% | 40% | 49% | 60% | 70% | 80% | 87% | 94% | 98% | 100% |
How do we convert dog years to human years?
We use the size-based chart published by AAHA and Zoetis (adapted from the Dog Owner’s Home Veterinary Handbook), with straight-line steps between birthdays and about 10 human years at 6 months. We also show the DNA-methylation formula from Wang and colleagues (Cell Systems, 2020): human age = 16 × ln(dog age) + 31, for dogs aged 1 and over only, because it was built from Labradors.
| Age | Small (20 lb or less) | Medium (21–50 lb) | Large (51–90 lb) | Giant (over 90 lb) |
|---|---|---|---|---|
| 1 | 15 | 15 | 15 | 15 |
| 2 | 24 | 24 | 24 | 24 |
| 3 | 28 | 28 | 30 | 32 |
| 4 | 32 | 33 | 35 | 37 |
| 5 | 36 | 37 | 40 | 42 |
| 6 | 40 | 42 | 45 | 49 |
| 7 | 44 | 47 | 50 | 56 |
| 8 | 48 | 51 | 55 | 64 |
| 9 | 52 | 56 | 61 | 71 |
| 10 | 56 | 60 | 66 | 78 |
| 11 | 60 | 65 | 72 | 86 |
| 12 | 64 | 69 | 77 | 93 |
| 13 | 68 | 74 | 82 | 101 |
| 14 | 72 | 78 | 88 | 108 |
| 15 | 76 | 83 | 93 | 115 |
| 16 | 80 | 87 | 99 | 123 |
| 17 | 84 | 92 | 104 | – |
| 18 | 88 | 96 | 109 | – |
| 19 | 92 | 101 | 115 | – |
| 20 | 96 | 105 | 120 | – |
Senior stage starts at about the last quarter of expected lifespan (2019 AAHA Canine Life Stage Guidelines). We use practical starting points of 10 years for small, 9 for medium, 8 for large and 6 for giant dogs.
How do we convert cat years to human years?
From the 2021 AAFP/AAHA Feline Life Stage Guidelines chart: 1 month ≈ 1, 3 months ≈ 4, 6 months ≈ 10, 1 year = 15, 2 years = 24, then +4 per year (so 10 years = 56). Life stages: kitten (under 1), young adult (1–6), mature adult (7–10), senior (over 10).
How do we estimate a dog’s due date?
- From breeding: 63 days, range 58–72 days (Merck Veterinary Manual). With several breedings, the window runs from the first date + 58 to the last date + 72.
- From ovulation: 63 days, ±2 (Cornell Riney Canine Health Center).
- From the LH surge: 65 days, range 64–66 (Merck).
How do we plan weight loss?
Dogs: 1.0 × RER at the ideal weight, with a floor of about 0.8 × RER, losing 1–2% of body weight a week (AAHA). Ideal weight from a 9-point body condition score: each point above 5 is about 10% extra weight. Cats: 0.8 × RER at ideal weight, and slower loss, with your vet, because of the risk of hepatic lipidosis.
How do we estimate water intake?
Healthy dogs: about 50–60 mL per kg per day, from all sources including wet food. Consistently more than 100 mL per kg per day is considered excessive and worth a vet visit.
How do we pick a crate size?
Minimum inside size = dog’s length (nose to tail base) + 2 in and height (floor to top of head or ear tips) + 2 in; roomy = + 4 in. We match that to the smallest standard crate whose typical inside length and height are both large enough. Heights vary by brand, so check the model’s listed inside height.
| Crate | Length (in) | Height (in) |
|---|---|---|
| Extra small (18") | 18 | 14 |
| Extra small (22") | 22 | 16 |
| Small (24") | 24 | 19 |
| Small–medium (30") | 30 | 21 |
| Medium (36") | 36 | 25 |
| Large (42") | 42 | 30 |
| Extra large (48") | 48 | 33 |
| XXL (54") | 54 | 45 |
What don’t we calculate?
We don’t offer medication doses or toxicity calculators. Those need a vet who knows your pet. See our medical disclaimer and editorial policy.