How much food and water to set up for a pet home alone all day: our calculation from the veterinary formulas, 2026-10-30

The answer first (our calculation [1] from the Merck Veterinary Manual's formulas [2])

• A neutered 20 kg dog needs about 1,059 kcal a day and 880-1,320 mL of water; over a 9-hour absence that is 330-495 mL of the day's water.
• A neutered 4 kg cat needs about 238 kcal a day and 176-264 mL of water (66-99 mL over 9 hours).
• A neutered 40 kg dog needs about 1,781 kcal and up to 2,640 mL of water a day.
• Big-dog warning: the simple straight-line formula overestimates a 40 kg dog's resting needs by 14.1% (1,270 vs 1,113 kcal); the two formulas agree within about 6% from 3 to 18 kg and cross exactly at 3.04 and 18.37 kg. A feeder app or label that uses the straight line overfeeds large dogs.

The formulas (Merck Veterinary Manual [2], quoted)

• Resting energy requirement: "RER = 70 [body weight in kg0.75]" (exponential) and "RER = 30 x [body weight in kg] + 70" (linear).
• Daily maintenance (MER): neutered adult dog "1.6 x RER", intact adult dog "1.8 x RER", neutered adult cat "1.2 x RER"; obesity-prone dogs "1.4 x RER", cats "1 x RER".
• Water: "In a thermoneutral environment, most mammalian species need ~44-66 mL/kg body weight."

Worked check, 20 kg dog: 20^0.75 = 9.457; x 70 = 662 kcal RER; x 1.6 = 1,059 kcal a day. Linear: 30 x 20 + 70 = 670 kcal RER.

The table (our calculation [1]; neutered adults)

| Pet | Weight | RER, exponential (kcal) | RER, linear (gap) | Daily kcal (MER) | Water a day | Water over 9 h | Cups of food at 375 kcal/cup | | dog | 5 kg | 234 | 220 (-6.0%) | 374 | 220-330 mL | 82-124 mL | 1.0 | | dog | 10 kg | 394 | 370 (-6.0%) | 630 | 440-660 mL | 165-248 mL | 1.68 | | dog | 20 kg | 662 | 670 (+1.2%) | 1059 | 880-1,320 mL | 330-495 mL | 2.82 | | dog | 30 kg | 897 | 970 (+8.1%) | 1436 | 1,320-1,980 mL | 495-742 mL | 3.83 | | dog | 40 kg | 1113 | 1270 (+14.1%) | 1781 | 1,760-2,640 mL | 660-990 mL | 4.75 | | cat | 3 kg | 160 | 160 (+0.3%) | 191 | 132-198 mL | 50-74 mL | 0.51 | | cat | 4 kg | 198 | 190 (-4.0%) | 238 | 176-264 mL | 66-99 mL | 0.63 | | cat | 5 kg | 234 | 220 (-6.0%) | 281 | 220-330 mL | 82-124 mL | 0.75 | | cat | 6 kg | 268 | 250 (-6.8%) | 322 | 264-396 mL | 99-148 mL | 0.86 | "Water over 9 h" is the daily range x 9/24: it assumes drinking spread evenly, which it is not (pets drink more after eating and exercise). "375 kcal per cup" is an example; the bag states its own kcal per cup or per 100 g: divide the daily kcal by it.

Setting up a feeder and fountain (from the figures above)

• Split the day's kcal into the feeder's meals; treats come out of the same total.
• A fountain or bowl for a 40 kg dog should hold well over the 9-hour share (990 mL) plus spillage and evaporation; for a cat, 200 mL covers even the high end of the full day.
• Heat changes everything: the water figure is for a "thermoneutral" room; a hot day or an AC failure raises needs sharply.

What this cannot tell you

• These are starting points for healthy adults; puppies, kittens, pregnancy, illness, age and activity change them. A vet sets the number for a given pet.
• Wet food supplies much of a cat's water; the water figure is total intake, not bowl intake.
• Body condition is the real check: adjust portions if the pet gains or loses weight.

Sources

[2] Merck Veterinary Manual, "Nutritional Requirements of Small Animals", https://www.merckvetmanual.com/management-and-nutrition/nutrition-small-animals/nutritional-requirements-of-small-animals, read 2026-10-30.