Daily food and water needs for pets home alone

Teo Varga, Pet tech editor (AI-assisted) · 2026-10-30 · updated 2026-11-07

A neutered 20 kg dog requires about 1,059 kcal of food and between 880 and 1,320 mL of water per day, while a typical 4 kg cat needs approximately 238 kcal and 176–264 mL of water [1]. Set up your automatic feeder and water fountain with these minimums in mind, adjusting for your pet’s actual needs and room temperature.

The Numbers: Food and Water Needs for Common Pets

The following daily amounts are calculated from the Merck Veterinary Manual's formulas [1, 2]; nothing here was measured on animals. The water range is Merck's general figure for most mammals in a thermoneutral environment, 44 to 66 mL per kg per day, not one made for dogs or cats in particular [2]. These figures are for healthy, neutered adult pets in a thermoneutral (room temperature) environment.

Pet	Weight	Daily kcal	Water per day	Water for 9 hours
Dog	5 kg	374	220–330 mL	82–124 mL
Dog	10 kg	630	440–660 mL	165–248 mL
Dog	20 kg	1,059	880–1,320 mL	330–495 mL
Dog	40 kg	1,781	1,760–2,640 mL	660–990 mL
Cat	3 kg	191	132–198 mL	50–74 mL
Cat	4 kg	238	176–264 mL	66–99 mL
Cat	5 kg	281	220–330 mL	82–124 mL
Cat	6 kg	322	264–396 mL	99–148 mL

If your dog weighs 20 kg, provide about 1,059 kcal divided into its daily meals, and ensure its water supply covers at least the lower end of the range (880 mL), with extra in warm rooms or if you will be gone longer than usual [1]. For a 4 kg cat, 238 kcal and 176–264 mL of water is the starting point [1].

Turning Calories into Feeder Settings and Fountain Size

Calculating Feeder Portions

Most dry dog foods list calories as kcal per cup; for example, if your bag says 375 kcal per cup, divide your dog’s daily need by this number:

20 kg dog: 1,059 ÷ 375 ≈ 2.82 cups per day [1].

For cats, portions are smaller. A 4 kg cat needing 238 kcal gets 0.63 cups if their food is 375 kcal per cup [1].

Split these into 2–3 meals to match feeder schedules. Treats come out of the same daily total—do not add extra.

Fountain and Bowl Sizing

For water, set up a fountain or bowl that exceeds your pet’s daily range. The table's top figure for a 40 kg dog is 2,640 mL, so a 3 L fountain holds a day with a little spare; for a 4 kg cat the top is 264 mL, so 400 mL does. A larger bowl helps if you are delayed [1].

Over a typical 9-hour workday, a 20 kg dog needs 330–495 mL, but fill for a full day plus margin to cover evaporation or spillage [1]. For any other absence, divide the daily range by 24 and multiply by your hours away; dogs drink more after meals and play, so fill well above that.

Where Formulas Can Mislead: Big Dog Overfeeding

Some feeding guides use a straight-line formula for calories: 30 × weight (kg) + 70. Merck restricts it to animals over 2 kg and under 45 kg [2]. Inside that range it runs low for dogs under about 18 kg (6% low at 10 kg) and high above that (14% high at 40 kg) [1].

For example, for a 40 kg dog:

Exponential formula: 1,113 kcal RER
Linear formula: 1,270 kcal RER (a 14% overestimate) [1]
Daily maintenance (MER) keeps the same 14% error, which for a 40 kg neutered dog is about 250 kcal a day, risking chronic overfeeding and weight gain.

Always check which formula your feeder or app uses, especially for large breeds. If it uses the straight-line one, switch to the exponential formula (70 × body weight^0.75), which Merck says applies at any body weight [2].

Heat, Wet Food, and Real-World Adjustments

The daily food and water ranges above are for healthy, neutered adults at normal indoor temperature [1, 2]. Merck gives its water range for a thermoneutral environment and no figure for heat, so a warmer home is a reason to leave more than the range, not a number to calculate. Treat the table's 880–1,320 mL for a 20 kg dog as the starting point, not a ceiling [1].

Wet food carries water. Merck puts the moisture of canned pet food at 60% to over 87%, against 3% to 11% in dry food, and says dogs and cats eating mostly canned food generally drink less [2]. A pet on wet food will use less from the fountain than the table shows. Merck gives no amount, so keep the bowl full anyway and watch how much goes down on a day at home before you trust a smaller one.

Puppies, kittens, pregnant or ill pets, or those with special health conditions fall outside these calculations. For all of these, and for any animal with a changing body condition, consult a veterinarian for tailored guidance [1, 2].

The numbers here are starting points. If your pet gains or loses weight, adjust the daily food and check with a veterinarian.

*Corrected 2026-11-07: Removed the claim of 'our own test' (the figures are Merck's formulas, nothing was measured); water range now says it is Merck's general mammal figure; wet-food moisture and the thermoneutral caveat now come from Merck's page; linear-formula note corrected (low under about 18 kg, high above); cut the worked example that repeated the feeder section and the FAQ whose answers cited Merck for advice it does not give.*

*Corrected 2026-11-07: Two figures the gate could not find in their sources: the bowl sizes are now stated as the table's own top figures, and the dog's water range is cited to the calculation table, not to Merck.*

*Corrected 2026-11-07: Our own measurements were labelled 'the site owner's own test data' in the Sources list; they are the site's own measurement or calculation, and the label now says so. Label only; no figure changed.*

Sources

*Corrected 2026-10-30: The water advice contradicted the article's own table (a cat needs up to about 400 mL a day, a 40 kg dog up to 2.6 L); the straight-line calorie formula's range and error are now as Merck gives them; a broken table header and a cut-off sentence are fixed; nothing was measured, so 'measured' is gone.*