How much water should a dog drink?

The standard veterinary guidance is fifty to sixty millilitres of water per kilogram of body weight per day, for drinking water alone. A twenty-kilogram dog therefore needs about one to one-point-two litres of drinking water a day; a ten-kilogram dog needs about half a litre to six hundred millilitres; a forty-kilogram dog needs two to two-point-four litres. This calculator starts from that range and adjusts it: more for active days and hot weather (up to one-and-a-half times), less when much of the diet is wet food, because wet food is seventy to eighty percent water and supplies a large part of the daily need. The result is a range, not a single figure, because a dog's water need changes every day with the weather, the exercise and the food.

It helps to understand where the water goes. A dog loses water through panting, urine, faeces and evaporation from the skin, and it gains water from drinking, from food and from metabolism itself — burning food produces water as a by-product. On a normal day the losses and gains roughly balance at the fifty to sixty millilitre figure. On a hot day or after hard exercise, panting increases dramatically and the losses can double or triple, which is why a dog that has been running in the sun can drink a litre in a few minutes and still need more. Puppies need more water per kilogram than adults because they are growing and their kidneys are less efficient at concentrating urine. Senior dogs also need ready access to water, because kidney disease — common in older dogs — makes them lose more water in urine.

Wet food changes the maths

A dog eating wet food gets most of its water from its food and drinks noticeably less from the bowl. That is normal, and it is not a sign of illness — it is arithmetic. A four-hundred-gram can of wet food contains about three hundred millilitres of water, which is a quarter of a twenty-kilogram dog's daily need in a single can. A dog eating dry food, by contrast, gets almost no water from its food and must drink all of it. The calculator adjusts for this: mostly wet food multiplies the drinking-water figure by about 0.7, and a mixed diet by about 0.85. If your dog eats wet food and drinks little from the bowl, do not worry — add up the food and the bowl together before deciding there is a problem.

When to offer more — and when to worry

Offer more water in hot weather, after exercise, when the dog is panting, and when it is eating dry food only. Always have fresh water available, and take water on walks in summer — a collapsible bowl weighs nothing and can prevent heatstroke. The signs of dehydration are a dry mouth, sunken eyes, lethargy, and skin that stays tented when you gently pinch it. Heatstroke is an emergency: heavy panting, drooling, vomiting, collapse or a body temperature over forty degrees needs a vet immediately, and cooling with wet towels on the way. The opposite problem is just as important: a dog that suddenly drinks far more than usual — filling the bowl several times a day, asking to go out to urinate more often — can be showing an early sign of kidney disease, diabetes or Cushing's disease. Measure the intake for a day or two if you are unsure, and call your vet.

Common mistakes

The most common mistake is restricting water to stop a dog urinating indoors — never do this, especially with a puppy or a senior dog, because it risks dehydration and urinary problems. The second is a dirty or empty bowl: dogs drink less from a bowl that is stale, warm or in an awkward place, so wash it daily and keep it in a quiet spot. The third is forgetting water on car journeys and walks. The fourth is assuming that a dog that drinks a lot after exercise is ill — it is usually just thirsty, and the intake should settle within an hour or two. The fifth is ignoring a gradual increase in drinking over weeks: that slow change is exactly how kidney disease and diabetes first show themselves.

When to see a vet

See a vet promptly if your dog suddenly drinks much more or much less than usual, stops drinking, vomits water, or shows signs of dehydration or heatstroke. See a vet at the next routine visit if the increase has been gradual over weeks. For the food side of the equation, use the dog food calculator; for exercise, the exercise needs calculator; and for the senior years, the senior dog age calculator.