There is a working baseline for a dog at rest: roughly an ounce of water per pound of body weight per day. For a 60 lb dog that is around 60 fl oz, a little under half a gallon.
On a hike, plan for meaningfully more than that. How much more is not a number anyone can honestly hand you, because it moves with heat, humidity, terrain, coat and conditioning, and with how hard the dog is actually working rather than how far you walked. Treat the baseline as the floor you start packing from, not a target to hit.
Why the number moves so much
Dogs shed heat mainly by panting, and panting costs water. A dog working in heat is evaporating water out of its airway continuously. That is the cooling system, and it runs on your water supply.
Which is why humidity matters as much as temperature. In humid air evaporation is less effective, so the dog pants harder and longer for the same amount of cooling and goes through more water doing it. A humid 80 degrees is harder work than a dry 90.
Terrain adds to it by a different route. A dog covers considerably more ground than its handler on any trail, quartering back and forth and checking things. Your mileage is not the dog's mileage, and the difference is not small.
Coat matters too. A thick double coat is insulation working in both directions, which helps in cold and costs in heat.
Little and often, not a big drink at the top
Offer water at every stop, in small amounts, from the start of the walk rather than from the point where the dog starts looking hot.
Two reasons. A dog that has been allowed to get properly thirsty on a hot day will gulp a large volume very fast, which is uncomfortable at best on a working dog. And hydration is far easier to maintain than to recover — once a dog has fallen behind, it tends to stay behind for the rest of the day.
Dogs will also refuse water they need, especially early on when they are excited and busy. Offer anyway, repeatedly. A dog declining water at the first stop tells you nothing about what it will need at the fourth.
Carry the dog's water separately
Sharing one bottle sounds efficient and works badly. You cannot see what the dog has actually taken, so rationing gets vague at exactly the point it starts to matter. Keep the dog's supply separate and you always know where you stand.
A dedicated dog bottle with a built-in trough solves the other half of the problem, which is delivery. Pouring water into a cupped hand or a collapsible bowl on a windy ridge wastes a surprising share of what you carried up there, and on a day when you are counting ounces, a bottle that returns what the dog leaves rather than dumping it on the trail is worth having.
What not to let a dog drink
Standing water is the one to be firm about. Puddles, stock ponds, stagnant margins and slow backwaters can carry Giardia and Leptospira, and warm still water can carry cyanobacteria — blue-green algae — which is genuinely dangerous and acts fast.
Moving water in a clean drainage is a lower risk, and plenty of dogs drink out of streams their whole lives without trouble. Lower is not zero. The practical point is that carrying enough of your own water is what puts you in a position to say no.
Salt water is the other one. Dogs playing in surf swallow more than people expect, and enough of it causes vomiting and worse.
How much can a dog carry itself
Water is heavy — a little over two pounds per quart — so it consumes a dog's carrying capacity quickly. A conditioned adult dog in a well-fitted pack can carry some of its own water, but the load has to be balanced side to side, built up over weeks rather than introduced on the day, and checked for rub points at every stop. Puppies and seniors should not be carrying anything.
Signs you have fallen behind
Thick ropy drool instead of thin saliva. Panting that does not settle during a rest. Tacky or dry gums. A drop in pace, or a dog that starts lying down at stops when it normally would not.
Some of these overlap with early heat exhaustion, because dehydration and overheating travel together. If you are seeing them, stop and get into shade, and read the signs of heat exhaustion on a hike — the step from tired to emergency can be quick, and heatstroke is a veterinary emergency rather than something to walk off.
The honest limitation
Any water rule of thumb, including the one at the top of this page, is a starting point for packing rather than a prediction. The only method that holds up is to plan the route around known water, carry more than the arithmetic suggests, and turn around on the water rather than on the miles.
The last mile is the one nobody plans for.
Our Canteen Trail Bottle dispenses one-handed into a fold-out trough and runs whatever the dog leaves back into the bottle instead of onto the ground. It is a walk and day-hike bottle rather than a water supply, and we say so on the listing. The rest of the trail kit is in Accessories.