Wet food is about 78 percent water; dry food is about 10 percent. For a 4 kg cat on 200 kcal a day that difference is 184 ml against 5 ml, before the cat drinks a drop. The daily target is 40 to 60 ml per kg, so 160 to 240 ml. One format nearly covers it from the bowl; the other leaves the cat to find 150 ml or more on its own, which most cats do not.
The two numbers: 78 percent and 10 percent
Canned pate, chunks in gravy and mousse run 75 to 82 percent moisture. Dry kibble is 6 to 10 percent, and freeze-dried raw is 5 percent or less. Those percentages are of the food as fed, not of dry matter, which is the number people usually quote by mistake. An 85 g tray at 78 percent water holds 66 ml of water; a 50 g scoop of kibble at 10 percent holds 5 ml.
The daily target: 40 to 60 ml per kg
Water need scales with weight, not with appetite. A 3 kg cat needs 120 to 180 ml, a 4 kg cat 160 to 240, a 5 kg cat 200 to 300, a 6 kg cat 240 to 360. That total includes water in food. Splitting it into food water and drinking water is the only way the maths works, and it is the step most feeding plans skip. A cat in a heated flat in winter, or one on diuretics, sits at the top of the range.
The table, computed for 200 kcal
| Format | Water (%) | kcal per 100 g | Grams for 200 kcal | ml water from food | ml per 100 kcal |
|---|---|---|---|---|---|
| Pate | 78 | 85 | 235 | 184 | 92 |
| Chunks in gravy | 82 | 75 | 267 | 219 | 109 |
| Mousse | 75 | 95 | 211 | 158 | 79 |
| Dry kibble | 10 | 390 | 51 | 5 | 2.6 |
| Dry, low moisture | 6 | 410 | 49 | 3 | 1.5 |
| Freeze-dried raw | 5 | 480 | 42 | 2 | 1.0 |
Reading moisture off the label
Moisture appears in the guaranteed analysis as a maximum: 75 to 82 percent for cans, 10 to 12 percent for kibble. Two traps. The number is as-fed, so 78 percent is 78 percent of the tin, not of the dry matter, which is why comparing it with a dry-matter protein figure makes no sense. And gravy or jelly formulas often reach 82 to 85 percent, which sounds better but dilutes the calories: the cat then needs 32 g more food to reach the same 200 kcal.
The gap, in millilitres
Take the 4 kg cat with a 160 to 240 ml target. On pate at 200 kcal it gets 184 ml from food and drinks 25 ml per kg, about 100 ml, so the day lands at 284 ml and easily clears the floor. On kibble it gets 5 ml from food and the same 100 ml of drinking: 105 ml against a 160 ml floor, a 55 ml shortfall that repeats every day for years.
The shortfall does not show up as a symptom. It shows up in urine concentration, in a bladder that sits with sediment, and eventually in a cat that visits the clinic at 9 years old with a stone. Twenty-five millilitres is about 5 teaspoons; the deficit is 11 teaspoons a day.
Urine specific gravity is the readout
Specific gravity measures how concentrated the urine is. A cat on dry food commonly reads 1.045 to 1.060; a cat on wet food reads 1.030 to 1.040. Below 1.035 is the usual target when urinary risk is the reason for the change. A dipstick at home is unreliable, but a clinic can run specific gravity on a free-catch sample in 2 minutes, and the number moves within 3 to 7 days of a diet change.
Three traps when adding water
First, adding 20 ml of water to kibble does not make it wet food: 20 ml is 12 percent of a 160 ml floor. Second, broth and tuna water carry sodium, and a cat topping out at 300 mg of sodium a day is not drinking more, it is drinking the same and retaining it. Third, a fountain helps only if the cat uses it: put 3 bowls in 3 rooms at 3 heights and measure what disappears overnight before buying anything with a motor.
Cost per 100 kcal and per 100 ml of water
At 3.00 per 100 g, pate at 85 kcal per 100 g costs 3.53 per 100 kcal. Dry food at 9.00 per 100 g and 390 kcal per 100 g costs 2.31 per 100 kcal, roughly 35 percent less per calorie. Then price the water: 100 ml from pate needs 109 kcal, so 3.85. The same 100 ml from kibble needs 3,846 kcal, which is 88.60. That is the real comparison, and it is why the cost tool asks for kcal per 100 g before it asks anything else.
Switching over 14 days
Change the water and the calories at the same time and the cat notices both. Replace 25 percent of the dry ration with pate every 3 to 4 days: 25 percent on day 1, 50 percent by day 5, 75 percent by day 9, all of it by day 14. Recalculate grams at every step, because 100 g of pate is 85 kcal against 390 kcal for 100 g of kibble, a 4.6 times difference per gram.
Five steps to close the gap this week
- Weigh the current food for 3 days and compute the water it delivers: grams times water percentage.
- Subtract that from the target for the cat's weight, and write the gap in millilitres on the fridge.
- Replace 25 percent of the dry ration with pate, keeping the calories identical, and recheck the gap.
- Add 1 to 2 tablespoons of warm water to any remaining kibble, and count it: 15 ml per tablespoon.
- Retest urine specific gravity at 7 days; if it has not moved below 1.040, the wet share is too small.
Mixing is where most people go wrong on calories: 100 g of pate is 85 kcal, so replacing 100 g of kibble with 100 g of pate cuts 305 kcal from the day. Keep the kcal constant and change only the water, as set out in the hydration maths and in the full wet versus dry comparison, and the cat loses nothing but urine concentration.
Frequently asked
Is a wet food diet enough water without a fountain?
For most cats, yes. Pate at 78 percent moisture delivers about 184 ml per 200 kcal, which covers most of a 4 kg cat's 160 to 240 ml target before any drinking. Check it with urine specific gravity 7 days after the switch: below 1.040 means the water side is working, and a fountain becomes optional rather than urgent.
How much water should I add to dry food?
Start with 1 to 2 tablespoons, which is 15 to 30 ml, and count every millilitre against the daily target rather than treating it as a bonus. Added water does not change the calories, so it is the cheapest intervention available. If the cat refuses soaked kibble, replace 25 percent of the ration with pate instead, keeping the calories constant.
Does wet food really prevent urinary blockages?
It lowers the concentration of urine, which lowers the risk of struvite and calcium oxalate crystals forming, and dilution is the mechanism most vets rely on. It does not remove a stone that already exists, and it does not replace imaging or a urine culture in a cat that is straining. A male cat that cannot pass urine is an emergency at any diet.