Yes, diet soda counts as water intake. It is roughly 99% water, and clinical research shows it hydrates your body about as well as plain water does. That said, there are reasons not to make it your primary fluid source.
How Diet Soda Compares to Water for Hydration
Researchers at Loughborough University developed a Beverage Hydration Index (BHI) to measure how well different drinks actually hydrate people compared to still water. They tracked urine output over four hours after participants drank equal volumes of 13 common beverages. Diet cola, regular cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water, and sports drinks all produced the same amount of urine as plain water. In other words, your body retained the same amount of fluid from diet soda as it did from water.
This makes sense when you consider what’s in the can. Diet soda is almost entirely water with small amounts of flavoring, carbonation, sweetener, and acid. The liquid your body absorbs from it functions the same way water does.
Why Caffeine Doesn’t Cancel It Out
The most common concern is that caffeine makes you urinate more, effectively undoing the hydration. The reality is more nuanced. Caffeine does act as a mild diuretic, but only at doses high enough to matter. A review of 11 studies found that caffeine doses above 250 mg could trigger increased urine production, while smaller amounts had no measurable diuretic effect. More recent research puts the strong diuretic threshold closer to 500 mg, or about 6 mg per kilogram of body weight.
A 12-ounce can of Diet Coke contains about 46 mg of caffeine. Diet Pepsi has around 35 mg. You would need to drink roughly six to ten cans in a short window to approach the dose where caffeine meaningfully increases fluid loss. At typical consumption levels, the caffeine in diet soda has no real impact on your hydration status.
What Counts Toward Your Daily Fluid Intake
The National Academies of Sciences, Engineering, and Medicine notes that about 80% of people’s total water intake comes from drinking water and other beverages, including caffeinated ones. The remaining 20% comes from food. There’s no requirement that your fluids come exclusively from plain water. Milk, juice, tea, coffee, soup, and yes, diet soda all contribute to meeting your daily fluid needs.
General daily water intake targets are around 3.7 liters (about 125 ounces) for men and 2.7 liters (about 91 ounces) for women, from all sources combined. If a couple of those servings come from diet soda, they count.
Reasons to Limit Diet Soda Anyway
Hydration isn’t the only thing to think about when choosing what to drink. Diet soda has some downsides worth knowing about, even if it does hydrate you.
Tooth Enamel Erosion
Diet soda is highly acidic. Diet Coca-Cola has a pH of about 2.98, which is only slightly less acidic than regular Coca-Cola at 2.75. Tooth enamel begins to break down in acidic environments, and regular exposure to liquids at this pH can soften and erode enamel over time. Plain water, by contrast, has a neutral pH and poses zero risk to your teeth. If you’re drinking diet soda throughout the day, your teeth are sitting in an acid bath for hours.
Kidney Health
Cola-type beverages specifically (both regular and diet) contain phosphoric acid, which has been linked to kidney problems. A large study found that drinking two or more colas per day was associated with a 2.3 times higher risk of chronic kidney disease. This applied equally to regular and artificially sweetened colas. Noncola carbonated beverages, which use citric acid instead of phosphoric acid, showed no increased kidney risk. Among men with kidney stones, those who continued drinking phosphoric acid-containing sodas had higher recurrence rates than those who switched to citric acid beverages.
Insulin Response
Even though diet soda contains no sugar, the sweet taste may still trigger biological responses. Research suggests artificial sweeteners can stimulate sweet-taste receptors in the gut, prompting the release of hormones involved in blood sugar regulation. This can lead to insulin being released even though no actual glucose entered the bloodstream. The long-term significance of this effect is still being studied, but it’s worth noting for people managing blood sugar levels.
A Practical Approach
If you enjoy diet soda and drink a can or two a day, it absolutely contributes to your hydration. You don’t need to mentally subtract it from your fluid intake or “make up for it” with extra water. From a pure hydration standpoint, your body treats it almost identically to water.
The practical issue is everything else in the can. The acidity wears on your teeth, the phosphoric acid in colas may stress your kidneys over years of heavy use, and the sweeteners trigger metabolic responses that plain water doesn’t. Making diet soda a supplement to your water intake rather than a replacement keeps you hydrated while minimizing those trade-offs. A reasonable pattern for most people: water as your default, diet soda as an occasional choice rather than an all-day habit.