Kidney Stone Risk: How Much Caffeine Is Too Much?

If you’ve had a kidney stone—or worry you might—you’ve likely been told to watch what you eat and drink. But one surprising factor may be your daily coffee habit. New research shows that people already at risk for kidney stones should think twice before drinking too much caffeine.

The short answer? Keep your daily caffeine below the amount in two cups of coffee—about 16 ounces total—to avoid increasing your risk. That includes all sources: tea, energy drinks, and other caffeinated beverages count toward the same limit.

What the Study Found

Researchers at the University of Washington and Columbia University tested 39 people with a history of kidney stones and 39 healthy individuals with no such history. After a 14-hour fast, participants drank water with added caffeine equivalent to roughly two cups of coffee. Researchers then collected urine samples two hours before and two hours after caffeine intake.

The results were clear: in both groups, caffeine caused an increase in urinary levels of calcium, sodium, magnesium, and citrate. But for those with a history of kidney stones, the rise in urinary calcium is especially concerning.

Why Calcium Matters in Kidney Stones

Kidney stones often form when certain compounds in urine—especially calcium—become too concentrated. Calcium is a major component of the most common types of kidney stones, so higher levels of it in urine can raise the likelihood of stone formation.

Linda Massey, a researcher from the University of Washington, explained that excess caffeine leads to more calcium being excreted in urine. For people already prone to forming stones, this extra calcium increases their risk.

How Much Caffeine Is Safe?

The study specifically measured caffeine intake equivalent to two cups of coffee, defined as 16 fluid ounces (about two standard “coffee pots” or mugs). That’s roughly the amount in two average 8-ounce cups, though serving sizes vary. Importantly, the limit applies to total daily caffeine from all sources—not just coffee.

Participants who consumed this amount showed a measurable spike in urinary calcium. Staying below this threshold may help reduce the risk of stone formation, especially for those with a known predisposition.

Frequently Asked Questions

How much caffeine is in two cups of coffee?

Two cups of coffee, as defined in the study, equal about 16 fluid ounces total—that’s typically two average 8-ounce servings. This usually translates to roughly 200–300 mg of caffeine, depending on brew strength, but the key measure is the volume: 16 ounces total from any caffeinated source counts toward the limit.

Does caffeine affect everyone the same way?

No. While both stone-formers and non-stone-formers showed increased urinary calcium and other minerals after caffeine intake, the study highlights that those with a history of kidney stones experience greater risk from the resulting calcium increase. Their baseline risk makes them more vulnerable to the effects of caffeine.

Are other caffeinated drinks also risky?

Yes. The 16-ounce caffeine limit applies to all sources, not just coffee. That includes tea, soda, energy drinks, and other beverages containing caffeine. All of these can contribute to the same increase in urinary calcium and should be included when calculating your daily total.

What should I do if I’ve had kidney stones before?

If you have a history of kidney stones, it’s recommended to limit total daily caffeine to the equivalent of less than two cups of coffee (16 ounces). Monitor all sources of caffeine, and consider discussing dietary adjustments with your doctor or a nutritionist familiar with kidney stone prevention.

Can I drink decaf coffee instead?

Decaf coffee contains minimal caffeine—usually less than 5 mg per cup—so it falls well below the 16-ounce caffeinated limit. However, other aspects of your diet, like calcium and fluid intake overall, also matter for kidney stone risk, so decaf isn’t a free pass but is a lower-risk choice.

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