Kidney Stone Risk Calculator

What Are Kidney Stones?

Kidney stones (nephrolithiasis) are hard mineral deposits that form inside the kidneys when urine holds more crystal-forming substances than the fluid can dilute, mainly calcium, oxalate, and uric acid.
They range from tiny specks to stones large enough to block urine flow, causing severe flank pain (renal colic), nausea, and blood in the urine.

About 1 in 10 people will develop a kidney stone during their lifetime.
In the US survey data it is about 11% of men and 7% of women, and women’s rates have been rising.

Types of Kidney Stones

• Calcium stones (about 80%), mostly calcium oxalate, formed when calcium combines with oxalate in urine
• Uric acid: linked to gout, diabetes, obesity and a lot of animal protein, all of which make urine more acidic
• Struvite: associated with urinary tract infections
• Cystine: rare; caused by a genetic disorder

The one number worth knowing: 2.5 litres of urine a day

The American Urological Association’s guideline for people who have had a stone asks for enough fluid to pass at least 2.5 litres of urine a day. Some water leaves through breath and sweat, so that usually means 2.5 to 3 litres of fluid, about 10 to 12 glasses of 250 mL, and more in heat or with physical work.
It works. In a five-year trial by Borghi and colleagues (1996), people who had passed a first calcium stone and kept their urine above 2 litres a day had a second one at less than half the rate of those left to drink as usual, 12% against 27%.

Water, coffee and tea all count. In the large US cohorts studied by Ferraro and colleagues (2013), coffee and tea drinkers had fewer stones and sugar-sweetened sodas went with more.

The other risk factors, and what is known about each

• A previous stone: the strongest predictor there is. The often-quoted “half within five years” is an old figure; a 2014 Mayo Clinic study of first-time formers found about 1 in 5 had another symptomatic stone within 5 years and nearly 1 in 3 within 10.
• Family history: in the Health Professionals Follow-up Study, men with a parent or sibling who had stones were about two and a half times as likely to form one.
• Obesity: in the same US cohorts (Taylor and colleagues, 2005), a BMI of 30 or more went with about a third more stones in men and roughly twice as many in women.
• A high-salt diet: sodium pulls calcium into the urine. The guideline limit is 2,300 mg a day.
• A lot of animal protein: raises uric acid and lowers citrate, a natural stone inhibitor.
• Hot, dry climate: sweat takes water that would otherwise leave as urine. The southern US “stone belt” has the highest rates in the country.

Why this page does not give you a score

An earlier version of this calculator gave each factor a number of points and turned the total into low, moderate or high risk. Those points were made up: no study has published weights for these factors added together, and the factors overlap (heat, salt and low fluid often travel together). So the page lists which apply, says what each is known to do, and puts a number only where there is one, on your fluid intake.

Prevention Strategies

• Drink enough to pass at least 2.5 litres of urine a day (urine should be pale yellow)
• Reduce sodium to under 2,300 mg/day
• Limit animal protein: moderate portions of meat, poultry, fish, and eggs
• Eat calcium-rich foods (do not avoid calcium: dietary calcium actually binds oxalate in the gut)
• Avoid high-oxalate foods if you have calcium oxalate stones (spinach, nuts, chocolate)
• Maintain a healthy weight through diet and exercise
• Speak to your doctor about potassium citrate supplementation if you have recurrent stones