How Much Water to Drink on a GLP-1: Daily Water Calculator

1. Enter your body weight in pounds, kilograms or stones.
2. Pick the bottle you carry to see the target as refills, and when you wake and go to bed to split it across the day. If you already drink a set amount, or were told one, enter it to compare.
3. If you eat noticeably less than before, say how much less to see the water your meals no longer carry. It is shown on its own, not added to the target.
4. On a day with nausea, vomiting or diarrhea, tick the box to see the signs of dehydration to watch for. If you have heart failure or kidney disease, tick that box instead of using the target.

The target in mL is your weight in kg multiplied by 35, kept between 2,000 and 4,000 mL. Liters are rounded to one decimal place. Cups are the target divided by 240 mL, the cup used on US nutrition labels (21 CFR 101.9). Fluid ounces are the target divided by 29.57 mL, the US fluid ounce, and pounds convert at 0.45359237 kg per lb (NIST Handbook 44, Appendix C).

For a 180 lb adult:

1. 180 × 0.45359237 = 81.647 kg.
2. 81.647 × 35 = 2,857.6 mL, rounded down to 2,857 mL and shown as 2.9 L, 12 cups or 97 fl oz.

Cups are rounded to whole 240 mL cups, so eight times the cup count can fall short of the fluid-ounce figure: 12 cups is 96 fl oz, against 97 fl oz for the same 2,857 mL.

The base and the range come from a 2025 expert consensus on nutrition with GLP‑1 therapy, which calls more than 2 L of fluid a day necessary and gives "2 to 4 L/day, or approximately 35 ml water/kg bodyweight" (Sievenpiper et al., 2025). It adds that people in hot, humid environments or doing heavy physical activity may need considerably more fluid.

At 70 kg, the target of 2.5 L falls between the IOM drink figures for women and men. The IOM report works out that moderately heavy exercise in the heat needs at least 0.72 L of sweat an hour for cooling (IOM, 2005). One such hour is more than a quarter of a 2.5 L day.

The Institute of Medicine’s Adequate Intake for total water counts the water in food as well as drinks. In its survey data for adults aged 19 to 30, drinks supplied about 81% of total water (IOM, 2005). The UK’s NHS advises 6 to 8 cups or glasses of fluid a day, and gives a check you can use on any day: drink enough that your pee is a clear pale yellow (NHS, water, drinks and hydration).

The 3.7 L and 2.7 L figures include food, so they are not drinking targets. The IOM adds that for a healthy person the Adequate Intake "should not be interpreted as a specific requirement", because a wide range of intakes keeps people normally hydrated.

Food supplied about 19% of total water intake in the IOM’s survey data (IOM, 2005). In a 20-week trial of 72 adults, the energy eaten at a free-choice lunch was 35% lower on semaglutide 2.4 mg than on placebo, 1,736 against 2,676 kJ (Friedrichsen et al., 2021).

If the water in your food fell by the same 35%, a man taking in the IOM’s 3.7 L would lose about 0.25 L a day from food (3.7 × 0.19 × 0.35) before any change in what he drinks. No study has measured this. The figure multiplies numbers from two separate sources, and the trial recorded one lunch rather than a whole day, without measuring water. On the weeks you eat least, add to what you drink.

The Ozempic, Wegovy, Mounjaro and Zepbound labels each warn of acute kidney injury due to volume depletion. The Ozempic label says most reported cases "occurred in patients who experienced gastrointestinal reactions leading to dehydration such as nausea, vomiting, or diarrhea" (Ozempic prescribing information, section 5.6). The same warning is section 5.3 of the Zepbound label and section 5.5 of both the Mounjaro label and the Wegovy label.

The Medication Guide for all four says "It is important for you to drink fluids to help reduce your chance of dehydration", without naming an amount (Ozempic Medication Guide). It also says to tell your healthcare provider right away about nausea, vomiting or diarrhea that does not go away.

The prescriber sections ask for kidney monitoring "especially during dosage initiation and escalation" (Ozempic prescribing information, section 5.6). On tirzepatide, most nausea, vomiting and diarrhea "occurred during dose escalation and decreased over time" (Mounjaro prescribing information, section 6.1). The dose titration planner dates your step-up weeks, and when Zepbound side effects peak explains the days after each shot.

NIDDK gives these signs of dehydration in adults (NIDDK, symptoms and causes of diarrhea):

• extreme thirst or a dry mouth
• urinating less than usual
• dark-colored urine
• feeling tired, dizzy or lightheaded
• pinched skin that does not flatten back right away
• sunken eyes or cheeks

With diarrhea, NIDDK advises seeing a doctor right away for any of them. Vomiting and diarrhea also cost salts; oral rehydration solutions are discussed in what to eat with GLP‑1 diarrhea.

A dated record of which days after each shot bring nausea or diarrhea tells you when to watch for the signs of dehydration above. Logging doses and symptoms in Pensday gives you that record without paying for Pensday Plus.

Fluids alone often do not settle GLP‑1 constipation. The 2025 joint advisory on nutrition with GLP‑1 therapy says "adequate fluids and fiber from foods should be encouraged, although additional strategies are often required" (Mozaffarian et al., 2025). The GLP‑1 fiber calculator sets the fiber side of that pair.