Chapter 01 · The method
Why per-kilogram beats a fixed number
A 110-pound person and a 220-pound person do not need the same volume of fluid, yet the eight-glasses rule hands them an identical target. Scaling intake to body weight, at roughly 35 mL per kilogram, fits the estimate to your actual size. Activity and climate then push the number up: heavy exertion can add a liter, and hot or humid conditions another half-liter through sweat losses.2
*12 month plan, billed once for the term when a physician approves. Renews each term.
| Scenario | Added daily | What it reflects |
|---|---|---|
| Sedentary, temperate | baseline only | Per-kilogram baseline |
| Light activity | +500 mL | Light exercise sweat loss |
| Moderate activity | +750 mL | Most-days training |
| Heavy or hot climate | +1,000 to 1,500 mL | Intense effort or heat |
How modifiers stack on the per-kilogram baseline. Values are added daily.
Chapter 02 · On treatment
Hydration on a GLP-1 medication
GLP-1 medications slow gastric emptying and reduce thirst, so fluid intake often drops without you noticing. Dehydration then amplifies the common GI side effects, and constipation is a leading reason patients stop the medication. Hitting your per-kilogram target plus an extra 500 mL daily supports tolerability for most patients on semaglutide or tirzepatide. Individual results vary.
Start here · one question
What's your goal weight?
Choose a goal to discuss with your clinician. You can change it in your intake.
On a GLP-1, thirst is a less reliable signal than usual. Aim for your target by the clock rather than waiting until you feel thirsty. Individual results vary.
Chapter 03 · When more is not better
When you should drink less
More is not always better. Heart failure, chronic kidney disease, and certain diuretics call for physician-supervised fluid limits, sometimes well below the calculator's estimate. Exercise-associated hyponatremia, dangerously low blood sodium from over-drinking during long endurance events, is rare but real. For most healthy adults at rest, the body's thirst signal is reliable; the calculator is a planning anchor, not a quota to force.
