The drink catalog
The app has 102 drinks to pick from. Each has a default serving, a hydration factor, its caffeine, its calories per 100 ml and its alcohol by volume. The website reads these values from the app's source code when the pages are built, so a drink table here cannot drift from the app. The full table is on the drinks page, and as CSV and JSON.
- Caffeine in coffee and tea counts by the shot or the cup, so a bigger cup of the same brew keeps the same caffeine. Sodas, energy drinks and cold brew scale with the volume.
- Calories are the value per 100 ml times the amount.
- Alcohol in grams is the volume times the alcohol by volume times 0.789 g per ml, the density of ethanol.
- Sources for caffeine: the values follow the Mayo Clinic caffeine chart. Cold brew follows a measured range of 153 to 238 mg per 12 oz cup instead, and cola uses the maker's figure of 33 mg per 330 ml can.
Hydration factors
A hydration factor says how well a drink hydrates compared with still water, which is 1.0. The factors are based on the Beverage Hydration Index trial. Each of 72 men drank a litre of still water and a litre each of three other drinks, in separate trials, and the researchers measured urine over the next four hours. Urine output after cola, diet cola, hot tea, iced tea, coffee, lager, orange juice, sparkling water and a sports drink did not differ from water. Full-fat milk (1.50), skimmed milk (1.58) and an oral rehydration solution (1.54) scored higher (Maughan, 2016).
The trial measured 13 drinks, still water among them. For drinks it did not test, the factor is a WaterChamp estimate, and the drinks page marks which is which. These are the trial drinks next to the factor the app uses:
| Drink | In the trial | WaterChamp factor |
|---|---|---|
| Water | The reference drink | 1.00 |
| Sparkling Water | No different from still water | 1.00 |
| Milk | Retained more than water (full-fat 1.50, skimmed 1.58) | 1.50 |
| Cola | No different from still water | 1.00 |
| Diet Soda | No different from still water (diet cola) | 1.00 |
| Black Tea | No different from still water (hot tea) | 1.00 |
| Iced Tea | No different from still water | 1.00 |
| Black Coffee | No different from still water | 1.00 |
| Beer | No different from still water (4% lager) | 1.00 |
| Orange Juice | No different from still water | 1.10 |
| Isotonic Drink | No different from still water | 1.05 |
How a drink counts toward the goal
Every drink counts its full volume toward the day's total. The factor then works in one of two ways:
- Below 1.0, the daily goal rises by the part the drink does not cover: the volume times one minus the factor. A 150 ml glass of red wine at 0.5 counts 150 ml and raises the goal by 75 ml. A 45 ml spirit at 0.0 counts 45 ml and raises the goal by 45 ml.
- Above 1.0, nothing extra is added to the total. The drink lifts the live hydration level further than the same amount of water would: 250 ml of milk at 1.5 counts 250 ml and moves the level half again as far.
The daily water goal
The goal covers drinks only, since food supplies the rest. The formula starts at 26 ml per kg of body weight and multiplies it by:
- sex: 1.05 for men, 0.95 for women, 1.0 when not set
- age: 1.05 under 30, 1.0 from 30 to 65, 0.95 over 65
- activity: 1.0 sedentary, 1.1 lightly active, 1.25 moderately active, 1.5 very active
- climate: 1.0 temperate, 1.1 warm, 1.2 hot, 1.3 hot and humid
Pregnancy adds 300 ml, breastfeeding 700 ml and both together 1,000 ml. The result is rounded to the nearest 50 ml and kept between 500 and 9,999 ml. The water intake calculator runs the same formula. In the app, a connected Apple Health or Health Connect account also adds 1 ml for each active calorie burned, up to 1,500 ml a day. That rate is a WaterChamp estimate.
For comparison, reference values that include the water in food are 2.0 litres a day for women and 2.5 litres for men from EFSA (EFSA, 2010), and 2.7 and 3.7 litres from the US National Academies (Institute of Medicine, 2004).
Caffeine
The caffeine curve is a one-compartment Bateman model for a drink taken by mouth. The level climbs while caffeine is absorbed, with an absorption half-life of 30 minutes, then halves with every elimination half-life. In the app you set that half-life from 2 to 8 hours in half-hour steps, starting at 5, and a pregnant profile doubles it. The caffeine calculator offers 3, 5 and 8 hours.
- The average half-life in adults is about 5 hours (StatPearls), and it ranges from 1.5 to 9.5 hours between healthy people (Institute of Medicine, 2001).
- In one study, pregnant women had an average half-life of 8.3 hours, against 3.4 hours in other adults (Knutti, 1981).
- The sleep line is 0.83 µg of caffeine per ml of plasma, the level above which heart rate during sleep fell in one study (Baur, 2024). It is turned into milligrams with a distribution volume of 0.7 litres per kg (Institute of Medicine, 2001), which gives about 41 mg for a 70 kg adult.
- The next-drink cutoff is the latest start time for one more drink that still leaves the level under that line at bedtime.
Blood alcohol
The BAC estimate uses the Widmark formula: grams of alcohol divided by body weight in kg times a factor r (Reiter, 2020). The app uses Widmark's own values of r, 0.68 for men and 0.55 for women, and 0.615 when sex is not set. Each drink is absorbed in a straight line over 45 minutes, and the level falls by 0.015% an hour, the average for moderate drinkers in a review of the measurements (Jones, 2010). The BAC calculator uses the same model. It is an estimate. Never use it to decide whether to drive.
How the site stays in step with the app
The calculators and drink pages run copies of the app's formulas. Automated tests in the app's code load the website's own code and compare it with the app's functions: drink nutrition, the water goal, the caffeine curve, threshold and cutoff, and the BAC curve. If a formula changes in one place and not the other, the tests fail. The site build also stops if the app's catalog no longer has exactly 102 drinks to pick from.
Sources
- Maughan RJ, Watson P, Cordery PA, et al. A randomized trial to assess the potential of different beverages to affect hydration status: development of a beverage hydration index. American Journal of Clinical Nutrition, 2016;103(3):717-723. PubMed
- EFSA Panel on Dietetic Products, Nutrition and Allergies. Scientific Opinion on Dietary Reference Values for water. EFSA Journal, 2010;8(3):1459. EFSA Journal
- Institute of Medicine. Dietary Reference Intakes for Water, Potassium, Sodium, Chloride, and Sulfate. National Academies Press, 2004. National Academies Press
- Institute of Medicine. Pharmacology of Caffeine, in Caffeine for the Sustainment of Mental Task Performance. National Academies Press, 2001. NCBI Bookshelf
- Evans J, Richards JR, Battisti AS. Caffeine. StatPearls, 2024. NCBI Bookshelf
- Knutti R, Rothweiler H, Schlatter C. Effect of pregnancy on the pharmacokinetics of caffeine. European Journal of Clinical Pharmacology, 1981;21(2):121-126. PubMed
- Baur DM, Dornbierer DA, Landolt HP. Concentration-effect relationships of plasma caffeine on EEG delta power and cardiac autonomic activity during human sleep. Journal of Sleep Research, 2024;33:e14140. PubMed
- Reiter GS, Boeckle M, Reiter C, Seltenhammer MH. The impact of total body water on breath alcohol calculations. Wiener klinische Wochenschrift, 2020;132(17):535-541. PMC
- Jones AW. Evidence-based survey of the elimination rates of ethanol from blood with applications in forensic casework. Forensic Science International, 2010;200(1-3):1-20. PubMed
- Mayo Clinic Staff. Caffeine content for coffee, tea, soda and more. Mayo Clinic
