Millilitres or US fluid ounces: choosing water units in HydraTag
HydraTag works in millilitres under the hood and can display everything in US fluid ounces instead. Switching is safe: your history is stored once, in millilitres, and only the display changes.
Pick a unit
During setup, or any time in Settings under Display units, choose Millilitres or US fl oz. When you switch in Settings, the goal and quick-add fields convert on screen immediately; tap Save preferences to keep the change.
How the app converts
- Every drink is saved as a whole number of millilitres.
- One US fluid ounce is treated as 29.5735 ml.
- Ounce displays are rounded to one decimal place, so 250 ml shows as 8.5 fl oz.
- An amount you type in ounces is converted to the nearest whole millilitre before saving, so 16 fl oz is saved as 473 ml.
- If you switch units and back without editing a field, the exact stored millilitres are kept; nothing drifts.
| Millilitres | Shown as US fl oz | Typical container |
|---|---|---|
| 250 ml | 8.5 | Small glass or cup |
| 350 ml | 11.8 | Mug or can |
| 500 ml | 16.9 | Standard bottle |
| 750 ml | 25.4 | Large bottle |
| 1,000 ml | 33.8 | One litre |
| 2,000 ml | 67.6 | Default daily goal |
| Typed in US fl oz | Saved as |
|---|---|
| 8 | 237 ml |
| 12 | 355 ml |
| 16 | 473 ml |
| 20 | 591 ml |
| 24 | 710 ml |
| 32 | 946 ml |
US versus imperial fluid ounces
The app's ounce is the US fluid ounce (about 29.57 ml). The imperial fluid ounce used on some older UK measures is smaller (about 28.41 ml). If a bottle label gives both millilitres and ounces, enter the millilitres to avoid the question entirely.
Ranges the app accepts
| Field | Allowed range |
|---|---|
| Daily goal | 100 to 10,000 ml (3.4 to 338 fl oz) |
| A single drink or quick-add button | 1 to 10,000 ml |
| Amount written to a sticker (per tap) | 10 to 2,000 ml (0.4 to 67.6 fl oz) |
Your goal is personal
The app's starting goal of 2,000 ml is a round number to begin with, not a recommendation. How much fluid a person needs varies with body size, activity, climate, diet and health, and some medical conditions call for limits rather than targets. HydraTag does not calculate a goal for you or give health advice. If you have a condition that affects how much you should drink, follow your clinician's guidance and set the app's goal to match it.
Next: set up quick-add buttons in your unit in manual logging, or choose a per-tap amount in NFC sticker logging.
HydraTag guide ยท Updated October 2, 2026