It is the most common question people ask once they try a fasting window such as 16:8. The short answer: it depends on what is in the cup and why you are fasting. There is no official, scientific cut-off for "breaking a fast". What we do know is how many calories common drinks contain, so you can decide for yourself.
Plain water, black coffee and plain tea contain almost no calories (plain brewed coffee has fewer than 5 kcal a cup). The calories arrive with what you add: milk, sugar, honey, cream, butter.
1. Pick your drink
2. Add anything?
3. Why are you fasting?
4. Cups a day: 2
Caffeine for 2 cups: about 180 mg
Typical values per cup: a splash of milk or oat drink (about 30 ml) is roughly 15 kcal, a teaspoon of sugar about 16 kcal, a teaspoon of honey about 20 kcal, a tablespoon of cream around 50 kcal, and a tablespoon of butter or oil about 110 kcal. Plant drinks differ a lot and some contain added sugar, so read the label. Flavoured syrups add roughly 10 to 20 kcal a pump.
The "bulletproof" style of coffee with butter or oil is a meal in a mug. It may suit some low-carb eaters, but it is not a fast.
For an everyday eating window such as 16:8, the point is usually to eat in fewer hours and get a routine you can keep. A black coffee or plain tea fits, and so does water. Whether a splash of milk "counts" is something people argue about. You will see a "50 kcal" rule of thumb online, but it is a rule of thumb, not a research finding. Research on fasting is mixed, and the benefits that are seen often come from eating fewer calories overall, not from the clock itself.
For a blood test it is different. Labs ask you to fast so that food does not change the result, and many ask for water only. The instructions come from your clinic, so follow them rather than a general rule.
Plain water, black coffee and plain tea are close to zero calories. Everything you add is where the calories come from.
Coffee and tea on an empty stomach can make some people jittery or give them reflux, and caffeine late in the day can hurt sleep. EFSA, the European food safety authority, says healthy adults can have up to 400 mg a day (single doses up to 200 mg), and pregnant and breastfeeding women up to 200 mg a day, from all sources. Typical values are about 90 mg for a cup of filter coffee, 80 mg for an espresso and 50 mg for a cup of black tea. The calculator above shows where your cups land.
This guide is general information, not medical advice. If you have a medical condition, are pregnant or breastfeeding, have or have had an eating disorder, or take medication (especially for diabetes or blood pressure), talk to your doctor before you change when you eat or drink, fasting included. Do not skip needed medication or food to keep a fast going.