Intermittent Fasting Calculator16:8, 18:6, OMAD and More: Your Eating Window
Choose a fasting protocol and your first meal time to see your eating window on a 24-hour clock, what happens during each hour of your fast, and a calorie-matched meal plan, with pharmacist advice on medication timing.
Built & medically reviewed by Manzoor Ahmad, Pharmacist & Calculator Developer · Updated
Based on time-restricted eating trials, including Sutton et al. 2018 and Lowe et al. 2020, and the NEJM review by de Cabo & Mattson.
How does intermittent fasting work?
Intermittent fasting means eating only within a set window each day. In 16:8, the most popular schedule, you fast for 16 hours and eat within 8, for example noon to 8 PM. After roughly 12 hours without food, your body relies more on fat for fuel. Weight loss comes from eating fewer total calories, so studies find results similar to standard dieting, but many people find it easier to stick to.
Your fasting plan
12:00 PM – 8:00 PM
Fast from 8:00 PM to 12:00 PM the next day: 16 hours.
What happens during your fast
Approximate phases for a typical adult. Timing varies with your last meal, activity and metabolic health.
Meal plan inside your window
Intermittent fasting schedules compared
| Protocol | Fast / Eat | Example window | Best for |
|---|---|---|---|
| 12:12 | 12 h / 12 h | 8 AM – 8 PM | Complete beginners, a gentle start |
| 14:10 | 14 h / 10 h | 9 AM – 7 PM | Women starting out, better sleep timing |
| 16:8 | 16 h / 8 h | 12 PM – 8 PM | Most people. The most studied daily schedule |
| 18:6 | 18 h / 6 h | 1 PM – 7 PM | Experienced fasters |
| 20:4 (Warrior) | 20 h / 4 h | 3 PM – 7 PM | Advanced. Harder to hit protein |
| OMAD | 23 h / 1 h | 6 PM – 7 PM | Short-term only for most people |
| 5:2 | 2 days at ~500–600 kcal | Mon & Thu | People who prefer weekly rather than daily rules |
Does intermittent fasting work?
- Weight loss: similar to everyday calorie restriction when calories are matched (several RCTs, including Lowe et al., 2020). Its main benefit is that some people find it easier to stick to.
- Earlier windows help more: early time-restricted eating (finishing by mid-afternoon) improved insulin sensitivity and blood pressure in Sutton et al. (2018).
- Protein matters: fewer meals make it easier to under-eat protein. Aim for 30–50 g per meal.
- Autophagy claims: mostly from animal studies. The exact timing in humans isn’t established.
Fasting and your medicines: timing matters
Some medicines must be taken with food and need to fall inside your eating window. These include NSAIDs (ibuprofen, naproxen), prednisolone, metformin and many antibiotics such as amoxicillin-clavulanate. Others are best on an empty stomach, which fits fasting well: levothyroxine 30–60 minutes before your first meal, alendronate, and some PPIs before your first meal.
Insulin and sulfonylureas can cause dangerous hypoglycaemia during long fasts, so adjust doses with your doctor first. The same applies to SGLT2 inhibitors, because of the ketoacidosis risk. People on blood-pressure tablets may feel dizzy in the first weeks.
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Intermittent Fasting calculator FAQs
16:8 (fasting 16 hours and eating within 8) is the most popular and best-studied daily schedule. Beginners often start with 12:12 or 14:10 and build up. The best schedule is the one that fits your sleep, work and training and that you can keep up.
Water, sparkling water, black coffee, plain tea (green, black or herbal) and unsweetened electrolyte drinks won’t meaningfully break a fast. Milk, cream, juice, sugar and most sweetened drinks will.
Typically 0.5–1 lb (0.2–0.5 kg) a week, the same as an equivalent calorie deficit. Intermittent fasting works by making it easier to eat less, not through any special fat-burning effect.
Fat burning rises steadily from about 8–12 hours after your last meal as glycogen runs low. Ketone production climbs noticeably after about 12–16 hours. Timing varies with your last meal and activity.
Generally yes for healthy adult women. Some find shorter fasts (14:10) better for energy, sleep and menstrual regularity. It’s not recommended during pregnancy or breastfeeding, or with a history of eating disorders.
Many medicines are fine, but some must be taken with food (NSAIDs, steroids, metformin) and others need dose changes during fasting (insulin, sulfonylureas, SGLT2 inhibitors). Check with your pharmacist or doctor before you start.
Light to moderate exercise during a fast is fine for most people. For hard or long workouts, train near the start of your eating window, or break the fast soon afterwards with protein and carbs.
Manzoor Ahmad
Pharmacist · Supplement & Nutrition Writer (9+ yrs) · Calculator Developer (13+ yrs)
I’m a practising pharmacist and DexoCalc’s developer. Patients who fast often ask me when to take their tablets, and that’s the gap this calculator fills. Alongside the eating window, it shows which medicines must sit inside it and which suit a fasting morning.
References
- de Cabo R, Mattson MP. Effects of intermittent fasting on health, aging, and disease. N Engl J Med. 2019;381(26):2541–2551. PubMed
- Sutton EF, et al. Early time-restricted feeding improves insulin sensitivity, blood pressure, and oxidative stress. Cell Metab. 2018;27(6):1212–1221. PubMed
- Lowe DA, et al. Effects of time-restricted eating on weight loss (TREAT RCT). JAMA Intern Med. 2020;180(11):1491–1499. PubMed
- Anton SD, et al. Flipping the metabolic switch. Obesity. 2018;26(2):254–268. PubMed
- Grajower MM, Horne BD. Clinical management of intermittent fasting in patients with diabetes mellitus. Nutrients. 2019;11(4):873. PubMed
Medical disclaimer: Fasting phases are approximate, and this tool is not medical advice. Don’t fast without medical supervision if you are pregnant, breastfeeding, under 18, underweight, have a history of eating disorders, or take insulin, sulfonylureas or SGLT2 inhibitors.