Most people aiming for weight loss do best with shorter, repeatable fasting windows (like 12–16 hours per day) rather than long multi‑day fasts, which should only be done rarely and with medical supervision. The “right” length is the one that creates a calorie deficit, is safe for your health, and is sustainable for months, not days.

Key safety note

  • If you have diabetes, take blood pressure meds, have a history of eating disorders, are pregnant/breastfeeding, under 18, or on multiple medications, fasting should be supervised by a clinician or avoided altogether.
  • If you ever feel faint, confused, chest pain, trouble breathing, or heart palpitations while fasting, you should stop and seek medical help.

Common fasting lengths for weight loss

These are typical patterns from medical and nutrition sources, not personal recommendations.

12–14 hours (gentle start)

  • Example: Finish dinner at 7 p.m., eat breakfast at 7–9 a.m. the next day.
  • Often suggested as a beginner approach because it mostly just cuts late‑night snacking and is easier to maintain long term.
  • Can still support weight loss if it helps you reduce overall calories and choose healthier foods.

16:8 (most popular “sweet spot”)

  • Example: Fast 16 hours, eat within an 8‑hour window (like 11 a.m.–7 p.m.).
  • Widely used because it’s long enough to boost fat burning and insulin sensitivity, but short enough for most people to stick with daily.
  • Large trials of 8‑hour time‑restricted eating show meaningful weight loss over months when combined with a generally healthy diet.

20:4 or OMAD (advanced, not for beginners)

  • 20‑hour fast with a 4‑hour eating window, or “one meal a day.”
  • Can create a big calorie deficit but is harder to sustain, may lead to overeating in the eating window, and can be problematic for blood sugar, mood, or training in some people.
  • Usually considered an advanced pattern and not a first step.

24‑hour fasts (1–2× per week)

  • Eat normally most days, then occasionally fast for 24 hours (e.g., dinner‑to‑dinner once per week).
  • Can help some people reduce weekly calories, but is tougher mentally and physically; you need to be very careful about hydration and not bingeing afterward.

36–48 hours and longer (rare, supervised only)

  • 36–48‑hour fasts can increase fat burning and certain cellular repair processes, but they are usually recommended at most 1–2 times per month, not weekly.
  • Multi‑day water fasts (5–20 days) can cause rapid weight loss, but much of it is water and lean mass (muscle) rather than just fat, and experts warn against doing more than about 5 days without medical supervision.
  • Longer fasts are not necessary for most people and carry more risk than benefit for simple weight loss.

What actually drives weight loss

Regardless of the fasting window, weight loss still comes down mainly to a sustained calorie deficit.

  • Time‑restricted eating (like 16:8) often helps people eat fewer calories by limiting the hours in which they snack or graze.
  • Studies comparing intermittent fasting to “regular” calorie‑restricted diets often find similar total weight loss when calories and protein are matched.
  • Fasting does not bypass basic energy balance; it’s a tool to make eating fewer calories easier for some people.

How to choose a fasting length

Use these steps to pick something both effective and realistic:

  1. Start short and build up
    • Begin with 12 hours overnight for 1–2 weeks (ex: 7 p.m.–7 a.m.).
 * If that feels OK, move to 14–16 hours (like skipping late‑night snacks and sometimes breakfast) and track your weight, energy, and mood for a few weeks.
  1. Aim for consistency, not extremes
    • Doing 16:8 most days will usually beat one heroic 48‑hour fast followed by a rebound binge.
 * Expect progress over months; very aggressive fasting often leads to regaining the weight, especially after long water fasts.
  1. Protect muscle and energy
    • Eat enough protein during your eating window, and include strength training to limit muscle loss.
 * Very long fasts tend to cost more lean mass, which is the opposite of what you want for long‑term metabolism.
  1. Watch your symptoms
    • “Normal” early effects: mild hunger waves, a bit of fatigue, some distractibility as your body adapts.
 * Red flags: dizziness that doesn’t pass, extreme weakness, confusion, blurry vision, heart pounding, or feeling like you might faint.

Forum & trending angles

  • In current forum discussions, many people report the best balance for weight loss as 16:8 or a similar 14–16‑hour daily fast because it fits work and social life while still cutting calories.
  • There is ongoing debate between “maximizing fat burning” with longer fasts vs. “maximizing adherence” with shorter, daily windows; research so far leans toward adherence being more important than the exact fasting length.
  • Longer “prolonged” fasts and water fast retreats are trending online, but clinical reviews emphasize that benefits often disappear after refeeding and that they are not a magic fix for long‑term weight management.

Practical example plans (non‑medical, for illustration)

These are examples , not prescriptions. Any major change should be discussed with a healthcare professional if you have medical conditions.

  • Beginner weight‑loss pattern
    • 12–14‑hour fast nightly.
    • Focus on whole foods, plenty of protein and fiber, and limiting sugary drinks and ultra‑processed snacks.
  • Intermediate pattern (common goal)
    • 16:8 on most days (e.g., 11 a.m.–7 p.m. eating window).
    • Keep protein high, lift weights 2–3× weekly, and keep portions modest inside the window.
  • Occasional longer fast (experienced, healthy adults only)
    • At most 24 hours once in a while if it truly feels good and does not trigger bingeing afterward.
* Avoid going beyond about 48 hours without medical supervision because of increasing risks and muscle loss.

TL;DR – “How long should you fast for weight loss?”

For most generally healthy adults, a 12–16‑hour daily fast is long enough to support weight loss and metabolic benefits, as long as it helps you eat fewer calories and you can stick with it for the long term. Very long fasts (several days) can cause rapid but often temporary weight loss, more muscle loss, and should not be done for more than about 5 days without medical supervision, and they are usually unnecessary for effective weight loss.