Intermittent Fasting Calculator
Pick a fasting protocol and your first-meal time to see exactly when your eating window opens and closes — plus a timeline of the metabolic phases across your fast.
8 h eating · 16 h fasting
What happens during your 16 h fast
Blood glucose and insulin rise as the last meal is absorbed.
Insulin falls; the body starts drawing on stored glycogen for fuel.
Glycogen runs low; lipolysis ramps up and ketone production starts.
Ketones become a major fuel; cellular clean-up (autophagy) increases.
Phase timings are approximate and vary with your last meal, activity and metabolism. Educational only — fasting is not appropriate for everyone (pregnancy, breastfeeding, a history of disordered eating, type 1 diabetes or glucose-lowering medication). Check with a clinician if you have a health condition.
Make fasting work with your body
ONDA Life tracks how your eating window affects sleep, energy and recovery — so you can find the fasting pattern your body actually responds to, not just the trendy one.
Download ONDA Life on the App Store →Common questions
What is 16:8 intermittent fasting?
16:8 means you fast for 16 hours and eat all your food within an 8-hour window — for example, eating between noon and 8 pm, then fasting until noon the next day. It is the most popular protocol because it fits naturally around sleep and a skipped breakfast, and is easy to sustain.
When does fat burning and ketosis start during a fast?
Insulin starts falling a few hours after your last meal, and the body shifts toward burning fat once liver glycogen runs low — typically around 12 hours in. Meaningful ketosis and increased autophagy generally build from roughly 16 hours onward, though the exact timing depends on your last meal, activity and metabolism.
Can I drink anything while fasting?
Yes — water, black coffee, plain tea and other zero-calorie drinks are fine and do not break a fast. Anything with calories (milk, sugar, juice, most "diet" additions beyond trace amounts) raises insulin and ends the fasted state. Staying hydrated and getting electrolytes helps with energy and headaches.
Which fasting window is best for beginners?
Start with 14:10 or 16:8. They deliver most of the metabolic benefit while being easy to live with — usually just delaying breakfast and not snacking after dinner. Tighter windows like 20:4 or OMAD push more time in the fasted state but make it harder to eat enough protein and micronutrients, so progress to them gradually if at all.
Is intermittent fasting safe for everyone?
No. Fasting is not recommended during pregnancy or breastfeeding, for people with a history of disordered eating, for type 1 diabetics or those on glucose-lowering medication without medical supervision, and for some other conditions. If you take regular medication or have a health condition, check with your clinician before starting.