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About this tool
Plan a two-meal 6-hour eating window with an 18-hour fast, plus a drink-by-drink hydration schedule and sweat sodium estimates.
The 18:6 Fasting Window Planner lays out a two-meal, 6-hour eating window and the 18-hour fast that surrounds it, then solves the problem a short window creates: hydration. Around 20% of daily water normally arrives with food, so a 6-hour window pushes most of the day's fluid into the fasting hours — the planner works from the EFSA adequate intake of 2.5 litres of total water a day for men and 2.0 for women, adds an allowance for training sweat at 0.4 to 0.8 litres an hour, and turns the result into timed drinks. It also estimates the sodium lost in sweat against the WHO guidance of under 2000 mg a day.
Open 18:6 Fasting Window Planner on AltFTool — it loads instantly in your browser.
Add your input to the workspace.
Adjust the options until the result looks right.
Copy or download the output and put it to work.
Accounts for the water you normally get from food but will not during 18 fasting hours.
Drinks are spread from waking until two hours before bed, so hitting the target does not cost you sleep.
Losses are shown against the WHO 2000 mg daily sodium guidance rather than as a reason to buy supplements.
Eating all food within a 6-hour window and fasting for the other 18 hours of the day, usually as two meals. It is a step up from 16:8, and the shorter window is the reason most people who follow it eat twice rather than three times.
Aim for roughly 2 litres of drinks a day if you are a man and 1.6 litres if you are a woman, before adding for exercise — those figures come from the EFSA adequate intake of 2.5 and 2.0 litres of total water minus the roughly 20% that normally comes from food. With an 18-hour fast most of that has to be drunk during the fasting hours, and training adds about 0.4 to 0.8 litres per hour of sweating.
Usually not. Daily time-restricted eating is short enough that normally salted food inside your window replaces what you lose, and a litre of sweat carries only about a gram of sodium — half the WHO daily guidance of 2000 mg, which most diets already exceed. Multi-day fasts are a different matter. Check with a doctor before taking sodium, potassium or magnesium supplements if you have high blood pressure, kidney disease or heart failure, or take diuretics.
Plain black coffee and plain tea have negligible calories and are the usual choices during the fast; milk, sugar, cream, honey and sweetened drinks supply calories and end it. Habitual coffee drinkers do not lose net fluid from moderate intake, so a black coffee still counts towards your drink total.