Most health advice arrives as a general rule — “cut off caffeine by 2pm,” “get enough protein,” “drink eight glasses of water.” The rules are fine, but the number that matters is yours. These tools run the arithmetic behind guidance already covered on this site, using your own inputs.
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How much caffeine is still in you at bedtime?
Caffeine has a half-life of roughly 5 hours in most adults, so half of what you drink is still circulating five hours later. This works out what is left when you try to sleep.
Half-life varies with genetics (CYP1A2), pregnancy, and some medications, so treat this as an estimate of the pattern rather than a precise blood level. Background and sources: Caffeine: how much is too much?
What time should you go to bed?
Work backwards from when you have to be up. Adults need 7–9 hours, and falling asleep takes most people 10–20 minutes — time that is easy to forget to budget for.
If you regularly need more than 30 minutes to fall asleep, or wake and cannot get back to sleep, that is a pattern worth addressing rather than scheduling around — see why you wake at 3am and when to get tested for sleep apnea. Background: an evening routine that actually works.
What is your daily protein target?
The RDA of 0.8 g/kg is the floor that prevents deficiency, not the amount most people do best on. This shows both, plus how to split it across meals.
Ranges follow the RDA plus commonly cited targets for active adults and older adults. People with kidney disease should get an individual target from their clinician rather than using a general calculator. Background and sources: How much protein do you actually need?
How much water do you actually need?
“Eight glasses a day” has no research behind it. The reference figures are total water — and about a fifth of that already comes from food.
These are population reference intakes, not targets to hit precisely. Thirst regulates this well in healthy adults; urine that is pale yellow is a better day-to-day signal than a number. People with kidney disease or heart failure are often on a fluid restriction and should follow that instead. Background and sources: how much water should you drink?
What is your Zone 2 heart rate?
Zone 2 is the conversational pace that builds aerobic base — roughly 60–70% of maximum heart rate. The most common mistake is going too hard.
220 − age is a population estimate with a standard deviation of about 10–12 bpm, so treat the range as a starting point and let the talk test correct it: in Zone 2 you can hold a full sentence but would rather not. Background and sources: Zone 2 cardio: a practical guide
A note on what these can and cannot tell you
Each calculator uses population averages — a 5-hour caffeine half-life, a 220 − age maximum heart rate, reference water intakes, protein targets per kilogram of body weight. Averages describe groups, not individuals. Your caffeine metabolism is partly genetic, your true maximum heart rate can sit 10–12 bpm either side of the estimate, and protein and fluid needs shift with kidney function, pregnancy, and illness.
Use these to understand the shape of the relationship — how slowly caffeine actually clears, how far the RDA sits below an athlete’s target, how much of your “water intake” is already coming from food, how easy Zone 2 is supposed to feel. Then adjust with what you observe in yourself, and talk to a clinician about anything that affects a medical condition or medication.