Methodology

How HydrationGuide calculators work

Every number on this site is attributed to a published source. This page lists what each calculator does, the math it runs, and what it doesn't try to do.

Created and maintained by Reede Taylor. Last reviewed August 6, 2026. I am not a doctor or a dietitian; see why I built this.

Editorial principles

Every calculator is built on published guidelines or peer-reviewed research: IOM, EFSA, ACSM, ISSN, WHO, NPS. Sources are named on the page that uses them.
Where the evidence gives a range, the result is a range. Single numbers to the decimal would be fake precision, so every model works in low/mid/high bands with the midpoint as the practical target.
All calculations run in your browser. Inputs are saved to your device only (localStorage); your inputs are never sent to us, and no account exists. We do collect anonymous, cookieless page-view statistics — see the privacy policy.
Estimates, not medical advice. Every tool carries that line, and conditions that change fluid needs (kidney, heart, and liver disease, some medications, pregnancy) are called out where they matter.

The calculators

Water Intake Calculator

Daily drinking target from body weight, exercise, and climate. Every output is a low/mid/high range because the underlying evidence supports ranges, not single numbers.

Model

baseline = weight_kg × 30–35 mL
exercise = minutes/30 × 250–475 mL
climate = +500–1,000 mL/day (hot or humid, non-acclimatized)
total = baseline + exercise + climate
drink target = total × 0.8 (about 20% of water arrives in food)

The 30–35 mL/kg clinical heuristic brackets the IOM adequate intakes (3.7 L/day total water for men, 2.7 L for women) and EFSA's slightly lower European values at typical adult weights. The exercise band reflects the ACSM's 0.4–1.0 L/hour sweat-rate range for most people in most conditions.

Sources

Creatine Water Intake Calculator

The same weight-and-training baseline as the pillar calculator, plus a supplement adjustment covering the water creatine pulls into muscle as stores saturate.

Model

base = water intake model above (weight, training)
creatine adjustment (maintenance, 3–5 g/day) = +250–500 mL/day
creatine adjustment (loading, 20 g/day) = +500–1,000 mL/day
drink target = base drink target + creatine adjustment

No clinical trial prescribes an exact "extra litres on creatine" figure, so this adjustment is an estimate built from what is documented: creatine is osmotically active and muscle stores roughly 1–2 litres of additional water as they saturate, fastest during a 20 g/day loading week. The adjustment is drunk water, so it lands fully on the beverage target rather than being split with food. The ISSN position stand finds no evidence that creatine causes dehydration or cramping.

Sources

Hiking Water Calculator

Water to carry on a hike from time on trail, effort, and heat. Deliberately has no body-weight input: trail guidance is expressed per hour, and weight-indexing it would be false precision.

Model

effort rate: easy 300–500, moderate 400–600, strenuous 550–850 mL/hour
heat add: cool +0, warm +100–200, hot +250–350 mL/hour
water to bring = (effort rate + heat add) × hours on trail

The moderate mid-point matches the American Hiking Society's half-litre-per-hour guidance; a strenuous hike on a hot day lands at the litre-per-hour upper bound the National Park Service uses for summer canyon hiking. Warnings trigger for heavy loads (a filter beats carrying 3+ litres where refills exist) and for multi-hour heat (sodium loss).

Sources

Homemade Electrolyte Drink Calculator

Turns a target electrolyte profile into kitchen measures for any batch size. Three profiles: sports-drink strength, LMNT-style zero sugar, and WHO-style rehydration.

Model

targets per litre: sports drink Na 500 mg / K 200 mg / 6% carbs;
  zero sugar Na 1,000 / K 200 / Mg 60; rehydration Na 1,150 / K 350 / 2.5% carbs
salt substitute tsp = K target ÷ 1,400 mg (sized to potassium first)
table salt tsp = (Na target − substitute sodium) ÷ 2,300 mg
sugar tbsp = grams ÷ 12.5
amounts round to ⅛ tsp / ½ tbsp; shown nutrition is recomputed from the rounded amounts

Ingredient constants come from labels and USDA data: table salt carries ~2,300 mg sodium per teaspoon; a 50/50 potassium salt substitute carries ~290 mg sodium and ~350 mg potassium per quarter teaspoon. Eighth-teaspoon rounding quantises sodium in ~288 mg/L steps at one litre, so the sports-drink preset delivers about 433 mg/L rather than its nominal 500 mg target: close to commercial sports drinks (~460 mg/L) and a little under the ACSM's 0.5–0.7 g/L band. The rehydration preset now reproduces the WHO/UNICEF reduced-osmolarity oral rehydration composition exactly (sodium 75 mmol/L, potassium 20 mmol/L, glucose 13.5 g/L); it previously carried materially weaker figures while still invoking WHO. Batch totals drive warnings covering potassium and kidney or blood-pressure medicines, whole-day sodium, sugar load, and the fact that a pharmacy ORS sachet is the right choice for illness.

Sources

Sweat Rate Calculator

The only calculator here that measures rather than estimates. The visitor brings real before-and-after weights from a session they did, and the arithmetic converts them into a personal hourly sweat rate.

Model

sweat loss = (pre-exercise mass − post-exercise mass) + fluid drunk
sweat rate = sweat loss ÷ hours trained
body mass lost % = (pre − post) ÷ pre × 100
sodium lost = sweat litres × 400–1,600 mg/L (band set by the saltiness answer)
post-session replacement = remaining deficit × 1.25–1.5

One kilogram of body mass lost is treated as one litre of fluid, the standard assumption in the field protocol. The sweat rate itself is reported as a single number because it is the visitor's measurement, not our estimate; only the sodium figure carries a range, because whole-body sweat sodium runs from about 18 to 71 mmol/L between people (roughly 400–1,600 mg/L) and is not predictable from body weight. Warnings trigger past the ACSM's 2% body-mass-loss performance threshold, when the visitor finishes heavier than they started (overdrinking, the hyponatremia direction), and at implausibly high rates that usually indicate wet kit on the scale.

Sources

Caffeine Hydration Calculator

How much of the coffee and tea you drank counts toward your daily fluid. Mostly it answers the question by disagreeing with its premise: at ordinary intakes the answer is all of it.

Model

volume and caffeine summed across servings from a drinks table
threshold by habit: daily 400 mg, a few times a week 300 mg, rarely 200 mg
diuretic offset = (caffeine − threshold) × 0.2–2.0 mL/mg, zero below threshold
offset is clamped so it can never exceed the volume drunk
counts toward fluid = volume − offset

The default is that a caffeinated drink contributes its full volume, which is what the evidence supports rather than a concession: Maughan's beverage hydration index found coffee, tea, and cola statistically indistinguishable from water, and Killer's trial found no hydration difference between four mugs of coffee a day and the same volume of water in habitual drinkers. The offset only applies above a per-day threshold and shrinks with habituation, since tolerance to the diuretic effect develops within days. The caffeine total is tracked separately against the 400 mg/day EFSA and FDA adult reference point, which is a caffeine-safety question rather than a hydration one.

Sources

How I use AI

HydrationGuide is drafted with AI assistance and read by hand before anything goes live. I use AI the way a working writer uses a research assistant: it speeds up the labor, it does not replace the judgment. Being specific about where that line falls matters more than sounding rigorous, so here it is.

What AI does

Produces first drafts and rephrasing options.
Proposes the sources that end up cited on a page.
Helps keep voice, structure, and terminology consistent across pages.
Flags edge cases and ambiguous wording worth documenting.

What AI does not do

Set the published formulas, ranges, or defaults.
Publish anything without me reading it first.
Decide what the site will and will not claim.

What I do not claim: I have not sat down with every cited paper and read it end to end. Sources are proposed during drafting and linked here so you can go to the original yourself, which is the entire point of citing them rather than asking you to take my word for it. If you follow a link and it does not say what the page says it says, that is a bug, not a difference of interpretation.

What does not depend on trusting either of us: every formula is published on this page, and every worked example on the site is computed by the same engine the calculator runs, so the tables cannot quietly drift away from the tool. When something here turns out to be wrong it gets changed, and the change is visible in the page history. Tell me and it gets fixed.

Limitations

These tools model averaged fluid physiology for healthy adults. Real sweat rates vary more than threefold between people and conditions; the ACSM band the exercise math uses (0.4–1.0 L/hour) covers most people most of the time, not endurance athletes in extreme heat, who should measure their own sweat rate. The creatine adjustment is an estimate over a documented mechanism, not a trial-derived prescription. The hiking calculator plans day hikes, not desert backpacking, where local ranger guidance wins.

HydrationGuide does not diagnose or treat anything. Kidney, heart, and liver conditions, diuretics and some other medications, pregnancy, and old age all change fluid and electrolyte needs; potassium-based salt substitutes can be dangerous with kidney disease and some blood-pressure medications. Ask your doctor.

How this page is maintained

Each calculator's engine is a small, framework-free module in the site's source; when a formula changes, this page and /llms.txt (the summary file AI search engines read) are updated in the same commit, and the review date above is bumped.

These tools are for informational purposes only and are not a substitute for medical advice.