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 5, 2026.

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); nothing is sent to a server, and no account exists.
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
Lite Salt tsp = K target ÷ 1,400 mg (sized to potassium first)
table salt tsp = (Na target − Lite Salt 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; 50/50 Lite Salt carries ~290 mg sodium and ~350 mg potassium per quarter teaspoon. The sports-drink sodium target sits in the ACSM's 0.5–0.7 g/L band for exercise over an hour; the rehydration profile sits in WHO oral-rehydration territory, where glucose actively assists sodium and water absorption.

Sources

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.