FREE Personalized Hydration Plan Calculator for Endurance Athletes

Optimize every mile with data‑driven hydration. Use our FREE hydration planner, powered by peer‑reviewed science and data from the hDrop wearable hydration sensor, to create a custom fluid and electrolyte plan for marathon running, cycling, or triathlon.

FREE hDrop Hydration Planning

Select the main sport for this plan
How would you describe yourself?
Morning body weight
Planned workout length
How hard will you push? [1 = Very Easy, 10 = Max]
Exact = Measured with an hDrop sensor or your scale.
Single lab tests miss ~40 % real‑world sweat sodium swings¹; an hDrop sensor measures live.
¹ Harris C., Blow A., Hew‑Butler T. (2024) link

Disclaimer: plan is an evidence‑based estimate and does NOT replace individual medical advice. You consent to follow‑up email from hDrop.

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Why Personalized Hydration Matters

  • Sweat rates vary 0.5 – 3.0 L·h⁻¹ depending on multiple factors like genetics, intensity, weather conditions…; sodium losses also range 230 – 3 000 mg Na⁺·L⁻¹. One generic rule cannot cover that spread.

  • Even 2 % body‑mass loss from dehydration slows endurance pace and elevates heart‑rate strain.

  • Over‑drinking plain water can dilute blood sodium, risking exercise‑associated hyponatremia.

  • Continuous monitoring with hDrop shows how intensity, temperature, and duration shift your sweat profile from session to session—insights a one‑time lab test misses.

Bottom line: a personalized plan protects performance and health.

Why We Break Down Your Hydration Plan (Pre • During • Post)

Tailoring intake to each phase of training or competition is grounded in exercise‑physiology research:

  1. Pre‑exercise (prime): Starting euhydrated expands plasma volume, lowers cardiovascular strain, and primes sweat mechanisms for heat dissipation — benefits documented in endurance trials and summarized by the ACSM Position Stand on Exercise & Fluid Replacement (Sawka et al., 2007) and the International Olympic Committee (IOC) Nutrition Consensus (Thomas et al., 2016). Controlled studies show that consuming 5–7 mL kg⁻¹ fluid with sodium 4 h pre‑event produces superior gastric emptying and thermoregulation compared with last‑minute “chugging.”

  2. During exercise (sustain): Matching a portion of ongoing losses — typically 0.4–0.8 L h⁻¹ fluid and 300–600 mg Na⁺ h⁻¹ — limits body‑mass loss to < 2 %, maintains serum sodium, and preserves pace/power. These intake ranges are synthesized from over 40 field/lab studies reviewed by Hew‑Butler et al. (2019, Curr Sports Med Rep) and the ACSM guidelines (Sawka et al., 2007).

  3. Post‑exercise (restore): Replacing ~125–150 % of the fluid deficit plus electrolytes over the first 2–6 h speeds plasma‑volume recovery and glycogen resynthesis (Maughan & Shirreffs, 2010; Thomas et al., 2016). Sodium of ≥1 g during this window enhances fluid retention versus plain water (Armstrong et al., 2020).

Breaking the plan into these time‑windows acknowledges that hydration goals shift from priming → sustaining → restoring. Continuous sweat data from hDrop® — and documented variability of sweat sodium with intensity (Harris, Blow & Hew‑Butler, 2024) — further demonstrate why one static “drink X per hour” rule is inadequate for optimized performance and safety.

Frequently Asked Questions

Q1 — I don’t know my sweat sodium concentration. Can I still use the calculator?

Yes. Select the “estimate” option—enter low, medium, or high based on visible salt residue or previous lab tests. For most athletes, medium (~900 mg Na⁺·L⁻¹) is a practical default until you gather sensor data.

Q2 — Will this plan stop muscle cramps?

Adequate electrolytes can reduce cramp risk in some athletes, but fatigue, pacing, and neuromuscular factors also contribute. Follow the plan and adjust based on personal response. For the best accurate data, you will have to track multiple activities and weather data with a sweat sensing device like hDrop.

Q3 — How do I avoid over‑hydrating?

Stick to the hourly volumes provided and listen to thirst cues. If your weight post‑race exceeds start weight, reduce intake next time. Always try not to drink more than what you lose.

Q4 — Is the calculator suitable for hot‑weather ultras?

Yes, but input longer duration and higher environmental temps to get higher intake targets. Re‑test with hDrop as you acclimatize; sweat sodium often decreases with heat adaptation.

Q5 — Does sweat sodium really change with intensity and conditions?

Yes. Sweat sodium concentration is dynamic. It tends to decrease during low‑intensity efforts and rise when exercise intensity, ambient temperature, or airflow demand increase. In trained cyclists, Harris, Blow & Hew‑Butler (2024) found sweat sodium was ~12 mmol L⁻¹ below passive baseline at 50 % of functional threshold power, but ~8 mmol L⁻¹ above baseline at 100 % FTP—a swing approaching 40 %. Hot, dry weather or heavy clothing can push values higher, while heat acclimation or cool, humid conditions can lower them. Record multiple sessions wearing hDrop to capture your full range. (Harris et al., 2024)

Q6 — I got a lab sweat test once; is that good enough?

Usually not. Harris, Blow & Hew‑Butler (2024) repeated sweat‑sodium testing weekly and observed swings of up to ~20 mmol L⁻¹ (≈40 % variation) across visits and intensities, even under controlled lab conditions. Real‑world factors like heat, terrain, and training status add further fluctuation. One snapshot can therefore under‑ or over‑estimate your race‑day needs by a large margin. Re‑testing in varied conditions—or wearing hDrop for session‑by‑session tracking—builds a reliable hydration profile. (Harris et al., 2024)