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Free Water Clearance Calculator

· Last updated：2026-08-06

This tool is for informational purposes only and does not constitute medical, financial, or legal advice; consult a qualified professional for advice.

Calculate solute-free water clearance (C_H₂O) and electrolyte-free water clearance (eC_H₂O) to differentiate hypo-/hypernatremia. Classic: C_H₂O = V·(1 − U_osm/P_osm); positive = dilute urine excreting free water (diabetes insipidus, polydipsia, water-load recovery), negative = concentrated urine conserving free water (dehydration, SIADH). Electrolyte-free: eC_H₂O = V·(1 − (U_Na + U_K)/P_Na), which better predicts the direction of serum sodium change — a negative eC_H₂O means the urine is electrolyte-free-water-rich and tends to raise serum Na⁺, informing IV fluid choice. Optional Na/K inputs enable the electrolyte-free calculation. Derived from Rose, Shimizu 2002 (Nephron), and NephSIM. Point estimate; interpret with full clinical context. Not medical advice.

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Tool usage guide

Learn when to use this tool, what it supports, and how real users apply it.

Key facts

Category
Health
Input types
number
Output type
json
Sample coverage
4
API ready
Yes

Overview

The Free Water Clearance Calculator estimates solute-free water clearance (C_H₂O) from urine flow and osmolality, with an optional electrolyte-free water clearance (eC_H₂O) calculation using urine sodium, urine potassium, and serum sodium. Use the results as point estimates alongside the full clinical context; this tool is not medical advice.

When to use

  • Estimate whether urine is dilute or concentrated from urine flow, urine osmolality, and plasma osmolality.
  • Assess free-water excretion patterns relevant to conditions such as diabetes insipidus, polydipsia, dehydration, or SIADH.
  • Calculate electrolyte-free water clearance when urine sodium, urine potassium, and serum sodium values are available.

How it works

  1. 1Enter a positive urine flow rate, urine osmolality, and plasma osmolality.
  2. 2The calculator applies C_H₂O = V · (1 − U_osm/P_osm).
  3. 3A positive C_H₂O indicates dilute urine excreting free water, while a negative value indicates concentrated urine conserving free water.
  4. 4Add urine sodium, urine potassium, and serum sodium to calculate eC_H₂O = V · (1 − (U_Na + U_K)/P_Na).

Use cases

  • Review urine concentration and free-water handling in nephrology education or clinical analysis.
  • Compare dilute and concentrated urine patterns in suspected diabetes insipidus, polydipsia, dehydration, or SIADH.
  • Estimate how electrolyte-free water clearance may relate to the direction of serum sodium change.

Examples

1. Estimating concentrated urine in SIADH

Nephrology learner

Background

A urine flow rate of 1 mL/min is measured with urine osmolality of 600 mOsm/kg and plasma osmolality of 260 mOsm/kg.

Problem

Determine whether the kidneys are excreting or conserving free water.

How to use

Enter urine flow rate 1, urine osmolality 600, plasma osmolality 260, and select the desired decimal places.

urineFlow=1; urineOsm=600; plasmaOsm=260

Outcome

C_H₂O is −1.3077 mL/min, indicating concentrated urine and free-water conservation.

2. Calculating free-water and electrolyte-free water clearance

Clinical educator

Background

A patient has a urine flow rate of 5 mL/min, urine osmolality of 80 mOsm/kg, plasma osmolality of 290 mOsm/kg, urine sodium of 20 mmol/L, urine potassium of 10 mmol/L, and serum sodium of 125 mmol/L.

Problem

Calculate both clearance measures to describe the urine dilution pattern and its relationship to serum sodium.

How to use

Enter all six laboratory and urine measurements, then review the C_H₂O and eC_H₂O results.

FAQ

What does a positive C_H₂O mean?

A positive C_H₂O means the urine is more dilute than plasma and the kidneys are excreting free water.

What does a negative C_H₂O mean?

A negative C_H₂O means the urine is more concentrated than plasma and the kidneys are conserving free water.

What is electrolyte-free water clearance?

eC_H₂O estimates water clearance relative to urine sodium and potassium compared with serum sodium.

Which inputs are required for C_H₂O?

Urine flow rate, urine osmolality, and plasma osmolality are required.

Which inputs enable eC_H₂O?

Enter urine sodium, urine potassium, and serum sodium in addition to the required C_H₂O inputs.

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urineFlow=5; urineOsm=80; plasmaOsm=290; urineSodium=20; urinePotassium=10; serumSodium=125

Outcome

C_H₂O is 3.6207 mL/min and eC_H₂O is 3.8 mL/min, indicating dilute urine with positive free-water clearance.