# Free Water Clearance Calculator

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.

> Canonical page: https://elysiatools.com/en/tools/free-water-clearance

- **Category:** Health

- **Keywords:** free water clearance, electrolyte-free water clearance, C_H2O, eC_H2O, SIADH, diabetes insipidus, hypernatremia, hyponatremia, sodium, osmolality, nephrology, renal

## 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.

## Inputs

- **Urine Flow Rate V (per time, e.g. mL/min)** (number): Urine flow rate V (volume per unit time). Must be positive. Use any consistent per-time unit; output shares it.
- **Urine Osmolality U_osm** (number): Urine osmolality U_osm. Must be positive.
- **Plasma Osmolality P_osm** (number): Plasma osmolality P_osm. Must be positive.
- **Urine Sodium U_Na (mmol/L, optional)** (number): Optional urine sodium U_Na (mmol/L) for the electrolyte-free water clearance. Must be ≥0.
- **Urine Potassium U_K (mmol/L, optional)** (number): Optional urine potassium U_K (mmol/L) for the electrolyte-free water clearance. Must be ≥0.
- **Serum Sodium P_Na (mmol/L, optional)** (number): Optional serum sodium P_Na (mmol/L) for the electrolyte-free water clearance. Must be positive.
- **Decimal Places** (number)

## 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

- Enter a positive urine flow rate, urine osmolality, and plasma osmolality.
- The calculator applies C_H₂O = V · (1 − U_osm/P_osm).
- A positive C_H₂O indicates dilute urine excreting free water, while a negative value indicates concentrated urine conserving free water.
- Add 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.

## Frequently asked questions

### 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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