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Calculates renal free water clearance (CH₂O) from urine flow, urine osmolality, and plasma osmolality to characterize renal water handling in polyuria, hyponatremia, and hypernatremia.
Clinical decision-support tool for qualified healthcare professionals. Results are not a diagnosis and should not replace professional clinical judgment.
Free water clearance is the classic renal physiology metric of water handling: it is the portion of urine flow that is free of osmotically active solute. It remains useful in the bedside evaluation of polyuria and dysnatremia.
Specialty
Internal MedicineCategory
NephrologyDifficulty
Basic
Estimated Time
30 seconds
Version
1.0
Last Updated
2026-08-16
Formula
Keywords
Normal Range
Positive (dilute urine, free water excretion); negative (concentrated urine, free water reabsorption)
| Classification | Range |
|---|---|
Positive — free water excretion dilute urine (Uosm < Posm); DI, polydipsia | > 0 mL/min |
Negative — free water reabsorption concentrated urine (Uosm > Posm); SIADH, dehydration | < 0 mL/min |
Calculates renal free water clearance (CH₂O) from urine flow, urine osmolality, and plasma osmolality to characterize renal water handling in polyuria, hyponatremia, and hypernatremia.
Free water clearance C(H2O) = V x (1 - urine osmolality / plasma osmolality), where V is the urine flow rate. Positive values indicate free water excretion and negative values indicate free water retention.
Urine flow rate in mL/min, the multiplier V in the formula.
Urine osmolality in mOsm/kg, used as the numerator of the ratio.
Plasma osmolality in mOsm/kg, used as the denominator of the ratio.
A positive CH₂O (dilute urine, Uosm < Posm) is seen in diabetes insipidus, primary polydipsia, and after water loading. A negative CH₂O (concentrated urine, Uosm > Posm) is seen in SIADH, volume depletion, and dehydration. Because urea contributes to osmolality but not tonicity, CH₂O can mislead in high-urea osmotic diuresis — use electrolyte-free water clearance there.
Free water clearance is a classic renal physiology metric that quantifies whether the kidney is excreting or conserving water, supporting the evaluation of polyuria and dysnatremia.
A 30-year-old man with suspected diabetes insipidus has a urine flow of 1.5 mL/min, urine osmolality 80 mOsm/kg, and plasma osmolality 290 mOsm/kg.
Inputs:
Expected Result: CH₂O = 1.5 × (1 − 80/290) ≈ 1.09 mL/min. This positive value indicates dilute urine with free water excretion.
A positive CH₂O means the kidney is excreting electrolyte-free water (dilute urine). This is appropriate after water loading and seen in diabetes insipidus and primary polydipsia.
In high-urea (osmotic) diuresis and when predicting the direction of serum sodium change, the electrolyte-free water clearance is more accurate because it excludes urea.
Disclaimer: This calculator is intended for educational and clinical decision support purposes only. Water balance decisions require the full clinical picture, including intake, insensible losses, and serum sodium trends.