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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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.

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SIADH: V=1, U_osm=600, P_osm=260 → C_H2O = −1.308 mL/min (concentrated)

C_H₂O = 1·(1 − 600/260) = −1.308 mL/min. Negative → concentrated urine conserving free water, consistent with SIADH. Electrolyte-free clearance omitted here.

{"result":"C_H₂O = −1.308 (Negative — concentrated urine). C_H₂O = V·(1 − U_osm/P_osm).","metadata":{"input":{"urineFlow":1,"urineOsm":600,"plasmaOsm":260},"result":{"osmolarClearance":2.3077,"freeWaterClearance":-1.3077,"category":"Negative — concentrated urine","interpretation":"C_H₂O < 0 (U_osm > P_osm): kidney conserves free water. Seen with hypovolemia/dehydration (appropriate ADH) or SIADH (inappropriate ADH with inappropriately concentrated urine despite low plasma osmolality).","method":"C_H₂O = V·(1 − U_osm/P_osm); C_osm = U_osm·V/P_osm","model":"Rose BD; Shimizu 2002 Nephron; NephSIM. Not medical advice."}}}

DI/polydipsia: V=5, U_osm=80, P_osm=290, U_Na=20, U_K=10, P_Na=125 → eC_H2O = +3.6 (dilute)

C_H₂O = 5·(1 − 80/290) = +3.621 mL/min. Electrolyte-free eC_H₂O = 5·(1 − (20+10)/125) = +3.800 mL/min. Positive → dilute urine excreting free water (DI/polydipsia); eC_H₂O positive means urine also tends to lower serum Na.

{"result":"C_H₂O = 3.6207 (Positive — dilute urine). eC_H₂O = 3.8. C_H₂O = V·(1 − U_osm/P_osm).","metadata":{"input":{"urineFlow":5,"urineOsm":80,"plasmaOsm":290,"urineSodium":20,"urinePotassium":10,"serumSodium":125},"result":{"osmolarClearance":1.3793,"freeWaterClearance":3.6207,"category":"Positive — dilute urine","interpretation":"C_H₂O > 0 (U_osm < P_osm): kidney excretes solute-free water. Seen with central/nephrogenic diabetes insipidus, primary polydipsia, or the recovery/diuretic phase after water load. Tends to raise serum Na⁺ unless water intake is sufficient.","electrolyteFreeWaterClearance":3.8,"electrolyteFreeCategory":"Positive — electrolyte-free water excretion (tends to LOWER serum Na)","electrolyteFreeInterpretation":"eC_H₂O > 0: urine is electrolyte-rich relative to plasma, so excreting it removes water and electrolytes proportionally → tends to lower serum Na⁺ (or fail to correct it).","method":"C_H₂O = V·(1 − U_osm/P_osm); eC_H₂O = V·(1 − (U_Na+U_K)/P_Na)","model":"Rose BD; Shimizu 2002 Nephron; NephSIM. Not medical advice."}}}

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7 options

SettingsAdjust formats, ranges, numbers, and modes.7

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

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1. Estimating concentrated urine in SIADH

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

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