Dry Weight & Fluid Balance Calculator

This calculator estimates how much fluid a hemodialysis patient needs removed during a session and whether the planned removal rate is safe. Interdialytic weight gain (IDWG) is simply pre-dialysis weight minus the clinically assessed target dry weight -- the euvolemic weight at which the patient has neither excess fluid nor is volume-depleted, judged by the care team from clinical signs rather than measured directly. Because roughly 1 kg of weight gain between sessions corresponds to about 1 liter of retained fluid, that same IDWG figure becomes the ultrafiltration (UF) volume the machine needs to remove. Spreading that volume over the planned session duration gives the UF rate in mL/hr, and dividing by dry weight gives UF rate per kilogram -- the number that matters most for safety, since pulling fluid faster than about 13 mL/kg/hr is associated with meaningfully higher risk of intradialytic hypotension as plasma refill can't keep pace with fluid removal.

The calculator separately estimates total body water (TBW) using the Watson formula (Watson et al. 1980), which differs by sex: men's TBW is estimated from age, height, and weight, while the women's version omits age entirely and uses only height and weight, reflecting the original regression equations derived from body-composition studies. Fluid overload as a percentage of TBW gives a sense of how large the interdialytic fluid gain is relative to the patient's whole-body water pool. This calculator does NOT replace bioimpedance spectroscopy or clinical dry-weight probing, does not adjust the 5% IDWG and 13 mL/kg/hr thresholds for cardiac or nutritional comorbidities, and treats the 1 kg-to-1 L fluid conversion as an approximation rather than a measured value.