Hypo- and hypernatraemic (dysnatraemic) disorders are among the most common electrolyte disorders encountered by primary care providers and nephrologists. They represent a diagnostic and therapeutic challenge, and inappropriate management can result in serious sequelae. Several formulas addressing the fluid prescription for dysnatraemic patients have been introduced. Many authors stress the importance of considering output as well as input in formulating a treatment plan for the dysnatraemic patient. However, currently available formulas fail to account for ongoing renal and extrarenal fluid and electrolyte losses. We propose a novel, versatile formula based on established principles governing the distribution of Na(+) in body fluids. The formula can be used in a simplified form for a quick but accurate estimate of the change in serum [Na(+)] for any infused fluid, while simultaneously accounting for renal losses. The formula can also be expanded to include more complex losses if desired. Importantly, it forces the caregiver to consider both output and input when formulating a prescription for the dysnatraemic patient.