Kt/V Calculator

Kt/V compares urea clearance (K, the dialyzer's urea clearance, multiplied by t, the session time) against V, the patient's total body water volume. The Daugirdas second-generation equation used here does not require K or V as separate inputs. Instead, it estimates Kt/V directly from the pre- and post-dialysis BUN ratio, the session length, the ultrafiltration volume, and the post-dialysis weight, which is the form most commonly used at bedside and in dialysis unit records.

R, the BUN ratio, is the single most important input. It is the post-dialysis BUN divided by the pre-dialysis BUN, so a larger drop in BUN produces a smaller R and, generally, a higher Kt/V. R must be strictly between 0 and 1 for this equation, since post-dialysis BUN is expected to be lower than pre-dialysis BUN. If post-dialysis BUN equals or exceeds pre-dialysis BUN, the calculator returns no result rather than a misleading number, because the underlying assumption of urea removal during the session no longer holds.

The 0.008 x t correction subtracted from R accounts for urea generation that continues during dialysis from ongoing protein metabolism. For long sessions or a small BUN ratio, this correction can push the value inside the logarithm to zero or below, which makes the natural log mathematically undefined. When that happens, the calculator returns no result instead of an invalid number, since a negative or zero value under a natural log has no real answer.

This is a single-pool model, which assumes urea is evenly distributed throughout body water at the moment BUN is sampled. In practice, urea concentration briefly rebounds after a session ends as urea in tissue and less-perfused compartments re-equilibrates with blood. Clinicians who need to account for that rebound use equilibrated Kt/V (eKt/V), which applies a further adjustment beyond what this single-pool calculation provides. This tool reports single-pool Kt/V only.

The ultrafiltration term, (4 - 3.5 x R) x (UF / W), adjusts for fluid removed during the session relative to post-dialysis body weight. Removing more fluid relative to body weight raises the calculated Kt/V somewhat, independent of urea clearance itself. Because this term uses post-dialysis weight, entering pre-dialysis weight by mistake will skew the result.

Published clinical guidance commonly cites a minimum single-pool Kt/V target of about 1.2 per session for patients on a standard thrice-weekly hemodialysis schedule, with many programs aiming somewhat higher to allow a margin for session variability. These are population-level reference figures drawn from published dialysis adequacy literature. They are not a personalized prescription, and they do not account for residual kidney function, session frequency other than three times weekly, body composition, comorbid conditions, or how a specific patient tolerates fluid removal.

Direct dialysate-side urea measurement and formal equilibrated Kt/V testing remain the more precise methods used in clinical practice when adequacy needs to be confirmed rather than estimated. This calculator is meant to help a patient or student understand how the single-pool Daugirdas equation works and how its inputs move the result. It is not a substitute for the adequacy monitoring performed by a dialysis unit's laboratory and care team.