Urine Output Calculator (ml/kg/hr)
Enter the patient weight, total urine collected, and the measurement window to calculate urine output in ml/kg/hr. Add fluid intake to see the net fluid balance. The tool automatically flags oliguria, polyuria, and KDIGO acute kidney injury stages, and switches thresholds between adult and pediatric patients.
What is urine output and why does it matter?
Urine output is the volume of urine produced by the kidneys over a defined time period, expressed either as an absolute volume (mL/hr) or normalised for body weight as mL/kg/hr. The weight-normalised rate is far more clinically useful because it accounts for differences in body size between patients. A 50 mL/hr output is reassuring in a 50 kg patient but could suggest oliguria in a 120 kg patient. Accurate measurement requires a urinary catheter in most ICU and perioperative settings, because self-reported or estimated voided volumes are too imprecise. Normal adult output falls between 0.5 and 1.0 mL/kg/hr (roughly 35 to 70 mL/hr for a 70 kg adult), maintaining roughly 800 mL to 2,000 mL per day. Any sustained fall below 0.5 mL/kg/hr is classified as oliguria and warrants prompt clinical assessment.
How the calculation works
The formula is straightforward: urine output rate (ml/kg/hr) = total urine collected (mL) divided by the product of patient weight (kg) and collection period (hours). For example, if a 70 kg patient produces 120 mL over 4 hours, the rate is 120 / (70 x 4) = 0.43 ml/kg/hr, which is oliguric. Fluid balance adds a second dimension: subtract total urine from total fluid intake over the same window. A strongly positive balance (more in than out) can reflect fluid overload, whereas a negative balance may indicate inadequate resuscitation or excessive losses. Note that insensible losses of roughly 400 mL per day in adults are not captured by urinary monitoring alone, so fluid balance as displayed here is an approximation.
KDIGO acute kidney injury staging by urine output
The Kidney Disease: Improving Global Outcomes (KDIGO) 2012 guidelines define three stages of AKI using either serum creatinine or urine output criteria - whichever gives the higher stage. Stage 1 requires urine output below 0.5 ml/kg/hr for 6 to 12 consecutive hours. Stage 2 requires the same rate sustained for 12 hours or more. Stage 3 requires output below 0.3 ml/kg/hr for 24 hours or more, or complete anuria for at least 12 hours. Because duration is critical, a single measurement cannot formally stage AKI; the clinician must document when oliguria began and whether it has persisted. This calculator uses the duration selector to give the most likely stage based on the rate and elapsed time, but serum creatinine data should always be incorporated into the final clinical assessment.
Pediatric considerations
Urine output thresholds differ substantially between adults and children. Pediatric oliguria is defined as output below 1.0 ml/kg/hr rather than the adult 0.5 ml/kg/hr, because children have higher baseline metabolic demands and proportionally larger obligate water losses. Normal output in children ranges from 1.0 to 3.0 ml/kg/hr, and polyuria is defined above 3.0 ml/kg/hr. Neonates and infants may have even higher baseline rates. This calculator switches to pediatric thresholds when you select "Pediatric (under 18 years)"; the core formula does not change, but the classification, insight, and reference table all reflect the appropriate age band.
Urine output clinical thresholds
| Rate (ml/kg/hr) | Category | Adults | Pediatric (<18) | KDIGO AKI Stage |
|---|---|---|---|---|
| < 0.3 | Severe oliguria | Possible AKI Stage 3 | Possible AKI Stage 3 | Stage 3 if >= 24 hr |
| < 0.5 | Oliguria | AKI Stage 1-2 | (Normal for pediatric) | Stage 1 if 6-12 hr; Stage 2 if >= 12 hr |
| 0.5-1.0 | Low-normal (adult) | Normal | Oliguria | No AKI staging |
| 1.0-3.0 | Normal | Normal | Normal | No AKI staging |
| 3.0-5.0 | High-normal (adult) | Normal | Polyuria | No AKI staging |
| > 5.0 | Polyuria | Investigate cause | Investigate cause | No AKI staging |
Standard clinical thresholds per KDIGO 2012 and common nursing assessment criteria. Duration of oliguria is required to stage AKI formally.
Frequently asked questions
What is a normal urine output in ml/kg/hr?
For adults, a normal urine output rate is 0.5 to 1.0 ml/kg/hr, which translates to roughly 35-70 mL per hour for a 70 kg adult, or about 1,200 to 1,700 mL per day. For children under 18, the normal range is higher at 1.0 to 3.0 ml/kg/hr because of proportionally higher metabolic water demands. Any sustained rate below 0.5 ml/kg/hr in adults (or below 1.0 ml/kg/hr in children) is classified as oliguria and should prompt clinical evaluation.
What is oliguria and when does it become AKI?
Oliguria means urine output below 0.5 ml/kg/hr in adults (or below 1.0 ml/kg/hr in children). It becomes an indication of acute kidney injury when it persists: KDIGO Stage 1 AKI is diagnosed when output stays below 0.5 ml/kg/hr for 6 to 12 hours; Stage 2 when it continues for 12 or more hours; Stage 3 when output falls below 0.3 ml/kg/hr for 24 or more hours, or when there is complete anuria for 12 or more hours. A single measurement showing oliguria is not sufficient to stage AKI - you need to track how long the rate has been reduced.
How do I measure urine output accurately?
In clinical settings, accurate measurement requires a Foley urinary catheter draining into a calibrated collection bag. The bag is emptied and zeroed at the start of each measurement period (typically every hour in the ICU, or every 4-8 hours on the ward), and the volume drained is recorded. Self-voided volumes collected in a urinal or hat are acceptable in alert, cooperative patients but are less precise. Urine output should be charted alongside the exact collection start time, end time, and patient weight so that the ml/kg/hr rate can be recalculated whenever weight changes.
What causes oliguria?
Oliguria has three broad categories. Pre-renal causes reduce blood flow to the kidneys - hypovolemia, hemorrhage, sepsis, heart failure, and excessive diuresis are common culprits. Intrinsic renal causes damage the kidney itself, including acute tubular necrosis (often from prolonged hypoperfusion or nephrotoxic drugs), glomerulonephritis, and interstitial nephritis. Post-renal (obstructive) causes block urine outflow - bladder outlet obstruction from an enlarged prostate, a kinked or blocked urinary catheter, or bilateral ureteric obstruction. A blocked or kinked catheter is one of the most common and easily fixed causes of apparent oliguria on the ward.
What is polyuria and what causes it?
Polyuria in adults is broadly defined as urine output exceeding 3 litres per 24 hours, or equivalently more than 50 mL/kg per day - rates that typically exceed 5 ml/kg/hr on an hourly basis. Common causes include excess IV fluid administration, the post-obstructive diuresis that follows relief of urinary obstruction, the polyuric recovery phase of AKI, diabetes insipidus (central or nephrogenic), and hyperglycaemia. Large urinary losses can deplete electrolytes rapidly, so patients with polyuria require regular monitoring of sodium, potassium, and fluid balance.
Should I use actual or ideal body weight?
For standard clinical urine output calculation, use actual body weight, because the kidneys filter actual circulating volume, which tracks actual mass more closely than ideal body weight. Some intensivists switch to adjusted body weight in morbidly obese patients to avoid diluting the rate, but this is not universally agreed. This calculator uses whatever weight you enter, so enter actual weight unless your institution has a specific protocol that specifies otherwise.