How to Calculate Urine Output
Urine output is calculated as milliliters of urine per kilogram of body weight per hour (mL/kg/hour). 1
Standard Calculation Method
The formula for calculating urine output is:
Urine Output = Total Urine Volume (mL) ÷ Patient Weight (kg) ÷ Time Period (hours)
Normal Thresholds and Clinical Significance
- Normal adult urine output: 0.5-1 mL/kg/hour 1
- Oliguria: <0.5 mL/kg/hour for 6-8 hours 1
- Anuria: <0.3 mL/kg/hour for 24 hours or no urine output for 12 hours 1
Measurement Periods for Different Clinical Scenarios
Different time intervals are used for measuring urine output depending on the clinical context:
Acute Kidney Injury (AKI) staging per KDIGO criteria 1:
- Stage 1: <0.5 mL/kg/hour for 6-12 hours
- Stage 2: <0.5 mL/kg/hour for ≥12 hours
- Stage 3: <0.3 mL/kg/hour for ≥24 hours or anuria for ≥12 hours
Critical care monitoring:
- Hourly measurements are standard in ICU settings
- Shorter collection intervals (1-6 hours) may be more sensitive for detecting early kidney dysfunction 2
Population-Specific Considerations
Urine output thresholds vary by patient population 1:
| Patient Population | Urine Output Threshold |
|---|---|
| Adults | 0.5 mL/kg/hour |
| Neonates | 0.5-1.0 mL/kg/hour |
| Burn patients | 0.5-1 mL/kg/hour |
| Home parenteral nutrition | 0.8-1 L per day |
Practical Measurement Tips
Measurement tools:
- Standard urine collection bags (hourly readings)
- Digital urine meters (for minute-to-minute measurements in critical care) 3
- Urinary catheter for accurate measurement in critically ill patients
Timing considerations:
- Record the exact time period over which urine was collected
- Ensure consistent measurement intervals for trend analysis
Clinical Pitfalls to Avoid
- Weight calculation errors: Always use actual body weight, not ideal body weight
- Diuretic influence: Note that diuretic use significantly alters urine output interpretation 4, 1
- Measurement inaccuracy: Small urine volumes in collection bags can lead to over- or underestimation 3
- Threshold interpretation: Recent research suggests the standard 0.5 mL/kg/hour threshold may be too liberal, with 0.3 mL/kg/hour over 6 hours potentially being more predictive of adverse outcomes 2
- Collection period: The optimal urine output threshold is linearly related to the duration of collection - shorter collection periods require lower thresholds 2
Documentation Requirements
For proper documentation and clinical decision-making:
- Record total urine volume
- Note patient's weight in kilograms
- Document exact collection time period
- Calculate and record mL/kg/hour
- Note any factors that might influence output (diuretics, fluid boluses, etc.)