A professional clinical tool for determining Urine Output (UOP) in mL/kg/hr to assess renal function and fluid balance.
Enter the total volume of urine collected.
Please enter a valid positive number.
kg
lbs
Enter the patient's current weight.
Please enter a valid weight > 0.
Time period over which urine was collected.
Please enter a valid time > 0.
Calculated Urine Output Rate
—mL/kg/hr
Waiting for input…
Total Volume
— mL
Weight (Standardized)
— kg
Hourly Average
— mL/hr
Formula Used: Rate = Total Urine (mL) ÷ [Weight (kg) × Time (hr)]
Fig 1. Patient Output vs. Clinical Thresholds
Parameter
Value
Clinical Note
Input Weight
—
Base metric
Collection Time
—
Duration
UOP Rate
—
Primary Indicator
Table 1: Summary of entered data and calculated renal function metrics.
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What is the "Calculate Urine Output by Weight" Metric?
When clinicians need to assess kidney function and fluid balance accurately, they rely on the metric calculate urine output by weight. This measurement normalizes a patient's urine production against their body mass and the time elapsed, resulting in a standard unit of milliliters per kilogram per hour (mL/kg/hr).
Unlike raw urine volume, which varies significantly based on body size, the weight-adjusted output provides a universal standard to detect conditions like Acute Kidney Injury (AKI), dehydration, or shock. It is a critical vital sign in intensive care units (ICU), pediatric wards, and post-operative recovery rooms.
Who should use this tool? This calculator is designed for nurses, medical students, physicians, and paramedics who need to rapidly stratify a patient's renal status as Oliguric, Normal, or Polyuric.
Formula and Mathematical Explanation
To calculate urine output by weight, you must perform a specific derivation that accounts for the three variables of renal function: volume, mass, and time.
The standard formula is:
UOP (mL/kg/hr) = Total Volume (mL) / [ Weight (kg) × Time (hours) ]
Variable Definitions
Variable
Meaning
Unit
Typical Range (Adult)
Total Volume
Amount of urine collected
Milliliters (mL)
30 – 100 mL/hr
Weight
Patient's body mass
Kilograms (kg)
40 – 150 kg
Time
Duration of collection
Hours (hr)
1 – 24 hours
Table 2: Variables used in the urine output calculation.
Practical Examples (Real-World Use Cases)
Example 1: The Post-Op Adult Patient
Consider a 70 kg male patient recovering from surgery. The nurse empties the Foley catheter bag after 4 hours and measures 140 mL of urine. To check for early signs of kidney failure, we calculate urine output by weight:
Interpretation: This result is exactly on the borderline of the minimum acceptable urine output (0.5 mL/kg/hr). The medical team would likely monitor this patient closely or consider fluid resuscitation.
Example 2: Pediatric Dehydration
A 15 kg child is admitted with vomiting. Over 6 hours, the child produces only 45 mL of urine.
Interpretation: While 0.5 is borderline for adults, pediatric targets are often higher (closer to 1.0 mL/kg/hr). This value indicates the child is likely dehydrated and requires intervention.
How to Use This Calculator
Our tool simplifies the math so you can focus on clinical decision-making. Follow these steps to calculate urine output by weight:
Enter Total Urine Volume: Input the cumulative amount of urine measured in the collection container (in mL).
Input Patient Weight: Enter the patient's weight. Use the dropdown to toggle between Kilograms (kg) and Pounds (lbs); the calculator converts lbs automatically.
Specify Time Duration: Enter the number of hours over which the urine was collected.
Review Results: The primary display shows the rate in mL/kg/hr. The status badge identifies if the rate is Oliguric ( 2.5).
Key Factors That Affect Urine Output Results
When you calculate urine output by weight, the number tells you what is happening, but not why. Several key factors influence this metric:
Fluid Intake & Hydration Status: The most direct correlate. Low intake leads to concentrated urine and lower volume (physiologic oliguria).
Mean Arterial Pressure (MAP): The kidneys require a certain perfusion pressure (usually MAP > 65 mmHg) to filter blood. Low blood pressure (shock) drastically reduces output.
Diuretic Medications: Drugs like Furosemide (Lasix) artificially increase urine output, potentially masking underlying renal issues when you calculate the rate.
Urinary Obstruction: Physical blockages (stones, enlarged prostate, kinked catheters) will result in zero or low output (Anuria/Oliguria) regardless of kidney health.
Insensible Losses: High fever or sweating diverts fluid away from the kidneys, lowering urine output naturally.
Renal Pathology: Intrinsic kidney damage (Acute Tubular Necrosis) prevents the formation of urine even if hydration and blood pressure are normal.
Frequently Asked Questions (FAQ)
What is considered "Normal" urine output?
For an average adult, 0.5 to 1.5 mL/kg/hr is considered normal. For children, the target is often 1.0 to 2.0 mL/kg/hr. Infants may require up to 2.0 mL/kg/hr.
What defines Oliguria?
Oliguria is clinically defined as urine output less than 0.5 mL/kg/hr for at least 6 consecutive hours in adults, or less than 1 mL/kg/hr in infants.
Why is weight-based calculation better than total volume?
A total volume of 30mL/hr might be normal for a small elderly woman (40kg) but represents severe kidney failure for a large man (100kg). To accurately assess risk, you must calculate urine output by weight.
How accurate does the weight need to be?
Ideally, use a "dry weight" or admission weight. If the patient is fluid overloaded (edematous), using their current heavy weight may artificially lower the calculated rate, making the situation look worse than it is.
Can I use this for dialysis patients?
No. Patients on End-Stage Renal Disease (ESRD) generally have little to no urine output. This calculator is for patients with functioning kidneys or acute kidney injury.
What is Anuria?
Anuria is the technical failure of the kidneys to produce urine, typically defined as less than 50-100mL over a full 24-hour period.
Does this calculator account for insensible loss?
No, this tool only calculates the raw output rate. Clinical judgment is required to balance this against insensible losses like sweat or respiration.
What is Polyuria?
Polyuria is excessive urine production, generally exceeding 2.5 to 3.0 mL/kg/hr. This can be seen in conditions like Diabetes Insipidus or during the recovery phase of renal failure.
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