Urine Output Calculator — mL/kg/hr & Fluid Balance (Metric & Imperial)

Urine Output Calculator — mL/kg/hr & Fluid Balance

Our urine output calculator helps clinicians, nurses, and caregivers quickly assess a patient's fluid status. Enter age, weight, collection time, urine volume, and fluid intake — and get the urine output rate in mL/kg/hr plus the fluid balance instantly.

Medical disclaimer: This tool is for educational and clinical-support purposes only. It cannot replace a professional doctor's assessment. Always consult a qualified physician for medical decisions.

How to Use the Urine Output Calculator

To calculate the urine output rate you need five data points:

1. Patient's age — determines the normal reference range (infant, child, adult).
2. Patient's weight — in kilograms (metric) or pounds (American/imperial).
3. Collection time — the period over which urine was collected, in hours.
4. Urine volume — total urine collected during that period, in mL or fl oz.
5. Fluid intake (optional) — total intake during the same period, used to compute fluid balance.

The calculator supports both metric (kg, mL) and American/imperial (lbs, fl oz) measurement systems. Simply toggle the unit system at the top of the form.

How to Calculate Urine Output in mL/kg/hr

The formula is straightforward:

Urine Output (mL/kg/hr) = Urine Volume (mL) ÷ (Weight (kg) × Time (hours))

Worked Example

A 20-year-old patient weighs 80 kg. Over a 24-hour observation period, 3,000 mL of urine was collected.

Urine Output = 3000 ÷ (80 × 24) = 3000 ÷ 1920 ≈ 1.56 mL/kg/hr

Since the patient is over 18, the normal adult range is 0.5 – 1.5 mL/kg/hr. A result of 1.56 mL/kg/hr is within — or just above — the normal upper limit, so no oliguria is indicated.

What Is Normal Urine Output Per Hour?

Oliguria

Oliguria means producing less urine than normal — in adults, that is below 0.5 mL/kg/hr (or roughly < 400 mL/day for a 70 kg adult). It can be a sign of dehydration, acute kidney injury (AKI), or circulatory failure.

Polyuria

Polyuria means producing abnormally high volumes of urine — generally above 3 mL/kg/hr (or > 3 L/day in adults). Causes include uncontrolled diabetes mellitus, diabetes insipidus, excessive fluid intake, or diuretic therapy.

Fluid Balance

Fluid balance tells you whether the patient is retaining or losing total body water:

Fluid Balance (mL) = Fluid Intake (mL) − Collected Urine (mL)

• Positive balance — the patient is retaining fluid (may indicate overhydration or impaired kidney excretion).
• Negative balance — the patient is losing more fluid than they are taking in (may indicate dehydration).

A typical person begins to show dehydration symptoms after losing approximately 5% of their body weight in fluids, or about 7% of total body water.

💡 Remember: patients also lose water through the lungs (insensible loss), skin (perspiration), and stool — especially when body temperature is elevated. These "insensible losses" are not captured by this calculator.

Acute Kidney Injury (AKI) Warning

According to the KDIGO (Kidney Disease: Improving Global Outcomes) criteria, oliguria in adults lasting ≥ 6 hours may indicate acute kidney injury:

• AKI Stage 1: < 0.5 mL/kg/hr for > 6 hours
• AKI Stage 2: < 0.5 mL/kg/hr for > 12 hours
• AKI Stage 3: < 0.3 mL/kg/hr for > 24 hours, or anuria for > 12 hours

The calculator alerts you when AKI risk criteria are met (adult patient, urine output < 0.5 mL/kg/hr, collection period ≥ 6 hours).

Frequently Asked Questions

What is a dangerous urine output?

For adults, urine output below 0.5 mL/kg/hr (oliguria) sustained for 6+ hours is considered clinically significant and warrants evaluation for acute kidney injury, dehydration, or circulatory compromise. Anuria (no urine) is a medical emergency.

How much urine is normal per day?

For a healthy adult of average weight, normal 24-hour urine output is approximately 800 – 2,000 mL/day. This corresponds to 0.5 – 1.5 mL/kg/hr for a 70 kg person.

Can I use this for pediatric patients?

Yes. The calculator automatically applies age-appropriate normal ranges: 1–2 mL/kg/hr for children and 2–4 mL/kg/hr for infants under 1 year.

What units does this calculator support?

Both metric (kg and mL) and American/imperial (lbs and fl oz) units are supported. The calculator converts automatically — urine rate is always expressed in mL/kg/hr regardless of the input units chosen.