Free Urine Output Calculator

Understanding Urine Output and Fluid Balance

The Free Urine Output Calculator provides a practical way to obtain a patient’s Urine Output per Hour expressed in ml/kg/hr, while also calculating their Fluid Balance. This tool doubles as a dedicated Fluid Balance Calculator and an Oliguria Calculator, automatically highlighting results that may indicate acute kidney injury or abnormal urine volumes such as oliguria (too little urine) or polyuria (excessive urine).

Data You Need to Enter

To compute the urine output rate, you supply the following information:

• Patient age and weight (in kilograms)
• The length of time over which urine was collected (hours)
• The total volume of urine collected (milliliters)
• The fluid intake during that same period (milliliters)

Based on these inputs, the calculator delivers the urine output rate and the net fluid balance. It then compares the rate against established thresholds to categorize the result as normal, oliguric, or polyuric.

Manual Calculation of Urine Output

If you prefer to verify the numbers manually, the core formula is:

$$\text{Urine Output (ml/kg/hr)} = \dfrac{\text{Urine Volume (mL)}}{\text{Weight (kg)} \times \text{Time (hr)}}$$

Example: A patient weighing 70 kg produces 300 mL of urine over a 6‑hour span.

$$\text{Urine Output} = \dfrac{300}{70 \times 6} = 0.71\ \text{ml/kg/hr}$$

For an adult, this rate meets the minimum requirement of 0.5 ml/kg/hr.

Understanding Fluid Balance

The Fluid Balance Calculator function uses the following equation:

$$\text{Fluid Balance} = \text{Fluid Intake} - \text{Urine Output}$$

A negative balance means the patient has lost more fluid than they have taken in, which can lead to dehydration. Clinically, dehydration symptoms often appear after a loss of roughly 7 % of total body water or about 5 % of body weight.

Reference Ranges Based on Age

Normal urine output depends strongly on age. The table below summarizes the standard thresholds that the calculator uses:

Age Group	Urine Output (ml/kg/hr)	Interpretation
Adults (≥ 18 years)	< 0.5	Oliguria
	0.5–5	Normal
	> 5	Polyuria
Children (< 18 years)	< 1	Oliguria
	1–3	Normal
	> 3	Polyuria

These reference values allow clinicians to quickly evaluate a patient’s Urine Output Rate and decide on further steps.

Important Limitation

This online tool is designed as a supportive resource. It does not replace the judgment of a qualified healthcare professional. If you have concerns about any medical condition, please consult a physician.