Urine Output Calculator (mL/kg/h, oliguria and KDIGO AKI criteria)

Q: What is oliguria?

Low urine output, usually under 0.5 mL/kg/h in adults or under about 400 mL a day.

Q: Which weight should I use?

Usually actual body weight; some units use ideal weight in obesity.

Q: Is urine output alone enough to diagnose AKI?

KDIGO uses either urine output or creatinine criteria, ideally both.

Understanding the urine output calculator — concept and principle

This calculator works out urine output in mL per kg of body weight per hour and checks it against the KDIGO urine-output criteria for acute kidney injury (AKI). Nurses and doctors use it to monitor fluid balance and kidney function in hospital, especially in intensive care.

mL ÷ kg ÷ hours: with the default inputs the result is 0.57 mL/kg/h (not below 0.5).

How it is calculated

Urine output (mL/kg/h) = volume (mL) ÷ weight (kg) ÷ hours

Urine output (mL/h) = volume ÷ hours; projected 24-hour volume = mL/h × 24

KDIGO criteria: stage 1 < 0.5 mL/kg/h for 6–12 h; stage 2 < 0.5 for 12 h or more; stage 3 < 0.3 for 24 h or more, or no urine for 12 h or more.

The principle

The kidneys respond quickly to falling blood flow or injury by producing less urine, often before creatinine rises. Expressing output per kilogram makes small and large patients comparable. KDIGO combined urine output with creatinine changes to define and stage AKI consistently.

Worked example

Default inputs: 240 mL over 6 hours, weight 70 kg.

Output = 240 ÷ 70 ÷ 6 = 0.57 mL/kg/h.
Per hour = 240 ÷ 6 = 40 mL/h; projected 24 h = 960 mL.
0.57 is not below 0.5 → Not oliguric.

Each point is the calculator re-run with only “Collection time (hours)” changed; the gold point is the default example. Change the inputs above and this chart follows your numbers.

Collection time (hours)	Urine output (mL/kg/h)
1.5	2.29
3	1.14
4.5	0.76
6	0.57
7.5	0.46
9	0.38
12	0.29

How to read the result

Where does your result fall?

About 0.5 mL/kg/h is widely used as the minimum acceptable output in adults (1 mL/kg/h in infants and young children). The calculator averages one period, so it can only say whether that average meets a criterion; KDIGO looks at the whole time course and at creatinine too. Low output prompts a check of fluid status, catheter position and causes of kidney injury.

Common mistakes

Forgetting to subtract bladder irrigation fluid.
Using a blocked catheter's volume without checking flow.
Ignoring that diuretics can keep output normal despite kidney injury.

Keep in mind: This is a monitoring aid for clinicians, not a diagnosis. Very low urine output needs prompt medical review.

Frequently asked questions

What is oliguria?