Urine Output Calculator - mL/kg/hr and Fluid Balance

The urine output calculator turns weight, collection time, and urine volume into the hourly read in mL per kg per hour, an age-aware band (oliguria, normal, or polyuria), and an optional fluid balance in mL.

• • Bedside AKI screening: Spot a low hourly read that meets the 0.5 mL per kg per hour adult or 1 mL per kg per hour pediatric oliguria cutoff.
• • 24-hour volume review: Read the published 800 to 2000 mL per day normal range against a real 24-hour collection.
• • Fluid balance on a fluid chart: Match the timed window of a fluid chart so the same window can also produce a balance in mL.
• • Critical care hourly monitoring: Use a short window, like 4 or 6 hours, to keep the hourly read on a trend.

The tool is a screening aid for clinicians, trainees, and careful patients or caregivers who already have a timed collection, a body weight, and an optional fluid intake in hand.

A flagged oliguria read on the same window should be paired with an estimated glomerular filtration rate, so GFR Calculator belongs in the same kidney workup sheet.

The hourly read in mL per kg per hour is urine volume divided by body weight times collection time. Multiply by 24 for a 24-hour per kg reading and a 24-hour total. When fluid intake is provided, the same window returns a balance as fluid intake minus urine volume.

• Body weight: Body weight in kilograms, used to normalize the per kg read.
• Collection time: Length of the timed window in hours. A 24-hour window matches the published reference data; a 1, 4, or 6 hour window matches critical care hourly monitoring.
• Urine volume: Total volume collected during the timed window, in milliliters. Catheter collections are most accurate; voided collections are acceptable when the team is confident in the capture.
• Fluid intake: Optional total fluid intake in milliliters covering the same timed window. Leave at 0 to skip the balance output.

All five inputs come from the same timed window. The age group switch lets one formula serve both adult and pediatric cutoffs. The 24-hour total is a planning estimate, and the balance output only shows a non-zero value when fluid intake is provided.

According to KDIGO 2012 AKI Guideline, acute kidney injury is defined in part by a urine output less than 0.5 mL per kg per hour for 6 to 12 hours, with the same threshold used as the standard oliguria cutoff for the bedside read.

Once the hourly read confirms a kidney workup is needed, Urine Anion Gap Calculator adds the urine chemistry workup that the same urine collection panel can support.

Four ideas cover why the formula normalizes per kg, why the cutoff switches by age group, why a low read can flag AKI, and why a balance needs the same window as the collection.

The most common trap is to compare an hourly read to the published 24-hour range without scaling. The 24-hour per kg range is about 20 to 50 mL per kg per day for a healthy adult, the same 0.83 to 2.08 mL per kg per hour window scaled up.

A sustained oliguria read on the same workup window is a strong reason to revisit long-term kidney trajectory, which is where Kidney Failure Risk Calculator adds the most value.

The calculator is most useful when the body weight, the timed window, the volume, and the optional fluid intake come from the same clinical window.

1. 1 Pick the patient age group: Use Adult for 18 years and older, and Child for under 18 years.
2. 2 Enter the body weight and collection time: Type the body weight in kilograms and the timed window in hours. Use measured weight, or estimated dry weight for a fluid overloaded patient.
3. 3 Enter the urine volume: Type the total volume collected during the timed window in mL. Leave at 0 only if the lab returned an undetectable result.
4. 4 Decide on a fluid intake value: If fluid intake was tracked for the same window, enter the total in mL so the calculator can return a balance. Leave at 0 for a urine-only read.
5. 5 Read the hourly output and the band: The primary number is the hourly read in mL per kg per hour. The band reads Oliguria, Normal, or Polyuria.
6. 6 Pair the band with a clinician review: An Oliguria band on an adult meets the published KDIGO AKI threshold. A Polyuria band should be paired with a glucose and a medication review.

An 80 kg adult with 80 mL over 4 hours returns 0.25 mL per kg per hour, an Oliguria band, and an Elevated AKI risk flag, fitting the published KDIGO Stage 1 threshold. The same case with 1,500 mL over 24 hours returns 0.78 mL per kg per hour and a Normal band.

When the hourly read drops during a fluid resuscitation, IV Flow Rate Calculator helps double check the actual fluid rate reaching the patient on the same timed window.

The calculator turns a timed collection into a number that fits cleanly into a kidney and fluid workup.

• • Published mL per kg per hour rate: Returns the hourly read in mL per kg per hour, the same unit used by the KDIGO AKI definition.
• • Age group aware cutoffs: Switches the oliguria and polyuria cutoffs by patient age group so the same per kg number reads correctly for adults and children.
• • AKI risk flag: Flags readings that meet the published oliguria cutoff as Elevated AKI risk so a low hourly read is not silent.
• • Optional balance in mL: Returns a balance in mL when fluid intake is added for the same timed window.
• • Pairs with the GFR workup: Returns the same per kg style as the GFR calculator, so a flagged oliguria read can be paired with an estimated GFR for the same workup window.

If the band is oliguria, the next steps are a repeat measurement, a serum creatinine, and a clinician review. If the band is polyuria, the next steps are a serum glucose, a serum sodium, and a medication review.

A negative balance that fits a hydration workup benefits from a matched intake estimate, which is where Daily Water Intake Calculator fits next to this read.

The hourly rate depends on the patient age group, the body weight, the collection method, and the broader fluid and kidney context.

• • The calculator does not measure glomerular filtration rate directly. Confirm a flagged oliguria read with a repeat measurement and a serum creatinine.
• • The 24-hour total is a planning estimate when the window is shorter than 24 hours. Use a real 24-hour collection when the decision depends on a precise 24-hour volume.
• • The balance is only as accurate as the intake record. Mixing a urine window with a different intake window will overstate or understate the balance in one direction.

The most common reason the hourly read and the clinical picture disagree is a body weight choice. Confirm the weight, window, and capture before changing the workup.

According to MedlinePlus NIH on Urine 24-hour Volume, the normal 24-hour urine volume is 800 to 2,000 mL per day, with reduced volume in dehydration and chronic kidney disease, and increased volume in diabetes insipidus, diabetes mellitus, high fluid intake, and diuretic use.

Adult oliguria is below 0.5 mL per kg per hour, adult polyuria is a 24-hour total above 3,000 mL, pediatric oliguria is below 1, and pediatric polyuria is a 24-hour total above 2,000 mL.

According to the MSD Manual on Polyuria, polyuria is a urine output above 3 L per day, which is the 3,000 mL per 24-hour threshold used in this calculator.

For pediatric cases, the same window pairs with a maintenance fluid target, which is the role Maintenance Fluids Children Calculator is built for.