Nitrogen Balance & Protein Target Calculator
Enter the delivered protein and 24-hour urinary urea nitrogen to estimate nitrogen balance. The calculator also reverse-solves the protein intake needed to reach a selected target balance.
Nitrogen balance (g N/day)0
- Nitrogen intake (g N/day)
- 16
- Estimated daily nitrogen loss (g N/day)
- 16
- BUN accumulation correction (g N/day)
- 0
- Balance classification
- Equilibrium
- Protein intake for target balance (g/day)
- 100
- Additional protein needed (g/day)
- 0
Protein intake and UUN should describe the same complete 24-hour period.
Routine fecal and skin losses are already included in the selected method.
How to use this calculator
- Enter protein actually delivered during the same 24-hour period as the urine collection.
- Enter the reported 24-hour urinary urea nitrogen value.
- Choose the classic loss estimate or the Dickerson multiple-trauma estimate.
- Add only abnormal measured or estimated nitrogen losses that the method does not already include.
- Turn on the BUN correction only when BUN changed materially during the collection.
How nitrogen balance is calculated
Nitrogen balance compares nitrogen intake with estimated nitrogen losses over a defined period, usually a complete 24-hour urine collection. Protein contains about one gram of nitrogen for each 6.25 grams of protein, so the calculator first converts delivered protein to nitrogen intake.
nitrogen intake = protein intake ÷ 6.25
The classic clinical estimate treats daily nitrogen loss as urinary urea nitrogen plus a fixed 4 gram allowance for routine non-urinary losses:
loss = UUN + 4 + additional abnormal loss
The Dickerson option uses the estimate specified for multiple-trauma patients:
loss = UUN ÷ 0.85 + 2 + additional abnormal loss
If changing BUN is included, the calculator estimates nitrogen accumulating in or leaving the body urea pool:
BUN correction = 0.6 × weight kg × (ending BUN − starting BUN) × 0.01
The final balance is:
nitrogen balance = nitrogen intake − estimated loss − BUN correction
How the protein target is reverse-solved
To estimate the protein intake for a target nitrogen balance, the calculator rearranges the balance equation:
target protein = 6.25 × (target balance + estimated loss + BUN correction)
If that algebra gives a negative intake, the displayed target is zero. Additional protein needed is the target protein minus current protein intake, floored at zero.
What moves the result most
The UUN value often drives the estimate because it stands in for urinary nitrogen loss. Protein intake has a direct effect through the 6.25 conversion. Abnormal losses from wounds, fistulas, burns, drains, dialysis, or continuous renal replacement therapy can also dominate the result when they are large. A rising BUN adds a positive correction and makes the balance lower; a falling BUN creates a negative correction and makes the balance higher.
What this calculator leaves out
This tool does not judge whether the urine collection was complete, whether the protein ordered was fully delivered, or whether UUN is a good proxy for total urinary nitrogen in the clinical situation. It does not independently estimate burn, wound, stool, fistula, drain, dialysis, or renal replacement therapy losses. The reverse-solved protein target is algebraic; real nitrogen balance may not respond linearly to added protein, especially when energy intake, kidney function, liver function, inflammation, hydration, or medications are changing.
Worked example
Suppose a patient received 120 g/day of protein, had a 24-hour UUN of 18 g/day, has 3 g N/day of added abnormal loss, and the Dickerson option is selected. Nitrogen intake is 120 ÷ 6.25 = 19.2 g N/day. Estimated loss is 18 ÷ 0.85 + 2 + 3 = 26.18 g N/day.
If body weight is 80 kg and BUN rose from 18 to 26 mg/dL, the correction is 0.6 × 80 × (26 − 18) × 0.01 = 3.84 g N/day. Nitrogen balance is 19.2 − 26.18 − 3.84 = -10.82 g N/day. For a target balance of +2 g N/day, the protein target is 6.25 × (2 + 26.18 + 3.84) = 200.10 g/day, so the additional protein needed is about 80.10 g/day.
Common questions
What does positive or negative nitrogen balance mean?
A positive nitrogen balance means estimated nitrogen intake is greater than estimated nitrogen loss for the collection period. A negative balance means estimated losses are greater than intake. The result is an estimate, not a diagnosis of nutritional status by itself.
How is protein converted to grams of nitrogen?
Clinical nitrogen balance calculations commonly use the factor 6.25 because mixed dietary protein is about 16% nitrogen. Dividing protein grams by 6.25 gives estimated grams of nitrogen intake.
Why does the classic equation add 4 grams to UUN?
The added 4 grams is a conventional allowance for routine non-urinary nitrogen losses, such as fecal and skin losses. Because that allowance is already included, routine losses should not be added again in the additional-loss field.
What is the difference between the classic and Dickerson methods?
The classic method uses UUN plus a fixed 4 gram allowance. The Dickerson option estimates total urinary nitrogen from UUN using a trauma-patient equation before adding the selected abnormal losses. It should be labeled as an estimate developed in multiple-trauma patients, not as a universal replacement for the classic approach.
When should changing BUN be included?
Use the BUN correction when BUN changed enough during the collection to materially affect interpretation. A rising BUN creates a positive accumulation correction and lowers the displayed balance. A falling BUN creates a negative correction and raises the displayed balance.
How complete must the 24-hour urine collection be?
The urine collection should cover the same 24-hour period as the protein intake and should be complete. Missed urine, extra time, timing errors, or mismatched intake records can make the balance misleading. Clinical teams often compare urine volume, creatinine excretion, and collection circumstances when judging reliability.