Nitrogen Balance Calculator
Nitrogen balance measures whether the body is in a net protein-building (anabolic) or protein-breaking (catabolic) state. It is calculated as nitrogen intake from dietary protein minus total nitrogen losses. Protein provides approximately 16% of its mass as nitrogen, so nitrogen intake = protein intake (g) / 6.25. Nitrogen loss is estimated from 24-hour urinary urea nitrogen (UUN) plus a correction of 4 g/day for fecal and insensible losses. A positive result indicates muscle growth or recovery; a negative result indicates catabolism.
Nitrogen balance formula
Nitrogen Intake (g/day) = Protein Intake (g/day) / 6.25
Nitrogen Loss (g/day) = UUN + 4 (correction for non-urinary losses)
Nitrogen Balance = Nitrogen Intake - Nitrogen Loss
The correction factor of 4 g/day accounts for fecal nitrogen loss (approximately 1-2 g/day) and insensible losses through sweat and skin. This formula is the standard clinical method described by the American Society for Parenteral and Enteral Nutrition (ASPEN).
Interpreting nitrogen balance
- Positive (above 0): Net protein anabolism; body is retaining more nitrogen than it loses.
- Zero: Nitrogen equilibrium; intake equals losses (maintenance state).
- Negative (below 0): Net protein catabolism; body is losing more nitrogen than consumed.
- Severely negative (below -5 g/day): Significant catabolism, common in critical illness, burns, or severe caloric restriction.
Nitrogen balance calculator: frequently asked questions
What is nitrogen balance?
Nitrogen balance is the difference between nitrogen intake (from dietary protein) and nitrogen loss (primarily through urinary urea, feces, skin, and other routes). A positive nitrogen balance means more nitrogen is being retained than lost, indicating net protein synthesis (anabolism). A negative nitrogen balance means more nitrogen is lost than consumed, indicating net protein breakdown (catabolism).
How is nitrogen intake calculated from protein?
Dietary protein contains approximately 16% nitrogen by mass. Therefore, nitrogen intake (g/day) = protein intake (g/day) / 6.25. For example, 100 g of dietary protein provides approximately 16 g of nitrogen.
What are the sources of nitrogen loss?
The primary route of nitrogen excretion is urinary urea nitrogen (UUN), which accounts for roughly 80-90% of total urinary nitrogen. Additional losses occur through feces (approximately 1-2 g/day) and insensible losses through skin and other routes (approximately 2-4 g/day). The standard clinical formula adds a fixed correction factor of 4 g/day for non-urinary losses.
What is a healthy nitrogen balance?
Healthy adults in maintenance are approximately in zero nitrogen balance. Athletes in muscle-building phases aim for a positive nitrogen balance of +2 to +4 g/day. A nitrogen balance of -5 g/day or more negative suggests significant protein catabolism, such as in critical illness, starvation, or severe injury. A highly positive balance (above +10 g/day) can indicate over-feeding.
How is nitrogen balance used clinically?
Nitrogen balance assessment is used in clinical nutrition to evaluate the adequacy of protein supplementation in hospitalized patients, particularly those in the ICU, post-surgery, or with burns. A positive or zero nitrogen balance is the goal of nutritional support. It is measured by collecting a 24-hour urine sample for urinary urea nitrogen and comparing it to protein intake.
Official sources
- American Society for Parenteral and Enteral Nutrition (ASPEN): Clinical Guidelines for Nutritional Assessment.
- National Institutes of Health, National Library of Medicine: Nitrogen Balance.
Reviewed by the CalculatorHub team, edited by James Graham, 14 June 2026. See our methodology.