Urine Sodium-Creatinine Ratio Calculator
Urine Sodium-Creatinine Ratio Calculator
Turn a spot urine sodium into a dilution-independent index of sodium output, with the daily sodium and salt figures it implies — and an honest account of how wide the error bars are on a single sample.
Urine Sodium-Creatinine Ratio
Sodium ÷ creatinineUrine sodium 90 mmol/L, urine creatinine 9 mmol/L
Formula
Estimated 24-hour sodium (mmol) ≈ ratio × expected daily creatinine (mmol)
Salt (g) = sodium (mmol) × 0.0584
- expected daily creatinine
- about 15.4 mmol/day in men and 11.1 mmol/day in women in a large European reference population, falling with age and with muscle mass. It is a population mean standing in for one person’s muscle, which is the single largest source of error in the daily estimate
- × 0.0584
- converts mmol of sodium to grams of sodium chloride: 1 mmol of sodium is 58.44 mg of salt. So 87 mmol of sodium is 5.1 g of salt, which is the WHO limit, and 100 mmol is 5.8 g
- why not the sodium alone
- a spot urine sodium of 30 mmol/L is avid retention in a dilute urine and unremarkable in a concentrated one. Creatinine removes the water, which is the whole point of a ratio
- per gram of creatinine
- multiply mmol/mmol by 8.84, since 1 g of creatinine is 8.84 mmol
- what this is not
- a fractional excretion of sodium. FENa needs paired plasma and urine sodium and creatinine and answers a different question — the proportion of filtered sodium excreted. For the pre-renal versus intrinsic question in acute kidney injury, use FENa or FEurea
Worked example
Urine sodium 90 mmol/L, urine creatinine 9 mmol/L
90 ÷ 9 = 10.0 mmol/mmol
In a man excreting about 15 mmol of creatinine a day: 10.0 × 15 ≈ 150 mmol of sodium per day
150 × 0.0584 ≈ 8.8 g of salt a day — well above the WHO limit of 5 g
Between 8 and 16 → typical to high sodium output, and worth acting on in hypertension, proteinuria or stone disease
In a woman excreting about 11 mmol of creatinine a day, the same ratio implies about 110 mmol of sodium — the ratio is identical, the daily figure is not
Ratio to daily sodium and salt, at reference creatinine output
| Ratio (mmol/mmol) | Man, ~15 mmol Cr/day | Woman, ~11 mmol Cr/day | Salt (man) |
|---|---|---|---|
| 2 | 30 mmol Na | 22 mmol Na | 1.8 g |
| 4 | 60 mmol Na | 44 mmol Na | 3.5 g |
| 6 | 90 mmol Na | 66 mmol Na | 5.3 g |
| 8 | 120 mmol Na | 88 mmol Na | 7.0 g |
| 12 | 180 mmol Na | 132 mmol Na | 10.5 g |
| 16 | 240 mmol Na | 176 mmol Na | 14.0 g |
What a low ratio means depends on why you asked
| Question | Low ratio suggests | Caveat |
|---|---|---|
| Is this patient adherent to a low-salt diet? | Intake is low — or the kidney is retaining sodium | One sample cannot distinguish the two, and cannot describe habitual intake |
| Is the effective circulating volume low? | Yes — hypovolaemia, heart failure, cirrhosis with ascites, hepatorenal syndrome | Diuretics invalidate it; a recent dose raises sodium output whatever the volume state |
| Is this acute kidney injury pre-renal? | Use the fractional excretion of sodium instead | The ratio is not normalised to filtered load, so it is not a substitute for FENa |
| Why is this patient hyponatraemic? | Depletional hyponatraemia rather than SIAD | A urine sodium under 30 mmol/L is the conventional cut for the hyponatraemia algorithms; use the concentration there, not the ratio |
A sodium concentration tells you about the drink; the ratio tells you about the kidney
A spot urine sodium is one of the most requested and most misread results in the laboratory, because it is a concentration and concentrations move with water. Dividing by creatinine is what makes a spot sample interpretable: a urine concentration on its own says as much about how much the patient drank as about what the kidney did. Thirty mmol/L in a concentrated morning sample is a kidney conserving sodium hard; the same 30 mmol/L in a dilute sample after two litres of water is unremarkable. Dividing by urine creatinine, which is excreted at a near-constant rate through the day, removes the water and leaves a figure proportional to sodium output.
That output can be read two ways. Clinically, a low ratio marks a kidney holding on to sodium — hypovolaemia, decompensated heart failure, cirrhosis with ascites, the hepatorenal syndrome — while a high one marks either a large sodium intake or a kidney that cannot keep sodium, as in recovering tubular injury, post-obstructive diuresis, a salt-wasting nephropathy or mineralocorticoid deficiency. Nutritionally, multiplying the ratio by the expected daily creatinine output gives an estimate of 24-hour sodium excretion, and in a steady state sodium excretion is sodium intake. About 15 mmol of creatinine a day in men and 11 in women are the reference figures; the World Health Organization limit of 2 g of sodium a day is 87 mmol, or 5 g of salt.
The error bars on that dietary estimate deserve stating plainly. The creatinine output used in the conversion is a population mean substituting for one patient’s muscle mass, and it falls with age and with frailty. Sodium excretion also varies substantially from day to day and across the day, which is why studies comparing spot samples with complete 24-hour collections — including in chronic kidney disease — find that spot estimates track group averages reasonably but misclassify individuals. A spot ratio is a screening and monitoring tool and a conversation opener about salt; it is not a measurement of habitual intake, and when the number will change management the 24-hour collection is still the reference method.
Two things this ratio is not. It is not a fractional excretion of sodium: FENa normalises urine sodium to the filtered load using paired plasma and urine creatinine, and it is FENa or the fractional excretion of urea, not this ratio, that addresses pre-renal versus intrinsic acute kidney injury. And it is not the input to the hyponatraemia algorithms, which are built on the urine sodium concentration with a cut around 30 mmol/L. This calculator supports a clinician’s assessment and does not replace it.
Frequently asked questions
How do I estimate 24-hour sodium intake from a spot urine sodium-creatinine ratio?
Multiply the ratio in mmol/mmol by the expected daily creatinine output — about 15 mmol/day in men and 11 mmol/day in women. A ratio of 10 mmol/mmol in a man implies roughly 150 mmol of sodium a day, which is about 8.8 g of salt. Treat it as an order of magnitude, not a measurement.
How many grams of salt is 100 mmol of urine sodium?
About 5.8 g. One mmol of sodium is 58.44 mg of sodium chloride, so multiply mmol of sodium by 0.0584 to get grams of salt. The WHO limit of 2 g of sodium a day is 87 mmol, or 5.1 g of salt.
Is a spot urine sodium-creatinine ratio as good as a 24-hour collection?
No. Spot estimates track average intake in groups but misclassify individuals, and studies in chronic kidney disease have shown they do not accurately reflect measured 24-hour sodium. Use the ratio to screen and to follow a trend; use a complete 24-hour collection when the number will change management.
Can this ratio replace the fractional excretion of sodium in acute kidney injury?
No. FENa normalises urine sodium to the filtered load using paired plasma and urine sodium and creatinine, which is what makes the pre-renal versus intrinsic comparison possible. The sodium-creatinine ratio is dilution-corrected but not load-corrected, so use FENa or the fractional excretion of urea for that question.
What makes the ratio falsely high?
Any sodium the patient did not eat: intravenous saline, sodium-containing antibiotics and effervescent preparations, and sodium bicarbonate. Diuretics raise it regardless of volume state, and a kidney that is losing sodium — recovering tubular injury, post-obstructive diuresis, mineralocorticoid deficiency — raises it while the patient is sodium depleted.
Related calculators
References
- Mann SJ, Gerber LM. Estimation of 24-hour sodium excretion from spot urine samples. J Clin Hypertens. 2010;12(3):174–180.
- Dougher CE, Rifkin DE, Anderson CA, et al. Spot urine sodium measurements do not accurately estimate dietary sodium intake in chronic kidney disease. Am J Clin Nutr. 2016;104(2):298–305.
- Forrest KYZ, et al.; Mensink GBM, et al. Urinary 24-h creatinine excretion in adults and its use as a simple tool for the estimation of daily urinary analyte excretion from analyte/creatinine ratios in populations. Eur J Clin Nutr. 2015;69(11):1246–1251.
- World Health Organization. Guideline: Sodium Intake for Adults and Children. Geneva: WHO; 2012.
Medical Disclaimer: The tools and content provided here are for educational and reference purposes only. They are not intended to substitute for professional medical advice, diagnosis, or treatment. Clinical decisions should always be based on the comprehensive assessment of a qualified healthcare professional.