Urine Specific Gravity to Osmolality Converter
Urine Specific Gravity to Osmolality Converter
Estimate urine osmolality from specific gravity using the last-two-digits rule, and know when the two measurements disagree.
Urine Specific Gravity to Osmolality Converter
Specific gravity → mOsm/kgSpecific gravity 1.018
Formula
- specific gravity
- e.g. 1.020
- 40,000
- empirical constant — the last two digits of SG × 40 gives the same result
Worked example
Specific gravity 1.018
(1.018 − 1) × 40,000 = 0.018 × 40,000 = 720 mOsm/kg
A specific gravity of 1.020, the value most often quoted, gives 800 mOsm/kg — the top of the intermediate band
Worked conversions
| Specific gravity | Approx. osmolality (mOsm/kg) | Interpretation |
|---|---|---|
| 1.005 | 200 | Dilute |
| 1.010 | 400 | Dilute-intermediate |
| 1.020 | 800 | Intermediate |
| 1.030 | 1200 | Concentrated |
A quick estimate with real limits
Specific gravity is easy to measure at the bedside with a refractometer or dipstick, while osmolality needs a laboratory osmometer, so a bedside estimate of one from the other is genuinely useful. The rule of thumb is simple: take the last two digits of the specific gravity and multiply by 40, or equivalently subtract 1 from the specific gravity and multiply by 40,000. A specific gravity of 1.020 gives about 800 mOsm/kg by either route.
The relationship holds only when the assumption behind it holds — that specific gravity and osmolality are both responding to the same population of small, similarly charged solutes, chiefly urea, sodium and its accompanying anions. Specific gravity is a measure of solution density, which depends on the mass and size of dissolved particles; osmolality is a measure of particle number, regardless of mass. Ordinarily these move together closely enough for the rule of thumb to work.
The two measurements diverge badly whenever large or heavy molecules are present in urine that contribute little to particle count: glucose, iodinated radiocontrast, mannitol, dextran and heavy proteinuria all raise specific gravity far more than they raise osmolality, because each adds substantial mass without adding many osmotically active particles. In any of these settings the estimated osmolality from this converter will be too high, and a directly measured osmolality is the physiologically meaningful value to trust.
Frequently asked questions
How do I convert urine specific gravity to osmolality?
Take the last two digits of the specific gravity and multiply by 40 — for example, 1.020 gives 20 × 40 = 800 mOsm/kg. This is equivalent to (specific gravity − 1) × 40,000.
Is a specific gravity of 1.020 concentrated?
It corresponds to roughly 800 mOsm/kg, which is in the intermediate range for a random urine sample — not dilute, not maximally concentrated.
Why do specific gravity and osmolality sometimes disagree?
Specific gravity depends on the mass of dissolved particles; osmolality depends on their number. Glucose, radiocontrast, mannitol, dextran and heavy proteinuria add mass without adding proportionate particle count, raising specific gravity far more than osmolality.
Which measurement should I trust if they disagree?
Osmolality. It is the physiologically meaningful measurement of solute concentration; specific gravity is only a convenient, density-based proxy for it.
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References
- Chadha V, Garg U, Alon US. Measurement of urinary concentration: a critical appraisal of methodologies. Pediatr Nephrol. 2001;16(4):374–82.
- Voinescu GC, Shoemaker M, Moore H, Khanna R, Nolph KD. The relationship between urine osmolality and specific gravity. Am J Med Sci. 2002;323(1):39–42.