Introduction
Serum osmolality tells you how concentrated your blood is. It measures the total number of dissolved particles — like sodium, glucose, and urea — in a sample of blood. Doctors use this value to check how well your body balances water and solutes. A normal serum osmolality falls between 285 and 295 mOsm/kg. Values outside this range can point to problems like dehydration, kidney disease, or poisoning from toxic alcohols.
This serum osmolality calculator uses a standard formula to estimate your result from lab values you already have. Enter your serum sodium, glucose, and blood urea nitrogen (BUN) to get a calculated osmolality right away. You can also add serum ethanol for a more accurate estimate when alcohol is involved. If you enter a measured osmolality from the lab, the tool will compute the osmol gap — the difference between the measured and calculated values. An osmol gap greater than 10 mOsm/kg may suggest unmeasured substances in the blood, such as methanol or ethylene glycol. Clinicians often evaluate the osmol gap alongside the anion gap to narrow down the cause of metabolic disturbances.
The calculator supports both conventional units (mg/dL, mEq/L) and SI units (mmol/L), shows a step-by-step solution with the full math, and displays a chart that breaks down how much each component contributes to the total osmolality.
How to Use Our Serum Osmolality Calculator
Enter your serum lab values below to calculate estimated serum osmolality in mOsm/kg. The calculator also shows the osmol gap, a step-by-step solution, and a chart of each component's contribution.
Serum Sodium (Na): Enter the sodium level from your lab results. You can switch between mEq/L and mmol/L. The normal range is 136–145 mEq/L. This field is required.
Serum Glucose (Glu): Enter the blood sugar level. You can switch between mg/dL and mmol/L. The normal range is 70–100 mg/dL. This field is required.
Blood Urea Nitrogen (BUN): Enter the BUN value from your lab panel. You can switch between mg/dL and mmol/L. The normal range is 7–20 mg/dL. This field is required.
Serum Ethanol (EtOH): Enter the blood alcohol level if it was tested. This field is optional and defaults to 0. Including it gives a more accurate result when alcohol is present.
Measured Serum Osmolality: Enter the lab-measured osmolality if available. This field is optional. When provided, the calculator compares it to the estimated value and displays the osmol gap. A gap greater than 10 mOsm/kg may point to unmeasured substances in the blood.
Press Calculate to see your results. Press Reset to clear all fields and start over.
What Is Serum Osmolality?
Serum osmolality measures how many dissolved particles are in your blood. These particles include salts, sugars, and waste products like urea. The concept is closely related to molarity, which measures solute concentration in a solution, but osmolality specifically counts all dissolved particles per kilogram of solvent. Your body works hard to keep this number in a tight range, usually between 275 and 295 mOsm/kg. When the number goes too high or too low, it can mean something is off with your hydration, kidneys, or hormone levels.
How Serum Osmolality Is Calculated
Doctors use a simple formula to estimate serum osmolality from a basic blood test:
Osmolality = 2 × Sodium + (Glucose ÷ 18) + (BUN ÷ 2.8) + (Ethanol ÷ 4.6)
Sodium is multiplied by 2 because it always travels with a partner ion, like chloride. Glucose and BUN are divided by their molecular weight factors to convert mg/dL into milliosmoles per kilogram. Ethanol is optional and only added when alcohol use is suspected.
What Is the Osmol Gap?
The osmol gap is the difference between the measured osmolality from a lab test and the calculated osmolality from the formula. A normal osmol gap is 10 mOsm/kg or less. A gap larger than 10 may mean there are extra substances in the blood that the formula does not account for, such as methanol, ethylene glycol, or isopropanol. These are toxic alcohols that need fast treatment, as they can cause severe metabolic acidosis and dangerous shifts in blood pH.
Why This Test Matters
Doctors order a serum osmolality test to help find the cause of several problems, including:
- Dehydration — too little water raises osmolality.
- Overhydration — too much water lowers osmolality.
- Low sodium (hyponatremia) — a common electrolyte issue.
- Diabetic emergencies — very high blood sugar pushes osmolality up. Tracking long-term glucose control with an A1C calculator can help manage diabetes before emergencies arise.
- Toxic alcohol poisoning — detected through a high osmol gap.
- Kidney disease — the kidneys control how much water stays in the body.
Key Values at a Glance
- Normal sodium: 136–145 mEq/L
- Normal glucose: 70–100 mg/dL
- Normal BUN: 7–20 mg/dL
- Normal serum osmolality: 275–295 mOsm/kg
- Normal osmol gap: ≤ 10 mOsm/kg
This calculator lets you enter your lab values in either conventional or SI units. It then shows the estimated osmolality, the osmol gap (if you provide a measured value), a step-by-step breakdown of the math, and a chart showing how much each component adds to the total. For a broader electrolyte assessment, you may also want to check your corrected calcium.