NaOH Concentration Calculator

20°C is the standard reference temperature used by essentially every published aqueous-density reference — it gives every table a common, comparable baseline. Density is temperature-dependent (hot caustic is less dense than cold caustic at the same concentration), so a reading taken well above or below 20°C will be off by a small, temperature-dependent amount. For routine dosing and QC this table is accurate enough as-is; for tight lab work, let the sample equilibrate to room temperature before measuring, or apply a separate thermal-expansion correction.

Density is mass per volume with real units (g/cm³). Specific gravity (SG) is a unitless ratio — the solution's density divided by the density of water under a reference condition, here 20°C water at 0.9982 g/cm³. Because water's density is so close to 1, density in g/cm³ and SG end up numerically close for aqueous solutions, but they aren't the same quantity: a hydrometer marked "SG" is reading that ratio directly, not grams per milliliter.

Caustic soda has long been sold and dosed by Baumé hydrometer reading. For liquids denser than water, the conversion is °Bé = 145 − (145 / SG); rearranged, SG = 145 / (145 − °Bé). A classic example: 50% NaOH (SG ≈ 1.53) works out to roughly 50 °Bé — "50° Baumé caustic" is exactly this grade, and it's the figure many suppliers still quote on a shipping tank. This calculator outputs density and SG directly — run either through the formula above if you need a Baumé figure to match an old chart.

If NaOH and water mixed with zero volume change, density would scale linearly with concentration. Real solutions don’t: as Na⁺ and OH⁻ ions dissolve, they pull water molecules into tighter hydration shells, so the mixture packs measurably denser than a simple weighted average predicts, and that packing effect itself changes with concentration. That is why the table uses closely spaced real measurements and this calculator interpolates linearly only between two adjacent rows, rather than fitting one formula across the whole 0–50.5% range.

A few small errors stack up on a hydrometer: the sample being off 20°C, a calibration drift in the float, or misreading the meniscus can each shift the result by a few tenths of a percent. Titration measures the NaOH content directly through a chemical reaction and is generally the more accurate method when the two disagree — treat the hydrometer/density route as a fast field check, not a lab-certified result.

Linear interpolation between two closely spaced, independently measured points is very close to the true curve — the error from treating a short segment as a straight line is far smaller than typical field-measurement error. The source data here steps in roughly 1-point-density increments, which is tight enough for process dosing, tank inventory, and QC checks; for certified analytical work, pair this with a lab titration.