NaOH Concentration Calculator

What 20°C buys you, and where the table stops

Every row behind this calculator was measured — or sits between two points that were measured — at 20°C. That's not an arbitrary lab convention: it's close to the near-universal reference temperature for aqueous-density literature, which is exactly why it's useful. A number read off this page lines up directly against a supplier data sheet, an old Baumé chart, or the hydrochloric acid or sulfuric acid calculators on this site, without a conversion step, because they're all anchored to the same 20°C basis.

Density still moves with temperature, though. Like most concentrated aqueous electrolyte solutions, caustic soda gets measurably less dense as it warms — on the rough order of a few tenths of a percent per 10°C in the mid-concentration range, enough to matter at tight tolerances even though it won't wreck a field check. If you're gauging a hot tank, a sample fresh off a heat-traced line, or a tote that just came off a delivery truck in July, let it settle to room temperature before you read it against this table, or expect the reading to run a little light.

The table stops at 50.5% w/w on purpose. That's the extent of the measured source data, and it also sits right at the practical ceiling for liquid caustic soda at ambient temperature: 50% caustic is the standard bulk commercial grade specifically because it's the strongest concentration that reliably stays liquid without heat tracing through an ordinary winter. Industry literature puts the freezing point of 50% caustic soda around 12°C (54°F) — already warm enough that most bulk tanks and rail cars carry insulation or trace heating regardless. Push past 50%, and NaOH solutions crystallize even more readily, which is why more concentrated product ships as solid flake or prill instead of solution. Don't extrapolate this calculator above 50.5% — you'd be reading a curve the source data never measured, in a strength band that barely exists as a liquid outside a heated vessel.

If your caustic lives in a bulk storage tank rather than a tote, the same idea — convert a physical reading into a quantity — carries over to inventory gauging. A strapping chart converts a dip reading to volume the same way this table converts a density reading to concentration, and some plants correct that strapping-chart volume for the actual density of what's in the tank rather than assuming water. See the tank strapping chart calculator and the guide to reading tank dip charts if that's the reading you're translating.

Two mixups that quietly wreck this calculation

% w/w vs. % w/v. Every figure on this page — in the table, in the calculator, on a technical NaOH spec sheet — is % w/w: grams of NaOH per 100 grams of total solution. That's a different quantity from % w/v (grams of NaOH per 100 mL of solution), and the two pull apart fast as the solution gets dense. Take the strongest row in the table: 50.5% w/w NaOH has a density of 1.53 g/cm³, which works out to 772.65 g of NaOH per liter of solution — divide by 10 and that's a 77.3% w/v figure for the same drum. Mix up which basis a spec sheet is quoting on a strong caustic solution and you're not off by a rounding error, you're off by more than half. The confusion does the most damage exactly where it matters most, because dilute solutions (density close to 1.00) make w/w and w/v nearly interchangeable — the very habit that gets people in trouble once they move up to commercial strength. Working from a stock solution rather than a fresh drum figure? The solution dilution calculator and the guide to dilution math in practice both work in mass or volume explicitly, rather than letting the two blur together.

Treating a warm reading as a 20°C reading. A hydrometer dropped into a tote straight off a delivery truck, or a sample pulled from a heat-traced day tank, reads warm — and because density falls as temperature rises, a warm sample of a given solution reads less dense than it would at 20°C. Run that reading through this table and you'll compute a % concentration that's too low for what's actually in the tank (a cold sample skews the other way, too high). For anything beyond a rough field check, let the sample reach room temperature first, or log the actual sample temperature alongside the reading so a wrong-temperature number doesn't get treated as gospel later in the paperwork.

Caustic soda has long been bought and sold by Baumé hydrometer reading rather than lab-measured density. For liquids denser than water (the "heavy" Baumé scale), degrees Baumé and specific gravity convert directly into each other:

This calculator's density and SG outputs plug straight into the second formula's SG term if you need to match a Baumé-marked hydrometer or an older supplier chart — that's how "50° Baumé caustic," the classic commercial grade of roughly 50% NaOH, gets its name.