Hydrochloric Acid Concentration Calculator

Muriatic acid — technical-grade HCl sold for pool chemistry, concrete etching, and masonry cleaning — has been graded by Baumé hydrometer reading for well over a century, not by a lab-measured %. For liquids denser than water (the "heavy" Baumé scale), degrees Baumé and specific gravity convert directly into each other:

20° Baumé is the standard commodity grade — the same drum as the worked example above, SG 1.160. Where it gets genuinely confusing is the % HCl printed alongside that Baumé number. This table's own 20°C interpolation puts SG 1.160 at roughly 31.7% w/w, but the figure that actually shows up on most supplier spec sheets for the same 20° Baumé grade is 31.45%. Both are correct; they're just not computed on the same basis. Industrial Baumé-to-SG conversion tables are conventionally referenced at 60°F (15.6°C) water rather than 20°C, and because HCl solutions run measurably denser when cold, the same SG maps to a slightly lower wt% under the colder convention. The gap — a few tenths of a percent — almost never matters for pool dosing or general acid work, and it disappears entirely if you work in SG instead of %: a hydrometer reads SG 1.160 either way. It matters more if you're pulling numbers off two different suppliers' sheets and expecting them to match to the decimal.

If you're stepping a drum like this down to a working dilution rather than using it neat, this page only tells you what the % or density number physically means — the volume math for getting from stock strength to a target concentration is a separate step, covered by the solution dilution calculator.

Every acid concentration calculator has some kind of upper limit, and for HCl it isn't set by how much you're willing to dissolve — it's set by vapor pressure. Hydrogen chloride is a gas at room temperature, and this calculator is really describing how much of that gas stays dissolved in water. There's a real limit to that at atmospheric pressure and ordinary room temperature: push past roughly 37–38% w/w and the equilibrium vapor pressure of HCl over the solution climbs high enough that gas visibly comes back out as a white fume the moment the container is opened. That's literally what "fuming hydrochloric acid" means, and it's why fuming-grade product needs different shipping and storage handling than ordinary concentrated acid. Reagent-grade "concentrated HCl" sold for lab and industrial use is conventionally 37% for exactly this reason — it sits just under the fuming threshold, which is also close to a practical ceiling for what's worth shipping as a stable, non-fuming liquid.

That's a different limit from HCl's other well-known concentration boundary, the constant-boiling azeotrope at roughly 20.2% w/w (boiling point 110°C at 1 atm). The azeotrope is a distillation limit, not a solubility one: boil a dilute HCl solution and water evaporates preferentially until the mixture converges on 20.2%; boil a concentrated one and HCl gas evaporates preferentially until it converges on the same 20.2% from the other side. You cannot concentrate aqueous HCl past that point in an open vessel by boiling alone, no matter how long you keep at it — it's a thermodynamic wall, not a matter of patience.

Practically, this means the reference table below extends to 40.0% because that's the extent of the independently measured 20°C data points behind it, but the top few rows already describe acid at or past the fuming threshold — most facilities never handle HCl that strong. Compare that with sulfuric acid, whose concentration calculator runs the full 0–100% range: H₂SO₄ isn't a dissolved gas to begin with, so it has no equivalent fuming ceiling anywhere near this one.

Two of this calculator's numbers do most of the real work in the field. Specific gravity is what a dip hydrometer actually reads off its floating scale, and molarity is what a stoichiometry calculation or a reagent recipe usually wants. Density in g/cm³ and concentration in g/L are useful intermediate figures, but rarely the number you started out needing — if you're unsure which output to reach for, SG answers "does this match the drum label" and mol/L answers "how much acid is actually in this liter."

Baumé and SG aren't interchangeable across trades without checking the modulus. The 145-modulus "heavy" formula above is for liquids denser than water — acids, brines, caustic. NaOH uses the identical heavy formula (see the NaOH concentration calculator for its own Baumé grades), but oils and alcohols are read on a completely different "light" Baumé formula with a different modulus and datum. Running the wrong one doesn't throw an error — it just hands you a confident, wrong number.

The rounded "37% = 1.19 g/mL" shorthand isn't a precise figure — see the FAQ below for how far off it actually runs from this table's own interpolated value at 37.00%.

Temperature drift matters more than it looks. A sample straight out of a metering pump, or a drum that's been sitting in direct sun, can easily be 10–15°C away from this table's 20°C basis. That shifts the density reading by a few thousandths of a g/cm³ — small in absolute terms, but enough to matter if you're checking a QC batch against this table rather than making a rough field estimate.

A wildly out-of-range number is almost always a unit mismatch, not unusual acid. Every realistic aqueous HCl solution falls between roughly 1.0 and 1.2 g/cm³ (1000–1200 kg/m³, about 8.3–10 lb/gal, SG 1.0–1.2). If an input or output lands far outside that band, check the unit selector before assuming the chemistry is unusual.

Diluting a drum down to a working strength is a separate calculation from anything on this page. This calculator tells you what a % or density number physically means; getting from stock strength to a target concentration is C₁V₁ = C₂V₂ math, worked through step by step in the dilution math guide. Whichever target you're working toward, the order you combine the two liquids in isn't optional — acid into water, never the reverse — and the heat and fume-handling side of that step is covered in the acid dilution safety guide.