KOH Concentration Calculator
Enter a KOH percentage to get density, specific gravity, g/L, and molarity — or enter a measured density or hydrometer reading to solve for % KOH.
Where these numbers come from, and where they stop being valid
Potassium hydroxide solutions don't follow a single tidy density formula across their whole range — as K⁺ and OH⁻ ions go into solution they reorganize the surrounding water into tighter hydration shells, and how tightly that packing happens itself shifts with concentration. So rather than fit one curve, this page's reference table lists independently measured density points at 20°C from the CRC Handbook, sampled every 5 wt%, and the calculator linearly interpolates between whichever two rows bracket your input. That's a coarser step than the ~1% spacing on this site's NaOH and HCl tables — not because the math needs it, but because open, finely-spaced published KOH data is harder to pin down than for NaOH, and a shorter table of numbers we're confident in beats a longer one padded with guesses.
The table runs 0–50% w/w. Below 0 there's nothing to compute, and above 50% the reference points this table draws on get thin — KOH itself stays soluble a bit further, up toward the low-50s% before the solution saturates at room temperature, but that's a narrow band, not open-ended headroom. If you need a specific solution above 50%, treat any number you find as a starting estimate and confirm it by hydrometer or titration rather than trusting extrapolation.
Once density is known, getting to molarity is a straightforward mass balance: a liter of solution weighs 1000 × density grams, the % w/w share of that mass is dissolved KOH, and dividing by KOH's molar mass — 56.106 g/mol — gives moles per liter.
M = 10 × density × wt% / 56.106
Commercial grades, for context
Unlike sodium hydroxide, which is overwhelmingly sold and quoted as 50%, bulk aqueous KOH shows up commercially at a couple of different strengths depending on the supplier and the cold-weather handling they're designed around:
| Grade | Density (20°C) | Specific gravity | Molarity |
|---|---|---|---|
| 45% ("liquid caustic potash") | 1.4524 g/cm³ | 1.4550 | 11.65 mol/L |
| 50% | 1.5106 g/cm³ | 1.5133 | 13.46 mol/L |
45% is the more common of the two on a shipping manifest — it stays liquid at temperatures where a 50% caustic soda tank would need heat tracing, which matters more to a distributor's logistics budget than the extra water weight matters to their freight bill. If you're pricing out bulk storage for either grade, the tank volume calculator handles sizing the vessel once you know the batch volume you're holding.
Worked examples
Batching a 30% KOH lye for liquid soap
A soapmaker's recipe calls for a 30% KOH lye solution to saponify a batch of coconut and olive oil into liquid soap, and they need the density to check their dilution with a hydrometer before adding oils.
- Concentration
- 30% w/w
- Basis
- 20°C
≈ 1.2878 g/cm³ (SG 1.290), 6.89 mol/L
Confirming an incoming KOH concentrate shipment
A small electrolyzer workshop receives a drum labeled "35% KOH" and checks it with a hydrometer before use, reading 1.34 g/cm³ — close enough to specification, but they want the exact figure for their log.
- Measured density
- 1.34 g/cm³
- Basis
- 20°C
≈ 34.90% w/w, 8.33 mol/L
Sizing KOH catalyst from a liquid concentrate
Small-batch biodiesel producers running transesterification often prefer KOH over NaOH because it dissolves faster and filters more cleanly out of the finished fuel, and titration against the feedstock's free fatty acid content is what tells you how much catalyst to add. Say a titration on a batch of used cooking oil comes back needing 7 g of KOH per liter of oil — a realistic number for oil with some free fatty acid content, above the roughly 3.5 g/L base rate for fresh, low-FFA oil.
If you're buying dry flakes, that's the end of the calculation — weigh out 7 g per liter and go. But a lot of shops buy bulk 45% liquid concentrate instead, because it's safer to handle and meters more precisely than scooping caustic flakes. That's where this calculator's g/L output earns its keep: interpolating 45% gives 653.6 g of KOH per liter of concentrate. Dividing your target — 7 g — by that figure tells you how much concentrate to add per liter of oil:
Scale that by your batch size and you have a dosing number straight from the drum, no flake weighing required. If you need to thin a stronger concentrate down to a specific working strength instead of dosing it directly, the general-purpose solution dilution calculator handles the C₁V₁ = C₂V₂ math for that step.
Sanity-checking the output, and where people misread it
Validation: interpolating this table at exactly 45.00% gives 1.4524 g/cm³ (SG 1.4550, 11.65 mol/L). Commercial 45% KOH solution technical and safety data sheets — the kind that ship with a drum of "liquid caustic potash" — typically quote specific gravity around 1.46 and molarity in the 11.6–11.7 mol/L range at room temperature. That's a match within the spec sheet's own rounding, which is the level of agreement to expect from a well-behaved interpolation against an independently measured commercial figure.
The mistake that actually bites people: confusing solid-KOH assay percentage with solution concentration (covered in the FAQ below) accounts for more bad numbers than any math error in the calculator itself. A close second is temperature — KOH solutions are noticeably temperature-sensitive, so a hydrometer reading taken on a solution straight off a hot mixing process will read lower density than the same batch at 20°C, understating the concentration. Let a sample equilibrate to room temperature before you trust a density reading against this table.
A smaller but real trap: reaching for the NaOH calculator out of habit and plugging KOH numbers into it, or vice versa. The two tables aren't interchangeable — same general chemistry, different molar mass, different density curve.
KOH density reference table (20°C)
| Concentration | Density | Specific gravity | g/L | Molarity |
|---|---|---|---|---|
| 0% | 0.9982 g/cm³ | 1.0000 | 0.0 g/L | 0.00 mol/L |
| 5% | 1.0439 g/cm³ | 1.0458 | 52.2 g/L | 0.93 mol/L |
| 10% | 1.0904 g/cm³ | 1.0924 | 109.0 g/L | 1.94 mol/L |
| 15% | 1.1379 g/cm³ | 1.1400 | 170.7 g/L | 3.04 mol/L |
| 20% | 1.1864 g/cm³ | 1.1885 | 237.3 g/L | 4.23 mol/L |
| 25% | 1.2364 g/cm³ | 1.2386 | 309.1 g/L | 5.51 mol/L |
| 30% | 1.2878 g/cm³ | 1.2901 | 386.3 g/L | 6.89 mol/L |
| 35% | 1.3411 g/cm³ | 1.3435 | 469.4 g/L | 8.37 mol/L |
| 40% | 1.3959 g/cm³ | 1.3984 | 558.4 g/L | 9.95 mol/L |
| 45% | 1.4524 g/cm³ | 1.4550 | 653.6 g/L | 11.65 mol/L |
| 50% | 1.5106 g/cm³ | 1.5133 | 755.3 g/L | 13.46 mol/L |
Sources: Density–concentration data interpolated from a standard CRC Handbook of Chemistry and Physics-style aqueous KOH density/concentration table at 20°C (finer-grained source, sampled here at 5 wt% marks), cross-checked against an independent electrolyte-density (Pitzer-model) calculation and commercial 45%/50% KOH solution technical/safety data sheets (e.g. ERCO Worldwide, INEOS; general figures consistent with Occidental Chemical's Potassium Hydroxide Handbook). 0% anchored to the standard density of water (0.9982 g/cm³ at 20°C).
Frequently asked questions
My bag of KOH flakes is labeled "90% assay" — do I put 90 into this calculator?
No, and this is the single most common way people misuse a KOH density calculator. "90% assay" describes the purity of the solid flakes themselves — the rest is mostly absorbed water plus a few percent potassium carbonate, since KOH is aggressively hygroscopic and pulls both moisture and CO₂ out of the air the moment a container is opened. This calculator's % w/w is the concentration of the finished aqueous solution — dissolved KOH mass divided by total solution mass, after you've weighed flakes into water. If you weigh out 100 g of 90%-assay flakes, you're adding roughly 90 g of actual KOH (plus a bit of carbonate) to the water, not 100 g. For anything where the extra few percent matters — a titrated biodiesel batch, a lab reagent — weigh flakes fresh from a sealed container and correct for assay before you compute your target %.
Why is bulk liquid KOH usually sold at 45%, when caustic soda is sold at 50%?
Partly tradition, partly handling. A 50% sodium hydroxide solution is notorious in the industry for freezing around 12°C (54°F) — cold-climate storage tanks and rail cars need heat tracing just to keep it pumpable in winter. Potassium hydroxide solutions of comparable strength stay liquid at noticeably lower temperatures, which is one practical reason 45% ("liquid caustic potash") became the common commercial grade rather than pushing to 50% the way caustic soda does — easier logistics outweigh the modest saving in shipped water weight. 50% KOH is available too, mainly where the buyer has heated storage and wants to minimize water they're paying to ship.
How does this calculator help with biodiesel catalyst dosing if the recipe calls for dry KOH?
Titration gives you a target mass of KOH per liter of oil, but a lot of small producers buy KOH as bulk liquid concentrate rather than dry flakes, because it's easier and safer to meter and doesn't cake in storage. The gap between "I need 7 grams of KOH" and "how many mL of my 45% drum do I pour" is exactly what this calculator's g/L output answers — see the worked walkthrough below the examples.
Why does KOH make soft or liquid soap, while NaOH makes hard bar soap?
Same saponification reaction — fat or oil plus strong base yields glycerin plus a fatty-acid salt (soap) — but the salt's physical form depends on which cation you used. Sodium fatty-acid salts pack into a dense, rigid crystal lattice, which is why NaOH soap sets into hard bars. Potassium fatty-acid salts, with a larger and more loosely coordinating K⁺ ion, don't pack the same way and stay soft, translucent, or fully liquid at room temperature — which is why virtually all commercial liquid hand soap and soft soap is made with KOH, not NaOH. The two aren't interchangeable gram-for-gram in a recipe either: KOH's molar mass (56.106 g/mol) is about 1.4× NaOH's (39.997 g/mol), so a recipe written for NaOH needs roughly 1.4× the mass in KOH to fully saponify the same batch of fat.
What KOH concentration should I use for an electrolyte — battery or electrolyzer?
Most alkaline electrolyte applications (alkaline battery fill, small water electrolyzers, some plating baths) land in the 25–30% w/w range, and that's not an arbitrary choice: aqueous KOH's electrical conductivity peaks around 27–30% at room temperature, then actually falls off again at higher concentrations as the solution gets too viscous and ion-crowded for current to move easily. Going stronger than that isn't "more conductive," it's usually just more corrosive and harder to handle for no electrical benefit. Confirm your specific equipment's spec, but if you're improvising, aim inside that band rather than at the top of this calculator's range.
I already used the NaOH calculator on this site — can't I just reuse those numbers?
No — density, molarity, and even the practical concentration ranges differ between the two, because KOH's molar mass (56.106 g/mol) is noticeably higher than NaOH's (39.997 g/mol) and the two ions hydrate water differently. A 30% NaOH solution and a 30% KOH solution have different densities and very different molarities per liter. If you're working from a recipe or SDS written for the other hydroxide, use this site's separate NaOH concentration calculator for NaOH figures and this page only for KOH — don't cross-reference the two tables.
This table only steps every 5%, not every 1% like the NaOH and HCl tables — is the interpolated answer still trustworthy?
Yes, for the same reason it works on the finer tables: linear interpolation over a short, smooth segment tracks the real curve closely, and KOH's density-vs-concentration curve has no kinks or inflections in this range to trip it up. The wider spacing here isn't a math limitation, it's an honest reflection of how finely the underlying published KOH data can be pinned down from open references — rather than interpolate between closely-spaced but shakier numbers, this table uses fewer points we're confident in. For routine dosing, soap batches, and electrolyte prep the result is accurate enough; for a certified analytical figure, confirm by titration.