Parts per million of what, in what? Without both, the number converts to nothing — and most charts answer as though it did.
Calculator
What is it in?
One part in a million — a milligram in a kilogram.
One part in a hundred.
Equals0.00009999999999999999 Percent (%)
Pure arithmetic on a ratio — no substance involved, so nothing here depends on what the parts are made of. That stops being true the moment a volume is introduced, which is what the other three tabs are about.
What this converter covers
Ratio conversions, concentrations in a named liquid, gas concentrations at a named temperature, and the four water-hardness units — each asking for the information the conversion actually needs.
ppm, ppb, ppt, percent, per mille and mg/kg
ppm to mg/L in six liquids, showing where the water shortcut fails
ppm to µg/m³ for eight gases, from their molar masses
The EU, EPA and STP reference temperatures side by side
Water hardness in ppm, grains, German, French and Clark degrees
Mass and volume kept apart Eight gases Reference conditions shown Four hardness units
Planning support from the values you enter — not professional advice.
One abbreviation, three quantities
Parts per million is a ratio, and a ratio needs to say what is being compared. Three different comparisons all get abbreviated the same way.
By mass — one milligram in a kilogram. This is what a laboratory usually means for a substance dissolved in a liquid, and it is better written as mg/kg, which cannot be misread.
By volume — one cubic centimetre in a cubic metre. This is what an air-quality or gas-detection reading means, and for an ideal gas it is the same as a mole fraction, which is why the molar mass is what converts it to a mass.
By moles — one molecule in a million. Used in chemistry, and equal to the volume fraction for gases but not for liquids or solids.
These are genuinely different numbers for the same mixture, and the SI itself takes the position that ppm should not be used without stating which is meant. Most published conversions skip that, which is why the same figure can be converted two ways and come out differently with neither being an arithmetic mistake.
Why ppm equals mg/L only in water
The most-repeated fact about ppm is that 1 ppm is 1 mg/L. It is true, in water, and it is not a property of the unit.
One ppm by mass means one milligram of substance per kilogram of solution. To turn a kilogram into a litre you need the density. Water is about 0.998 kg per litre, so a kilogram and a litre of it are the same thing to within a fifth of a percent, and the two units collapse into each other. That coincidence is the entire basis of the shortcut.
Change the liquid and it stops working. Ethanol is 0.789 kg/L, so 1 ppm by mass is 0.79 mg/L — a 21% error if the water rule is applied. Petrol is further out again. Mercury is 13.5 kg/L, where the shortcut is wrong by more than a factor of thirteen. And seawater, which looks close enough to water to not think about, is 2.5% denser — enough to matter against a regulated limit.
The habit is worth breaking even for water, because the reason it works is invisible from the shortcut itself. If a result matters, write mg/kg or mg/L rather than ppm and the ambiguity disappears at the source.
In air, the gas decides the answer
Air-quality limits are published in two units. The EU sets them in µg/m³; US and occupational standards often use ppm or ppb. Converting between them is not a fixed factor, because ppm in a gas is a volume fraction and µg/m³ is a mass concentration, and the bridge between them is the molecule’s own weight.
What 1 ppm becomes in µg/m³ at 25 °C, for four regulated gases
Gas
Molar mass (g/mol)
1 ppm in µg/m³
Methane (CH₄)
16.04
656
Carbon monoxide (CO)
28.01
1145
Nitrogen dioxide (NO₂)
46.01
1880
Sulphur dioxide (SO₂)
64.07
2619
A meter reading 1 ppm of SO₂ and one reading 1 ppm of NO₂ show the same number and hold 39% different amounts of matter. Against methane the spread is a factor of four. So a limit expressed in ppm and a limit expressed in µg/m³ cannot be compared at all without naming the gas — and any converter that offers a single ppm-to-µg/m³ box without asking which gas it is has already given a wrong answer.
The reference temperature nobody quotes
There is a second variable hiding in the same conversion. A volume fraction becomes a mass concentration through the molar volume of the gas, and the molar volume depends on temperature: warmer air is less dense, so the same ppm is fewer micrograms in every cubic metre.
This would be a footnote if everyone used the same reference. They do not. The EU’s ambient air quality directive standardises gaseous pollutant limits to 20 °C. US EPA and occupational limits conventionally use 25 °C. Chemistry often uses 0 °C. For the same gas at the same ppm those give figures 1.7% apart between the first two, and 9% apart against the third.
None of that is large next to a measurement uncertainty, and all of it is large enough to matter when a reported value sits just under or just over a limit — which is exactly when someone reaches for a conversion. The tool above lets you pick the reference condition and shows all three together, because a converted figure without a stated temperature is not a complete answer.
Water hardness has four national units
Hardness is a concentration of dissolved calcium and magnesium, conventionally reported as though it were all calcium carbonate. One quantity — and four units in common use, three of them named after countries.
The four hardness units, each in mg/L of calcium carbonate
Unit
Equals
Where it is used
ppm / mg/L as CaCO₃
1
United States, and most laboratory reporting
Grain per gallon (gpg)
17.12
US water softeners
German degree (°dH)
17.85
Germany and central Europe
French degree (°fH)
10
France
Clark degree (°e)
14.25
England, historically
Two of those figures are worth understanding rather than memorising. The German degree is 17.85 rather than a round number because it is defined as 10 mg/L of calcium oxide, not carbonate, and the two have different molar masses. The Clark degree is a grain per imperial gallon while the American gpg is a grain per US gallon, so the same idea in two countries gives 14.25 against 17.12 — a 20% difference that comes entirely from the gallon.
Practically: a softener sized in grains per gallon and a water report in French degrees are describing the same water, and the conversion above is exact. What is not exact is the classification — “hard” is a band, not a measurement, and the bands here are the US Geological Survey’s. Other bodies draw them in slightly different places.
Related calculators
Other conversions that need to know what the substance is:
Scientific NotationScientific, engineering and decimal forms with significant figures counted — and ambiguous inputs flagged rather than silently resolved.
Volume to WeightGallons to pounds, cubic yards to tons, litres to kilograms — by substance, with an honest range rather than one invented number.
WeightKilograms, pounds, ounces, stone and tonnes, listing the short, long and metric ton separately — three masses, one word.
VolumeLitres, gallons, pints and cubic units — with US and imperial named apart, because a UK gallon is 20% larger than a US one.
TemperatureCelsius, Fahrenheit, kelvin and Rankine — and temperature CHANGES, which convert differently from readings.
CookingCups, spoons and sticks to grams for 16 ingredients — with all five cups, the 20 mL Australian tablespoon, and flour as a range.
Every factor on this page is derived from a molar mass and the molar volume of an ideal gas rather than copied from a conversion table, which is what lets the temperature be a variable instead of a footnote. The sources settle the parts that are conventions rather than arithmetic: that ppm is not an SI unit and is ambiguous without naming the quantity, which reference temperatures the EU and the US actually specify, and where the water-hardness bands come from.
Three limits worth stating. First, the gas conversions assume ideal-gas behaviour at 101.325 kPa. That is accurate to well under a percent for the dilute pollutants here at ordinary temperatures, but it is not exact, and it is not appropriate at high pressure, near a gas's condensation point, or for a concentrated mixture. Second, this page converts units; it does not assess exposure or safety. A concentration only means something alongside an averaging period, a measurement method and the relevant limit — an hourly mean and an annual mean of the same pollutant are different regulated quantities, and converting an instantaneous reading does not make it comparable to either. Third, ppm in a solution is taken here as a mass fraction, which is the usual laboratory convention but not universal: some fields use a volume fraction and some a mole fraction, and the three give different numbers for the same mixture. Where a result matters — a regulatory submission, a clinical or environmental decision, a water treatment specification — read the unit definition in the governing standard rather than assuming this one. For drinking water or workplace air specifically, consult the relevant authority or a qualified professional.
Published the PPM Converter with four modes, because ppm names three different quantities and a single input box would misrepresent that.
Asks which liquid a solution is in. The familiar 1 ppm = 1 mg/L shortcut is a property of waters density rather than of the unit -- it is 21 percent wrong in ethanol, 2.5 percent wrong in seawater, and out by more than a factor of ten in mercury.
Asks which gas. In air ppm is a volume fraction, so the mass concentration depends on the molar mass: 1 ppm of sulphur dioxide is 39 percent more ug/m3 than 1 ppm of nitrogen dioxide and about four times more than methane. A converter offering a single ppm-to-ug/m3 box has already answered wrongly.
Asks which reference temperature, and shows all three together. The EU standardises gaseous pollutant limits to 20 C and the US EPA uses 25 C, which moves the same figure by 1.7 percent; against 0 C it is 9 percent.
Covers water hardness in all four national units. The German degree is 17.85 mg/L rather than a round number because it is defined in calcium oxide, and the Clark degree differs from the US grain-per-gallon only by which gallon is meant.
Every gas factor is derived from a molar mass and the molar volume of an ideal gas rather than transcribed, which is what allows temperature to be an input. Verified by 87 automated cases including exact round-trips for all eight gases at four temperatures.
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