PPM Calculator

What a part per million means

A part per million is a ratio: one unit of something mixed into a million units of everything else. As a fraction that is 1/1,000,000, as a decimal 0.000001, and as a percent 0.0001. Turn it around and 1 percent is 10,000 ppm, so a 2 percent salt brine holds 20,000 ppm of salt and a 0.9 percent saline drip holds 9,000 ppm.

The unit survives because it states amounts too small for a percent. NIST Special Publication 811 does not accept ppm as an SI unit and asks authors to write 2.0 microliters per liter or 2 x 10^-6 instead, but the same guide allows ppm where a law or regulation requires it. Drinking water rules are the clearest case: the EPA lists limits in milligrams per liter and notes on the same table that mg/L is equivalent to ppm.

One warning before any arithmetic. ppm is a mass ratio in water, soil and food, a volume ratio in air, and a mass ratio again in consumer products. The number always compares parts, so you have to know which parts are being counted before you convert anything.

Water: 1 milligram per liter is 1 ppm

A liter of water weighs about 1 kilogram at room temperature, and a kilogram is 1,000,000 milligrams. Drop 1 mg of anything into that liter and you have 1 mg per 1,000,000 mg, which is 1 ppm. The EPA's data reporting guidance spells out both equivalences: 1 mg/kg is 1 ppm, and 1 mg/L is 1 ppm for water samples.

So 5 mg of fluoride in 2 L of water is 2.5 mg/L, or 2.5 ppm. Doubling the water halves the reading, which is the whole point of the unit: it tracks concentration, not the total amount. If you have 5 mg in 2 L and then pour it into 8 L, you still have 5 mg but the strength falls to 0.625 ppm.

Regulatory limits are written in the same units, so a ppm reading can be compared to them directly. The table below takes the numbers from the EPA's National Primary Drinking Water Regulations, page updated August 31, 2026.

The lead row carries a date you should keep with it. The EPA notes that from the compliance date of November 1, 2027, systems must meet a revised lead action level of 0.010 mg/L, which is 10 ppb. A page or a lab report that quotes 15 ppb without a year is describing the rule as it stands before that date.

Water operators work in pounds as well as milligrams. Because a gallon of water weighs 8.34 lb, one ppm across a million gallons is 8.34 lb of substance, a figure the USGS water school uses when it weighs a million gallons at 8,340,000 lb. That is why a dose chart reads lb = ppm x mgd x 8.34.

Solids, soil and food: 1 milligram per kilogram

For anything that is not water, the denominator is a mass. One milligram of a contaminant in one kilogram of soil is 1 ppm, and the EPA keeps that equivalence in the same guidance note. A lab report that says 250 mg/kg is telling you 250 ppm, no conversion needed.

Worked example: a soil sample weighs 0.5 kg and the lab extracts 250 mg of a metal from it. That is 250 mg divided by 0.5 kg, or 500 mg/kg, so the result is 500 ppm. The same sample reported as a percent would be 0.05 percent, which shows how quickly percent loses its usefulness at these sizes.

Percent and ppm convert with a factor of 10,000 in either direction. A 0.9 percent saline solution is 9,000 ppm. A pesticide tolerance written as 3 ppm is 0.0003 percent. When a label gives you a percent and a limit is written in ppm, multiply the percent by 10,000 and you can compare the two numbers directly.

One trap belongs here. A mg/L reading only equals ppm when the liquid has a density close to water. Brine, syrup and solvent weigh more than a kilogram per liter, so the same milligrams per liter is a lower ppm by mass. For a home calculation the difference is small, but it matters in a plant that doses by weight.

Diluting a stock solution

Dilution keeps the mass of solute and adds volume, so the concentration falls in proportion. The working equation is C1 x V1 = C2 x V2, where the first pair is the stock and the second is the mix you end up with. Units only have to match within each pair, so milliliters and liters can be mixed as long as you stay consistent.

Example: 200 mL of 50 ppm stock is brought up to 1,000 mL with water. The stock carries 50 x 200 = 10,000 ppm-mL of solute, and spreading that over 1,000 mL gives 10 ppm. Check it the other way: 10 x 1,000 is also 10,000 ppm-mL, so nothing was lost or invented.

Run it backwards to plan a mix. To make 5 L of 2 ppm from 50 ppm stock, the stock volume is 2 x 5,000 / 50, which is 200 mL, topped up to 5,000 mL. The table repeats the move for a few common targets.

Tank and pool dosing uses the same idea with pounds. Raising 10,000 gallons by 2 ppm takes 2 x 0.01 x 8.34, which is 0.1668 lb, or 2.67 oz of active ingredient. Ten thousand gallons is 0.01 million gallons, which is where the 0.01 comes from. Read the product label before dosing anything you drink or swim in, since the active ingredient is rarely 100 percent of the package.

Air and gases: ppm by volume

In air, ppm counts volumes, not masses, so the same word means a different ratio. The EPA's indoor air unit conversion note is blunt about it: gas readings should be called ppmV, one milliliter of vapor per cubic meter of air. Converting a mass reading takes the molecular weight and the molar volume.

At 25 C and one atmosphere, a mole of gas fills 24.45 L. That gives ppmV = mg/m3 x 24.45 / molecular weight. Ammonia has a molecular weight of 17.03, so 35 mg/m3 works out to 50.25 ppm, which is the pair the OSHA Table Z-1 lists for its 50 ppm limit (the table rounds the mass column to 35 mg/m3).

The EPA example with benzene makes a good check. Benzene has a molecular weight of 78.11, and 123.45 micrograms per cubic meter is 0.12345 mg/m3, so 0.12345 x 24.45 / 78.11 gives 0.0386 ppmV. The EPA note prints 0.0386 as well.

Temperature changes the constant. At 0 C a mole fills 22.414 L, so the same mass reading becomes a larger ppm number: 35 mg/m3 of ammonia is 46.07 ppm at freezing instead of 50.25 ppm at room temperature. State the temperature with the number, or the comparison to a limit is meaningless.

Percent, ppb and the units people mix up

Four units cover almost every case, and they differ only by the size of the million, billion or trillion they divide by. Knowing the factor for each one prevents the 1,000x mistakes that show up in water reports and lab sheets.

The abbreviation ppt is the one to watch. In the United States it usually means parts per trillion, while older British and some industry writing used it for parts per thousand. When a document matters, ask for the exponent instead of the abbreviation, or read the definition the document uses.

Percent by volume and percent by mass are a second source of confusion. A 5 percent bleach solution made by volume is not 50,000 ppm by mass unless the density is 1. The dilution pages linked below handle that case with the bleach strength written as a percent of hypochlorite.

Checks that catch a wrong ppm number

Most wrong ppm answers come from four slips: using the wrong denominator, forgetting the final volume, mixing up mass and volume units, or applying a limit that has changed. Each one leaves a fingerprint you can spot before you publish or dose anything.

Two sanity checks finish the job. A ppm value cannot exceed 1,000,000, and anything above 10,000 ppm is more than 1 percent, which is unusual for a trace contaminant and worth a second look at the inputs. If your answer moves by exactly a factor of 1,000, suspect a ppb and ppm swap rather than an arithmetic error.

Finally, keep the basis with the number. A result is complete when it says what was measured (mass or volume), what it was measured in (water, soil, air) and the date of the limit you compare it to. The calculator above returns the first two and leaves the third to the rule you are checking.