PPM Calculator
Enter a concentration in ppm, ppb, ppt, percent, mg/L, µg/L or molarity and see it in every other unit at once, with the working shown. The one assumption that matters — the density of the solution — is a field you can change. Free, no sign-up.
1.000 is right for dilute aqueous solutions; water is 0.9982 g/mL at 20 °C. It links mass fractions (ppm) to mg/L.
Case matters: Co is cobalt, CO is carbon monoxide. Hydrates use · or *, charges use ^ (Fe^3+).
Results
250.0
ppm
250.0
mg/L
0.02500
%
The same concentration in other units
| Unit | Value |
|---|---|
| ppm | 250.0 ppm |
| ppb | 250000 ppb |
| ppt | 2.500 × 10⁸ ppt |
| % | 0.02500 % |
| g/L | 0.2500 g/L |
| mg/L | 250.0 mg/L |
| µg/L | 250000 µg/L |
| ng/L | 2.500 × 10⁸ ng/L |
| mol/L | 0.004278 mol/L |
| mmol/L | 4.278 mmol/L |
| µmol/L | 4278 µmol/L |
| nmol/L | 4.278 × 10⁶ nmol/L |
The working
250 mg/L = 0.25000 g/L
ppm = 0.25000 g/L ÷ (1000 mL/L × 1 g/mL) × 10⁶ = 250.0
mol/L = 0.25000 g/L ÷ 58.44 g/mol = 0.004278 mol/L
How to Use the PPM Calculator
- 1. Enter the concentration and pick its unit. The unit list covers ppm, ppb, ppt, percent, mg/kg, µg/kg, g/L, mg/L, µg/L, ng/L and molar units.
- 2. Check the density. It defaults to 1.000 g/mL, which suits dilute water solutions. Change it for brine, seawater, a solvent or any dense solution.
- 3. Add the substance for molarity. Type the solute's formula or its molar mass; leave the box ticked to see mol/L, mmol/L and µmol/L as well. Converting from a molar unit requires it.
- 4. Read the table. Every unit is listed with the value rounded to the significant figures of your data, and the working shows the two conversions that were used.
What ppm, ppb and ppt Mean
| Unit | Fraction | Equivalent mass ratio |
|---|---|---|
| 1 ppm | 10⁻⁶ | 1 mg per kg |
| 1 ppb | 10⁻⁹ | 1 µg per kg |
| 1 ppt | 10⁻¹² | 1 ng per kg |
| 1 % | 10⁻² | 10 g per kg (10,000 ppm) |
Parts per million is a mass fraction: grams of solute per million grams of solution. ppb and ppt use the short scale, where a billion is 10⁹ and a trillion is 10¹²; some older sources in languages that use the long scale mean something different, so check for a unit definition when a document is not English. In this calculator ppm is always a mass fraction, so 1 ppm = 1 mg/kg.
Why Density Decides Whether ppm Equals mg/L
Water analysis reports concentrations as mg/L, which is mass of solute per volume of solution. A litre of solution weighs 1,000 × ρ grams, where ρ is the density in g/mL, so the two are linked by ρ:
ppm to mg/L
mg/L = ppm × ρ (g/mL)
mg/L to mol/L
mol/L = (mg/L ÷ 1000) ÷ molar mass
For a dilute aqueous solution ρ is within a fraction of a percent of 1.000 g/mL (pure water is 0.9982 g/mL at 20 °C), so ppm and mg/L are equal for practical purposes. For seawater, brine or an organic solvent they are not: at ρ = 1.2 g/mL, 100 ppm is 120 mg/L. The calculator keeps the two apart and says so when the density is more than half a percent from 1.
Worked Examples
mg/L to everything else (the default inputs). A 250.0 mg/L sodium chloride solution at ρ = 1.000 g/mL is 250.0 ppm, 250,000 ppb, 0.02500% and 0.2500 g/L. With M = 58.44 g/mol, 0.2500 g/L ÷ 58.44 g/mol = 0.004278 mol/L, or 4.278 mmol/L.
ppb to mg/L. A trace contaminant at 15 ppb in water is 0.015 ppm = 0.015 mg/L = 15 µg/L = 1.5 × 10⁻⁶ %. For lead (207.2 g/mol) that is 15 × 10⁻⁶ g/L ÷ 207.2 g/mol = 7.2 × 10⁻⁸ mol/L, which shows why trace analysis is quoted in ppb and not molarity.
Gases use parts per million by volume
Air-quality figures such as "400 ppm CO₂" are usually parts per million by volume (ppmv), which for an ideal gas equals micromoles per mole. That is a mole fraction, not a mass fraction, and turning it into mg/m³ needs the temperature, the pressure and the molar mass — see the Ideal Gas Law Calculator. This tool converts solutions only.
Common Mistakes
- Assuming ppm = mg/L everywhere. It holds only near ρ = 1.000 g/mL.
- Mixing the scales. 1 ppb is 1,000 times smaller than 1 ppm, and 1 µg/L is 1 ppb in water, not 1 ppm.
- Forgetting to convert grams to milligrams. 0.015 g/L is 15 mg/L; the calculator handles prefixes, but a hand calculation often drops a factor of 1,000.
- Reporting false precision. A converted value cannot be more certain than the measurement; check the digits against the Significant Figures Calculator.
Frequently Asked Questions
- How do you convert ppm to mg/L?
- For a dilute aqueous solution the numbers are the same: 1 ppm is 1 mg/L. In general mg/L equals ppm times the solution's density in g/mL, so at 1.2 g/mL, 100 ppm is 120 mg/L. Enter the density and the calculator does it.
- How do you convert ppm to ppb?
- Multiply by 1,000, because a part per billion is a thousand times smaller than a part per million: 0.5 ppm is 500 ppb. Going the other way, divide by 1,000: 15 ppb is 0.015 ppm.
- How do you convert ppm to molarity?
- Turn ppm into g/L (ppm ÷ 1000 × density), then divide by the molar mass. For 100 ppm of NaCl at 1.000 g/mL, 0.100 g/L ÷ 58.44 g/mol = 1.71 mmol/L. The molar mass is needed because molarity counts particles and ppm counts mass.
- What is the difference between ppm and percent?
- Percent is parts per hundred and ppm is parts per million, so 1% equals 10,000 ppm and 1 ppm equals 0.0001%. Use percent for major components and ppm or ppb for trace levels.
- Is ppm by mass or by volume?
- In solution chemistry it normally means by mass (mg per kg), and in air-quality work it means by volume (ppmv). The same label hides two different quantities, so state which one you are using. This calculator treats ppm as a mass fraction.
- Why does the calculator ask for density?
- Mass fractions such as ppm are per kilogram of solution while mg/L is per litre, and density is the link between them. For dilute water solutions it is almost exactly 1.000 g/mL, which is why many references treat ppm and mg/L as interchangeable.