PPM Calculator

The ppm formula

For a dilute aqueous (water-based) solution, parts per million is simply mass over volume:

approximate ppm = solute mass (mg) ÷ solution volume (L)

This shortcut uses the approximation that 1 L of dilute aqueous solution has a mass of about 1 kg, or 1,000,000 mg. True ppm is a ratio of like quantities (usually mass/mass or volume/volume), while mg/L is mass/volume; they are numerically close here because of the assumed density.

• 1 ppm = 1 mg/L = 1000 ppb
• 1 ppm = 0.0001 % = 1 µg/g
• percent (%) = ppm ÷ 10,000

Worked example

Suppose you dissolve 5 mg of fluoride in 2 L of water:

• ppm = 5 ÷ 2 = 2.5 ppm
• ppb = 2.5 × 1000 = 2,500 ppb
• percent = 2.5 ÷ 10,000 = 0.00025 %

This arithmetic does not determine whether that concentration is safe. Health or process limits depend on the substance, exposure route, jurisdiction and application.

Quick reference

Common pitfalls

• Volume in milliliters, not liters. A 250 mL beaker is 0.25 L. Convert before entering, or your ppm will be off by 1000×.
• Non-aqueous or concentrated solutions. The mg/L = ppm shortcut assumes density ≈ 1 g/mL. For brines, solvents or syrups, use mass/mass ppm (mg of solute per kg of solution) instead.
• Confusing ppm and percent. They differ by a factor of 10,000, a 0.5 % solution is 5,000 ppm, not 0.5 ppm.
• Mixing mg and g. Solute mass here is in milligrams. If your label lists grams, multiply by 1000 first.