mg/kg to PPM Converter
Common Conversions
| mg/kg | ppm |
|---|---|
| 0.01 | 0.01 |
| 0.1 | 0.1 |
| 1 | 1 |
| 5 | 5 |
| 10 | 10 |
| 50 | 50 |
| 100 | 100 |
| 400 | 400 |
| 1000 | 1000 |
| 5000 | 5000 |
| 10000 | 10000 |
Why this conversion matters in chemistry
For solid samples — soil, sediment, food matrices, biota — mg/kg and ppm describe the same thing. Both are milligrams of analyte per kilogram of sample. The factor is exactly 1, but the conversion still earns its keep at the boundary between an analytical method and a regulatory report. A 150 mg/kg total petroleum hydrocarbons result on a soil sample is 150 ppm in EPA risk-assessment language. The identity holds rigorously for mass per mass; for liquid samples reported in mg/L, the equivalence depends on density being close to 1 g/mL.
Formula
Where the factor comes from
A factor of one still has a derivation, and this one is worth writing out because the coincidence is not an accident of notation. Milli is 10⁻³ and kilo is 10³, both exactly, so a milligram per kilogram is 10⁻³ g over 10³ g — one part in 10⁶ before anyone says ppm. Parts per million on a mass basis is defined as precisely that ratio. The two expressions describe one number in different alphabets, and the identity follows from the definitions of the prefixes rather than from any measurement. What it does depend on is ppm carrying a mass denominator. The three letters are not SI and declare nothing about their basis; volume and mole readings of ppm are equally common, and neither is numerically equal to mg/kg.
Precision and significant figures
Multiplying by one is the only conversion that cannot degrade a result, so whatever the digits meant on the analytical report they mean afterward. That makes added precision easy to spot: 12 mg/kg is 12 ppm, not 12.0 ppm. Everything limiting the number sits upstream — the mass of solid actually digested, the volume the digest was made up to, and where the result falls in the calibration range. Trace-metal work on a well-homogenized matrix supports three figures; a coarse or fibrous sample often does not support two, since subsampling variance can exceed the instrument's repeatability by an order of magnitude.
Worked Examples
The identity itself — one milligram of analyte per kilogram of solid sample is one part per million by mass.
The long-standing EPA residential soil screening level for lead, expressed in either notation.
A sub-ppm contaminant — at the edge of routine ICP-MS sensitivity for many trace metals in solid matrices.
A moderately elevated contamination level — well above background for most regulated compounds.
Common mistakes
Treating the identity as basis-independent
The equality holds only when the ppm was built on mass. A headspace or emissions figure in ppm is a mole fraction of gas, and leaving that number unchanged to call it mg/kg produces nonsense dressed as arithmetic. When a result arrives as a bare ppm, find out what it was divided by before accepting the one-to-one.
Losing the prefix between mg/kg and µg/kg
Low-level limits are frequently quoted in µg/kg, which is ppb rather than ppm. A single character in the unit string separates a result from one a thousand times larger, and in a table mixing both prefixes the eye slides over it. Read the prefix off the column header rather than off memory of what the analyte usually runs at.
Skipping the digest volume and sample mass
The instrument reads the digest in mg/L; the mg/kg on the sheet comes from that reading times the final volume divided by the mass taken. A 0.5 g sample made to 50 mL carries a factor of 100, so a 1 mg/L instrument reading is 100 mg/kg. Renaming mg/kg as ppm at the end does nothing to catch a dilution factor dropped at the start.