By Gaurav Agarwal · Last updated 7 November 2026 · 6 minute read
A lab report gives a pesticide residue as 25 mg/kg. A regulation limit is written in mg/L. Both describe how much of a substance is present, but they measure it against different things: mass in one case, volume in the other. To compare them you need one extra number, the density of the liquid.
This guide gives the formula, shows why it works, and flags the cases where the conversion should not be used at all. The worked examples were recalculated before publishing.
For a liquid, mg/L = mg/kg × density (kg/L). For water the density is close to 1 kg/L, so the two numbers are almost identical. For anything lighter or heavier than water they are not, and the gap can reach 15% or more.
What mg/kg and mg/L mean
mg/kg is milligrams of a substance per kilogram of the whole sample. It is a mass ratio, so it does not change with temperature. mg/L is milligrams of a substance per litre of liquid. It is a mass-per-volume figure, and the volume of a liquid shifts slightly as it warms or cools.
The term ppm adds to the confusion. When it refers to mass, 1 ppm is 1 mg/kg. In dilute water solutions, 1 ppm and 1 mg/L are treated as equal because one litre of water has a mass of about one kilogram. That shortcut fails for other liquids.
The formula and why it works
mg/L = mg/kg × density in kg/L and the reverse: mg/kg = mg/L ÷ density in kg/L
The reasoning is short. Take 1 kg of liquid containing 10 mg of the substance, so the concentration is 10 mg/kg. If the density is 0.785 kg/L, that 1 kg fills 1 ÷ 0.785 = 1.274 L. The same 10 mg now sits in 1.274 L, which is 10 ÷ 1.274 = 7.85 mg/L. Multiplying by the density gives the same answer in one step.
Worked examples
Example 1: isopropanol. A solution of a dye in isopropanol (density about 0.785 kg/L) measures 25 mg/kg. Then 25 × 0.785 = 19.6 mg/L.
Example 2: seawater. A sample of seawater (about 1.025 kg/L) contains 10 mg/kg of a dissolved substance. Then 10 × 1.025 = 10.25 mg/L.
Example 3: going the other way. A diesel sample (about 0.84 kg/L) is reported at 42 mg/L. Divide by the density: 42 ÷ 0.84 = 50 mg/kg.
The table shows what a 10 mg/kg result becomes in mg/L for different liquids. Densities are approximate and depend on temperature.
| Liquid | Approx. density (kg/L) | 10 mg/kg equals |
|---|---|---|
| Water at 20 °C | 0.998 | 9.98 mg/L |
| Seawater | 1.025 | 10.25 mg/L |
| Vegetable oil | 0.92 | 9.2 mg/L |
| Diesel | 0.84 | 8.4 mg/L |
| Isopropanol | 0.785 | 7.85 mg/L |
If a result is in grams or micrograms, or the volume is in millilitres or gallons, bring everything to mg, kg and litres before applying the formula. The Weight Convertor handles milligrams, grams and kilograms. The Volume Convertor handles litres, millilitres and gallons.
When not to use it
- Solids. A result for soil, food or sludge in mg/kg is per kilogram of solid. It cannot be turned into mg/L of water without knowing how the sample was extracted and how much liquid was used. Regulators usually specify a leaching test for that.
- Gases. Concentrations in air follow different rules. Use the conditions given in the method, such as temperature and pressure.
- Unknown density. If you only guess the density, say so in your report. A wrong density gives a wrong conversion without any warning.
- Temperature. Density moves with temperature. Use the density at the temperature where the volume was measured.
Frequently asked questions
Is mg/kg the same as mg/L?
Only when the density of the liquid is 1 kg/L. Water is close to that, so the numbers are nearly the same. Oils, solvents and brines are not.
Is ppm the same as mg/L?
In dilute water solutions, yes, to a good approximation. In other liquids ppm by mass equals mg/kg, and mg/L must be calculated with the density.
How do I convert mg/L to mg/kg?
Divide the mg/L value by the density in kg/L. A result of 19.6 mg/L in a liquid of 0.785 kg/L is 25 mg/kg.
Where do I find the density?
Use the supplier’s data sheet or a safety data sheet for the product, measured near the temperature of your sample. For water, 0.998 kg/L at 20 °C is a standard figure.
Sources
- Omni Calculator: ppm to mg/L, including the density formula
- Inch Calculator: mg/L to ppm with the isopropanol example
Densities in the table are typical reference values and vary with temperature and composition. For regulatory or lab reporting, use measured values and the method your standard specifies.
