Depends on the substance. Conversions between a mass and a volume are problematic. If the substance was pure water, each mg = .001 X ml so 25 mg are .025 ml.
To convert mg to kg, divide by 1,000. 45654 mg = 45.654 kg.
0.25 kg is equal to 250,000 mg. To convert kilograms to milligrams, you multiply by 1,000, since there are 1,000 milligrams in 1 gram and 1,000 grams in 1 kilogram.
To convert milligrams (mg) to kilograms (kg), divide the value in milligrams by 1,000 because there are 1,000 milligrams in 1 kilogram.
To convert milligrams per liter (mg/L) to milligrams per kilogram (mg/kg) for sand, you need to know the density of sand to account for the volume change. Once you have the density, you can multiply the mg/L value by 1000 (since 1 liter of water is approximately 1 kilogram) and divide by the density of sand to get the mg/kg value.
1 kilogram = 1000 grams and 1 gram = 1000 milligrams so that 1 kg = 1000000 mg. You now have all the information required to work out, for yourself, the answer to this and similar questions.