250 milligrams is equal to 0.0525 teaspoons of dry cooking ingredients, such as salt, sugar and spice. One gram, or 1,000 milligrams, is roughly equal to 0.21 teaspoons. 250 milligrams is a quarter of 1,000 milligrams, so, dividing 0.21 by four yields 0.0525.
This is a mass to volume conversion. Such conversions are not very accurate, but for the sake of everyday domestic use, such as cooking, the differences are normally regarded as neglible enough to ignore. Ingredients have different mass values at the same volume; therefore, a teaspoon of one ingredient can be heavier than a teaspoon of another.
To make an accurate conversion, a chef needs a large database of ingredients, preferably at different temperature points. This is necessary because many materials expand with an increase in temperature while mass remains constant, leading to inaccurate conversions.