250 grams of water is equivalent to 250 milliliters of water at 4 degrees Celsius. Because certain substances change density and therefore volume in response to temperature fluctuations, 250 grams may be slightly more or less than 250 milliliters. However, the two measurements are roughly equivalent.
Initially, the gram and the milliliter were equivalent amounts. One liter was defined as one kilogram of water. However, this original definition was changed permanently in 1964 in response to changes in scientific knowledge. Today, a liter is defined as the space occupied by one cubic decimeter, which means that one milliliter is a thousandth of a cubic decimeter.