Pure water has its highest density 1000 kg/m3 or 1.940 slug/ft3 at temperature 4°C (=39.2°F).
What is the real density of water?
The usual value used in calculations is 1 gram per milliliter (1 g/ml) or 1 gram per cubic centimeter (1 g/cm3). While you can round the density to 1 gram per milliliter, here are more precise values for you. The density of pure water actually is somewhat ?less than 1 g/cm3.
The slug is a derived unit of mass in the weight-based system of measures, most notably within the British Imperial measurement system and in the United States customary measures system. A slug is defined as the mass that is accelerated by 1 ft/s2 when a force of one pound (lbf) is exerted on it.
The US gallon, which is equal to approximately 3.785 L, is legally defined as 231 cubic inches. A US liquid gallon of water weighs about 8.34 pounds or 3.78 kilograms at 62 °F (17 °C), making it about 16.6% lighter than the imperial gallon.
A cubic metre of pure water at the temperature of maximum density (3.98 °C or 39.16 °F) and standard atmospheric pressure (1013.25 kPa) has a mass of1000 kg, or one tonne. At 0 °C (32 °F), the freezing point of water, a cubic metre of water has slightly less mass, 999.972 kilograms.
ρ (rho) = density, m = mass, V = volume. The SI unit of density is kg/m3. Water of 4 °C is the reference ρ = 1000 kg/m3 = 1 kg/dm3 = 1 kg/l or 1 g/cm3 = 1 g/ml.
Water has the highest specific heat capacity of any liquid. Specific heat is defined as the amount of heat one gram of a substance must absorb or lose to change its temperature by one degree Celsius. For water, this amount is one calorie, or 4.184 Joules.
In general, we say that the density of water is 1000 kg/m^3 (or 1 g/cm^3). But you're right that it does vary a little bit with temperature. It is exactly 1000 kg/m^3 at 4 degrees Celsius. At 20 degrees Celsius it is 998.23 kg/m^3 ( or 0.99823 g/cm^3).
The mass of water is expressed in grams (g) or kilograms (kg), and the volume is measured in liters (L), cubic centimeters (cm3), or milliliters (mL). Density is calculated by the dividing the mass by the volume, so that density is measured as units of mass/volume, often g/mL.
Pure water has a density of 0.99823 grams/cubic centimeter at 1 atm pressure and a temperature of 20 degrees Celsius (68 degrees Fahrenheit).