Whereas the imperial system of measurements uses at least three units for volume, the metric system has only one: the liter. It's defined variously as the volume of a kilogram of water at 4 degrees Celsius and atmospheric pressure, as one one-thousandth of a cubic meter or as one thousand milliliters. That makes a milliliter equal to one one-thousandth of a liter, or exactly one cubic centimeter. To calculate volume in milliliters directly from measurements, you must make the measurements in centimeters. If you have already calculated volume in other units, you can use standard conversion factors to convert to milliliters.
TL;DR (Too Long; Didn't Read)
Milliliters are the units of volume in the CGS (centimeter, gram, second) metric system. To calculate volume in milliliters from measurements, convert the original measurement to centimeters before making the calculations.
Handy Conversion Factors
It's easy to convert between milliliters and other metric volume units. All you have to do is multiply by the appropriate power of ten.
I milliter (ml) = 1 cubic centimeter (cc)= 0.001 liters (l) = 0.000001 cubic meters (m3).
Converting between milliliters and imperial volume units requires specific conversion factors. Here are some of the most used:
1 ml = 0.061024 cubic inches (in3) ; 1 in3 = 16.4 ml
1 ml = 0.000035 cubic feet (ft3); 1 ft3 = 28,317 ml
1 ml = 2.64 x 10-4 U.S. gallons (gal); 1 gal = 4.55 x 103 ml
Calculating Volume From Measurements
Volume is a three-dimensional quantity, so you generally need three measurements to calculate it. Exceptions include a cube, which has three sides of equal length, and anything with a circular cross-section, such as a cylinder or sphere. You calculate the volume of a cylinder by squaring the radius (r) of its cross-sectional area and multiplying by π and its height (h): V = πr2h. You only need to know the radius of a sphere to calculate its volume, because V = (4/3)πr3.
As long as you make all the measurements in centimeters, thus getting the volume result in cubic centimeters, you can calculate volume directly in milliliters, because a milliliter is defined as a cubic centimeter. If you make measurements in other units, convert them to centimeters before calculating volume to get the result in milliliters.
Calculating Volume From Density
If you know the density of a liquid or solid, you can calculate its volume by weighing it. This is possible because density is defined as mass per unit weight. In the metric system, mass and weight are expressed in the same units, so a one-kilogram weight also has a mass of 1 kilogram. The units of mass (weight) in the metric system are kilograms and grams. Once you know both the density and the mass, divide the mass by the density to find the volume.
If you want to calculate volume in milliliters, measure the weight in grams. If you already know the weight in kilograms, convert to grams by multiplying by 1,000 (103), because the definition of a kilogram is one thousand grams. You also need to express the density in CGS units, which you can look up in a table.