Running cables through access holes in joists and down from drop ceilings is easier if the cables are bundled together. A cable bundle diameter is a little bit larger than the sum of the diameters of the cables in question, to give room for the pull cord and to give room for the individual cables to slide against each other. The cable bundle diameter is also used to generate the diameter of the hole you're running them through in your structure.
Get a two-foot length of each cable you intend to run into the bundle and bundle them together. Wrap the bundle with cord to keep it bound together.
Flex the bundle a few times, then pull on one end. Your goal is to get the bundle of cables to behave as if it is been tugged and pulled through a hole, bent around a shallow curve once or twice.
Wrap a tape measure around the circumference of the largest, loosest point of the bundle. Write this number down.
Divide the circumference by 3.1415 (pi). This will give you the diameter of the cable bundle. For the diameter of the hole, add a quarter of an inch to this number.
Make a list of the types of cables you intend to run through the joists. The three most common cable types are CAT5 (Ethernet) cables, RG6 Quad Shielded (cable TV) cables, and 24-gauge RJ11 Twisted Pair (telephone) cables, which have diameters of 0.237", 0.298", and 0.0201", respectively.
Find the sum of the smaller diameters of the cables and divide that number by 2; this gives a rough average of your smaller cables, and assumes that they will be distributed evenly around your largest cable. For example, if you're running a bundle of two RG6 cables, three CAT5 cables and three RJ11 cables, the numbers would work out as follows: (0.237 * 3) + (0.0201) * 3 = 0.7711". Divide by 2. The result is 0.386 inch.
Add your largest cable's diameter to this number. Continuing the previous example, with the two RG6 cables, add 0.596" of diameter to the cable run. Adding 0.596 to 0.386 gives a final cable bundle diameter of 0.982". Unlike the first method, you do not need to add a quarter of an inch to this number to compensate for wiggle room on the cables.
The second method is a significantly less precise method of calculating cable bundle diameter and relies on a geometry trick (closely packing cylinders is very similar to packing hexagons). Do not run telephone or Ethernet cable in the same bundles as electrical cables; the current running through the electrical cable can generate a magnetic field that will interfere with the signals carried by the other cables.