Determining the CORRECT Timpani head size
The new timpani heads arrive and you find out that the heads are either too big or too small for the drums! What happened? In most cases "standard" school timpani (Ludwig and Yamaha instruments) are fairly predictable when it comes to determining the head size. For these drums that have extended collars (the distance from the lip of the timpani bowl to the counterhoop) you generally add 2" to the bowl size. In other words, for a 32" drum you would order a 34" head or #3400 head.
The adventure comes when you have to determine the appropriate head size for custom instruments (usually Dresden timpani) or timpani that have very little collar if any. There are a couple of things you should check BEFORE ordering new timpani heads. First, check to see if there is a number listed on the old head that you're replacing. If you see a "2600" on the head chances are the head diameter is 26" but measure the outside diameter of the old head to be sure. However, if no numbers are present the best plan of action is to measure the diameter of the counterhoop (not the bowl).
Keep in mind that there are generally two different types of counterhoops; L-shaped and Square. The L-shaped hoop covers the top and side of the head and the Square hoop (also sometimes referred to as a "German hoop" or 7/16" hoop) is a bit heavier and covers the top part of the timpani head only (you should be able to see any visible numbers on the existing head with a Square hoop).
For L-shape hoops measure the total outside diameter (see picture above) but take into account that the side piece of the hoop will eat up about 1/16" or 2 1/16"). The drum pictured above takes a 26" head or #2600 head. For Square hoops simply measure the total outside diameter and that should give you the appropriate head size.
There may be some instances where your measurements might seem quite unusual. A year and a half ago I was replacing some heads on some old Ringer timpani and discovered some very unusual head sizes. I measured the outside diameter of the hoops (and was also able to find some older "spare" heads that had the appropriate numbers on them to verify the correct size) and came up with the following measurements:
31" drum = 3300 (33" head)
It was obvious that these were actually centimeter head sizes that were converted to inches. The #2502 and #3012 heads had to be special ordered from Remo.
Once you determine which head sizes are correct be sure to write this information down and keep it in a safe place. In my current job I am responsible for maintaining three different sets of timpani - each with slightly different head sizes.
If you're still not sure if you are measuring the instrument correctly try to consult a professional timpani player or the manufacturer. Chances are they will be able to determine the proper head for the instrument.