Originally Posted by
SweetLou
Computers are nice, but not necessary. I use one that doesn't have cadence capabilities. I just count the revolutions for 30 seconds, then double. Now I don't do that, since I know the feel of my cadence if I am pedaling too fast or slow. You can have a friend drive next to you and see how fast you are going. Remember this and what gear you are in, check Sheldon Brown's Gear Calculator and determine your cadence. More work, but can be done.
If you really want to do the math, you could use gear inches and speed to determine your cadence.
I find a cadence meter to be especially helpful when I'm tired on a long ride or when fighting a head wind and my body doesn't feel quite right. Using a cadence meter I can find a gear that will put me back into that higher efficiency range and pretty soon I'm feeling better and my speed may increase even though I've selected a lower gear.
I'm sure that if I tried to calculate my cadence by counting rotations for 30 seconds by the time I've made the calculations my cadence will have changed up or down 5 to 10 rpm, maybe more.
Computers with cadence are very inexpensive, less than the cost of a new tire.