Easy way to figure out my cadence?

I'm riding 42/16 on 700x23's... any way to guesstimate my cadence at ~20 or 25 mph?

100-120 RPM
https://sheldonbrown.com/gears/

Oh yeah, and cranks are 165s....ooh awesome, thanks for the linky.

here's a handy online calculator...
https://www.panix.com/~jbarrm/cycal/cycal.30f.html

if you want to get all crazy just get a cyclometer that has a cadence counter.

Or, just count the number of rotations of the pedal in 15 seconds and multiply by four. Like measuring your heartrate.

Pedal fast enough so you almost spin out. Too much and you spin out, and too little and you go slow.

Low tech solution that works for any gear: count how many times you pedal in 15 seconds and multiply by 4. ( or count for 10 seconds and multiply by 6)

Or count six seconds and multiply by ten. A bit easier, but you might sacrifice accuracy.

If you are musical, try singing a song with a known tempo in time with your pedaling.

Here is a handy chart:
https://www.ssqq.com/information/speedcha.htm

For instance, "Bad to the Bone" is 96 beats/minute. "Sultans of Swing" is 147 and "In the Mood" is 81.

Even easier method: count strokes for 6 seconds and add 0 - no multiplication needed

Very sneaky, but that's still multiplication. If you want to avoid multiplication, just count for 60 seconds.

Now you could be off by as much as 12 rpm in either direction.

I find that 6 seconds is not very accurate because it just doesn't give you a lot of time to count each pedal stroke.

I usually do a 15 sec cadence count on one leg and multiply by 4. It is accurate if you're timing it exactly using a stopwatch. I've timed cadence using a stopwatch, using a metronome, and using a cyclocomputer. At most, you'll be off by maybe 2 RPM if you don't do it EXACTLY right. But it's a good, rough estimate. And it's not going to give average cadence, just your instant cadence. If you want average cadence, you'll either spend your entire ride staring at a clock and multiplying in your head (unrealistic), or you'll get a bike computer and let it estimate out your cadence for you.

Koffee

just update your computer with one that does speed and cadence...

What makes you say that?

I normally ride my trainer one day a week which tells me my cadence. As a result I can "feel" fairly accurately what my cadence is on the road. I find I have a tendency to push bigger gears at a lower cadence if I don't ride my trainer. Then on the road I count my strokes from time to time for a minute to verify that I am in the range I want to be. I rest at 90-95, spin at 95-100, and push from time to time at 105-110.

Here's a gear ratio & cadence spreadsheet I've posted before. As far as crank length, when your chainring makes one revolution so does your crank arm. No matter how long or short it is. It is the same thing as when people ask where should the magnet for a computer be placed on the spoke? Closer to the hub or closer to the rim? It makes no difference. The magnet will always make one revolution for each wheel revolution no matter where it's placed. Crank length affects mechanical advantage and the real speed of your pedaling effort, but not revolutions.

Think inaccuracy of 1 or 2 rpm multiplied by 10.