Converting revolution to rounds per minutes

Question

I have a DC motor, I want to measure its speed (e.g. A rotary encoder tells me that a full revolution takes 24ms). How can I convert this number to one with units of Rounds Per Minute?

I think $\text{RPM} = \text{ms} / 6000$, but this for $24 \ \text{ms}$ per revolution gives the strange result of $400\times 10^{-9}$. What am I doing wrong?

Answer 1

$$ \dfrac{1 \ \text{rev}}{24 \ \text{ms}} \cdot \dfrac{1000 \ \text{ms}}{1 \ \text{sec}} \cdot \dfrac{60 \ \text{sec}}{1 \ \text{min}} = 2500 \dfrac{\text{rev}}{\text{min}} $$

Answer 2

Use the rule of three:

One revolution takes 24 ms, so in 12 ms you have $\frac12$ revolution, and in a minute ($60,000$ ms) you have $5000$ times more, i.e. it corresponds to $$\frac{5000}2=2500 \,\text{ rpm}.$$