This is the most simple question in the world, but it is surprisingly difficult to explain to a student. The heart rate is measured in bpm, beats per minute. Minutes are a true unit. Beats are not a true unit. However, we are using beats as if it where a unit. I'm not sure how to explain this to a student, and I'm confusing myself. Please help.

  • 4
    $\begingroup$ It's a dimensionless quantity. Thus the dimension of "beats per minute" is $\frac 1{\text{time}}$. $\endgroup$
    – lulu
    Dec 11, 2020 at 22:42
  • 1
    $\begingroup$ How do you reckon a beat is not a unit? Sure, it is not one of the standard SI/imperial units, but it is clear enough what it refers to and is measuring/describing, surely? $\endgroup$ Dec 11, 2020 at 22:43
  • $\begingroup$ @lulu But in the medical world, it is allways written as bpm. If you write it as 1/time, no one will understand. $\endgroup$
    – Chris
    Dec 11, 2020 at 22:45
  • $\begingroup$ Are you familiar with hertz (Hz)? A unit is $s^{-1}$, or in plain English, per second. $\endgroup$ Dec 11, 2020 at 22:48
  • $\begingroup$ @Eevee I agree that we all clearly understand the concept. However, if a beat is not one of the standard SI/imperial units, then it is not a unit. There are no other units. I applogize if we are going in circles. $\endgroup$
    – Chris
    Dec 11, 2020 at 22:48

2 Answers 2


BPM means "beats per minute" but that doesn't mean that "beats" is a "unit." Otherwise we could have an infinite number of "units" based on counting other things.

For instance: How many red two-door cars with sagging taillights and broken headlights with expired license plates pass by a point on this road in a minute? Oh, $7$, you say? Well I suppose the rate is 7 RTDCSTBHELPPM."

So now "RTDCSTBHELP" is a "unit"?!?!


  • $\begingroup$ Thanks for the laugh! Let me please pretend I'm the student. I would say to you that you can treat "beats" mathematically as if they were a unit. All the same operations apply. So, isn't it some kind of effective unit? $\endgroup$
    – Chris
    Dec 11, 2020 at 23:01

@Stork led me to the answer, but he didn't quite get there. A "beat" is a "counting unit". This is the term I've been looking for! A "dozen" is also a counting unit. If you google "counting unit", you will find it is a fairly common term. So, if a "beat" is a counting unit, then what kind of unit is a "second". A "second" is a "measuring unit". This implies that counting and measuring should be treated as distinct activities, which makes sense. I had never really stopped to realize that counting and measuring are distinct.

I do apologize for dwelling on this, but I think this terminology will clarify matters for students.

ps. This ancient paper appears to discuss the distinctions.


You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .