A few years ago someone told me that all mammals, including humans, have a lifespan of a billion heartbeats. I believed him, didn’t question the number, assumed that he knew what he was talking about. Some time later I decided to check the numbers, to work out how many heartbeats there might be in a human lifetime of 80 or 100 or 120 years. I was on a tube train and started with some calculations in my head – no room to use a pen and paper.

At rest our hearts beat at something like 60 or 70bpm (beats per minute). Top athletes might have a much lower heart rate. Back in the 1970s we heard that the great New Zealand runner John Walker had a heart rate of 40bpm. When I was a child my father’s was 48bpm. I have just tested mine and am surprised to find that it is only 58bpm; I thought it was higher than that.

To get a rough figure let’s use a bpm of 60. That makes 60 heartbeats per minute, 3,600 (60×60) in an hour and 86,400 (24×60) per day. You can round up or down to get approximate numbers for a week or a year after that: 7 x 90,000 makes around 630,000 heartbeats in a week; 50 x630,000 gives us around 30 million heartbeats in a year. At that rate you get through your first billion heartbeats in about 33 years. My heart has gone through more than a billion beats, and I’m well on the way to two billion. At these rates a person who lives to 100 will have had over three billion (3,000,000,000) heartbeats. Does your heart have a built-in limit like certain pieces of machinery? Can it go on beating four billion times or more, as long as everything else is working? I hope so.

Of course I had to take my calculations further and work out some figures on a spreadsheet, so here they are.

With a heart rate of 60bpm you will get to 1 billion beats a few months before your 32nd birthday, and you’ll get to 3 billion beats just after your 95th birthday. If you want to live that long with a heart rate of 80bpm you will need your heart to beat 4 billions times. A billion heartbeats is not enough for most of us.