Cottage Q&A: Calculating swimming distance

Updated: June 25, 2019

By Kzenon/Shutterstock

We have a buoy in our lake that we like to swim to, and we’re curious how far a swim it is. Is there a way to calculate this distance? Can you do something other than use a long rope?—Billie Dominic, Huntsville, Ont.

Sure. There are apps and gadgetry—a golfer’s range finder, for example—that can tell you. But, “I’m a Luddite,” says Lynn Marshall, a Masters Swimming world-record holder and coach. “I don’t even own a cellphone. Personally, I would just swim to the marker and figure out how far it was from how long it took.” 

Of course! It’s a simple physics problem, solvable with the Distance Speed Time formula: Distance = Speed x Time. As long as you know those two variables—speed and time—you can calculate the distance.

First you need to figure out your average swimming speed. Time yourself at an indoor pool—the size of the pool is known, it’s usually marked, and it’s accurate. “In physics we say, ‘You need to calibrate your device,’ ” says Joanne O’Meara, a professor in the department of physics at Ontario’s University of Guelph. “This is how you’d calibrate your measurement.” 

Swim at your regular, steady pace. “When I’m doing lengths in a pool, I will usually do 50 lengths of 25 metres—so 1,250 metres—in about 40 minutes,” says O’Meara. That would make her average speed just over 30 metres per minute (1,250 metres divided by 40 minutes). 

To then find the distance to your buoy, she’d time herself swimming to it. “If it takes me five minutes to swim there, that means the buoy is roughly 150 metres away,” says O’Meara. 

For more accurate results, do several swims on a wind-free day, and calculate the average time. Is this method as precise as a golf range finder? Of course not. But it won’t cost you any money, and it’s probably more fun.

Got a question for Cottage Life’s Cottage Q&A? Send it to answers@cottagelife.com.

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