Why the number on your bathroom scales changes so much from day to day.

Image: iStock.

Even if you think you have a good idea of how much you weigh, there’s no way to tell exactly what number will appear when you step on a set of bathroom scales.

Those digits can oscillate wildly with no obvious reason, which makes things incredibly frustrating (infuriating, even) if you’re trying to keep an eye on your weight. If it makes you feel any better, you’re not the only one whose scales seem to have a mind of their own.

Guardian writer Martin Robbins has also noticed this pattern and decided to investigate the possible reasons behind this variance by weighing himself every hour on the hour over a long weekend.

Robbins also weighed every gram of food and drink he consumed over the almost 90-hour period, measured the amount of urine passed, and noted down how much exercise he did, to determine how or whether these factors influenced his hourly weight fluctuations. (Post continues after gallery.)

Interestingly, while Robbins consumed 14.86kgs of food and drink, he was 1.86kg lighter by the end of the weekend that he was at the beginning. Of that sum figure, 9.2kg of consumed mass was lost through urine and bowel movements, and Robbins lost one kilogram of sweat over two running sessions, but he was perplexed by what happened to the rest of it.

“On average, I lost 69 grams every single hour that couldn’t be explained by anything I’d measured. Over the whole weekend, that added up to nearly six kilos of unexplained weight loss, 1.65kgs every 24 hours,” he writes.

In fact, much of the weight we lose from our bodies is ejected in the carbon dioxide we breathe out — Robbins estimates someone of his size with his level of exercise could breathe out up to a kilo of carbon over one day, which explains where that excess mass might have got to.

All of this is very interesting, but the main takeaway is this: the number on your bathroom scales is not a reliable indicator of your weight at a given moment, because there are so many variables at play.

"Weight measurements are like opinion polls – individual results don’t tell you anything because there’s just too much random noise, error and variation," Robbins concludes.

"On any given day my weight varied by about four pounds [around 1.8 kilos], with a dozen pounds passing in and out of the giant meat tube that is me at only vaguely predictable times... While I was generally lighter in the mornings and heavier after meals as you’d expect, my exact weight at any moment was really just a crap shoot."

Instead of weighing yourself once a week to track any progressive weight loss, maintenance or gain, he recommends hopping on the scales each morning, looking at the average over a seven to 14 day period, and then over time determining whether this average measurement is changing.

Do you measure yourself with scales? Do you find the number varies significantly each time?