Short answer: yes, pooping lowers the number on the scale, usually by 100 to 250 g and occasionally by 0.5 kg. No, it is not weight loss. You are weighing what you ate a day or two ago on its way out. The reason it matters is not the amount. It is that the effect is invisible unless you write it down, so it makes two otherwise identical mornings look different.
How much a bowel movement weighs
Stool is mostly water (around three quarters), plus bacteria, undigested fibre and some fat and protein. A systematic review of faecal output (Rose et al., 2015, Critical Reviews in Environmental Science and Technology) put the median for adults in high-income countries at about 128 g per person per day, with higher figures on high-fibre diets. A urine void, which most people have before the morning scale anyway, adds another 0.2 to 0.4 kg.
So, depending on what you ate, the toilet can account for most of a typical day-to-day swing on the morning scale. My own mornings move 0.66 kg on average (the data is here), and a bowel movement explains a good part of that on any given day.
Why I log it with every weigh-in
Every evening, my weight prediction model guesses my fasted weight for the next morning. It has no way of knowing whether I will go to the toilet before I weigh. On 23 October it predicted 87.86 kg and I weighed 87.0 kg, a 0.86 kg miss. Part of that was a bowel movement and a long ruck the day before, and the model had no way to see either.
Since 25 October, every weigh-in carries a yes or no: was there a bowel movement since the last weigh-in? The model uses it in two ways:
- When grading a prediction. If I went before weighing, the log shows the error with and without that factor. The raw miss stays on the record, so the model cannot quietly flatter itself.
- When building the trend. A reading taken after a movement is adjusted upwards before it enters the 10-day trend. Otherwise one lighter morning would drag the next prediction down.
The size of that adjustment started at 0.3 kg, a textbook-sized movement. It is refitted after every scored morning from my own data, and currently sits at 0.26 kg after 5 scored mornings. That is still far too few to read anything into. The live value is in the coefficient table on the log.
What to do with this
- Pick one routine and keep it. Weigh after the toilet if your mornings make that natural. If not, weigh before, every time.
- Note the exceptions. A day without a movement, or two in one morning, explains a 0.3 kg surprise better than any theory about water.
- Do not chase it. Fibre, coffee and laxatives change the timing, not your body fat. A lower number from an earlier toilet visit is not progress.
Frequently Asked Questions
How much weight do you lose when you poop?
Usually 100 to 250 g, the weight of the stool itself. A review of studies on faecal output found a median of about 128 g per person per day, higher on high-fibre diets. A large or delayed movement can reach 0.5 kg.
Does pooping make you lose weight?
It lowers the number on the scale, but it is not weight loss. Stool is undigested food, fibre, bacteria and water. The calories you absorbed were absorbed already, and no fat leaves with it.
Should I weigh myself before or after pooping?
After, if your mornings allow it, and always the same way. Consistency matters more than which choice you make. If you sometimes go before and sometimes after, note it, because it explains a 0.2 to 0.4 kg difference between two mornings.
Why do I weigh more when I am constipated?
Because a day or two of stool is still inside you, plus the water it holds. It can add 0.5 kg or more and disappears as soon as things move.
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