Why your daily weight fluctuates
Most of the daily fluctuation is water. Glycogen, the carbohydrate store in muscle and liver, is stored with about three to four parts water (Kreitzman and colleagues 1992). An evening with a lot of pasta fills the stores and shows up on the scale the next morning. Salt also binds water, a full gut weighs more than an empty one, and after hard training a worked muscle holds fluid for repair.
Fat, on the other hand, changes slowly. One kilo of fat takes a surplus of several thousand kilocalories. A jump of one kilo overnight is therefore practically never fat.
Weighing correctly
The more similar the conditions, the less noise. For a trend weight, daily measurements are better than weekly ones, because more values make the smoothing more stable. In a study by Steinberg and colleagues (2015), daily weighing was also associated with more weight loss. If you find weighing stressful, look only at the trend and ignore the daily value.
- In the morning after using the toilet, before eating and drinking
- Always the same scale on the same firm surface
- With as little clothing as possible, the same every day
- Log every value, including the unexpectedly high ones
What a trend weight is
A trend weight is a smoothed curve of your measurements. The simplest form is the moving average over 7 or 14 days. Also widespread is the exponential smoothing that John Walker describes in The Hacker's Diet: each new value moves the trend by only a small share, and older values gradually lose weight.
Every kind of smoothing has the same price: it lags behind a real change. Stronger smoothing means a calmer line that reacts later; weaker smoothing reacts faster and jitters more. For bulking and cutting, where you judge changes over weeks, this delay is not a problem.
Typical outliers
Some jumps repeat and can be predicted. A salty restaurant meal, a day with lots of carbohydrates, a hard leg session, starting creatine, a trip or, for many women, the menstrual cycle push your weight up for a few days without anything changing in your body fat.
Log such days anyway. A well-smoothed trend absorbs single outliers; leaving them out is more likely to distort the curve. A note on the day helps, so that you know later where a spike came from.
Reading the pace
The most important number is the change of the trend per week, ideally as a percentage of your body weight. That lets you compare goals regardless of weight: for a lean bulk, the literature gives about 0.25 to 0.5 percent per week, for a cut about 0.5 to 1 percent (more on this in the guides to building muscle and dieting).
Judge the pace over two to three weeks, not over one day. Only when the trend clearly deviates from the goal over that period do you adjust calories.
More than the scale
Weight does not tell you whether you are losing or building fat or muscle. So add waist circumference, photos and your strength numbers. A rough estimate of body fat percentage comes from the US Navy circumference method (Hodgdon and Beckett 1984), which only needs a tape measure. BMI is only of limited use for muscular people, because it does not distinguish between muscle and fat.
Tools
With the weight trend calculator you enter your values and see the trend, weekly rate and curve; your data stays in your browser. If you track daily, you can also have the trend weight kept automatically, for example with the Lean is law app.