Why your weight goes up and down every day
This article is general information, not medical advice. It is not a substitute for a doctor, dietitian or other qualified professional who knows your circumstances. If you have a medical condition, are pregnant, or are changing your diet or exercise significantly, talk to one first.
You eat sensibly all week, step on the scale and it is up by a kilogram. Almost always, that is not fat. Body weight swings by one to two kilograms (two to four pounds) from day to day for reasons that have nothing to do with fat gain or loss. Understanding them makes the scale far less stressful, and it is the reason the weight tracker draws a trend line rather than celebrating or mourning each weigh-in.
How much fluctuation is normal
For most adults, day-to-day changes of 0.5 to 2 kg are normal, and larger swings happen after holidays, travel, illness or a change in diet. To put that in perspective, gaining one kilogram of body fat requires eating roughly 7,700 kcal more than you burn, several days' worth of food for most people. A one-kilogram jump overnight is therefore almost entirely water and the contents of your digestive tract.
Water, salt and carbohydrate
About 50 to 60 per cent of body weight is water, and the amount you carry changes constantly. A salty meal makes the body hold extra water to keep sodium concentration steady, which can add half a kilogram or more by the next morning and disappear a day or two later. Carbohydrate has a similar effect: the body stores carbohydrate as glycogen in muscles and liver, and each gram of glycogen is stored with about three to four grams of water. That is why the first week of a low-carbohydrate diet often shows a dramatic drop that is mostly water, and why a pasta dinner can show up on the scale the next day. Dehydration works the other way, temporarily lowering weight.
Food, hormones and exercise
- Food and drink in transit. A day's meals and drinks can weigh one to two kilograms before they are digested and excreted; constipation adds to it.
- The menstrual cycle. Many people retain water in the days before a period, often adding one to two kilograms that goes once the period starts; the period tracker makes the pattern easy to see.
- Exercise. A hard or new workout causes muscle inflammation and repair that holds water for a few days, so weight can rise after you start training.
- Sleep, stress and illness change fluid balance and appetite.
- The scale itself. Home scales can differ by a few hundred grams depending on the floor and where you stand; use the same scale on a hard, level surface.
How to weigh yourself
The goal is to keep conditions the same so changes reflect your body rather than the circumstances. Weigh at the same time of day, ideally in the morning after using the toilet and before eating or drinking, in similar clothing or none. Daily weighing, or several times a week, gives the smoothest trend; weighing once a week risks catching a high or low day and drawing the wrong conclusion. If daily numbers make you anxious, keep weighing but only look at the trend.
Reading the trend
A moving average cuts through the noise. The weight tracker plots each weigh-in as a dot and draws a 7-day average line through them; it also fits a straight line to the last four weeks to estimate your rate of change in kilograms or pounds a week. If the line is falling slowly, your plan is working, whatever yesterday's number said. Judge progress over two to four weeks, not days, and remember that body-weight changes after a big shift in diet or training take a week or two to settle. How fast should you lose weight? What a safe, lasting rate looks like covers what a realistic rate looks like, and the BMI calculator puts your trend weight in context.
Sources and further reading
The claims in this guide rest on these references, which were checked when the guide was last updated. Spotted an error? The contact page says how to report it.