How often should you weigh yourself?

The short answer#

Daily, at the same time, and then ignore every individual number.

That sounds contradictory and it is not. More weigh-ins make the underlying trend clearer, because the noise averages out faster. Fewer weigh-ins make each one matter more, which is exactly the wrong way round — a single morning is the least reliable number in the whole system.

If daily weighing makes you miserable, weigh less often. A trend built from three mornings a week is still a trend. A habit you have abandoned is not.

What a scale actually measures#

Not fat. Everything: bone, muscle, organs, blood, the water in all of it, the contents of your gut and bladder, and whatever you are wearing.

Most of what moves day to day is water. A few things that shift it by a kilogram or more, none of which are fat:

Salt. Sodium holds water. A takeaway dinner can show up as a kilogram the next morning and be gone two days later.
Carbohydrate. Glycogen is stored with water attached — roughly three grams of water per gram of glycogen. This is why the first week of a low-carb diet produces a dramatic number and why it stops.
What has not yet left you. Food and drink in transit weigh what they weigh.
Hormonal cycle. Water retention across a menstrual cycle routinely moves the scale by one to two kilograms, on a schedule that has nothing to do with what you ate.
Hard exercise. Muscle repair holds water. A first session back can add weight for several days.

A kilogram of body fat is about 7700 kcal. To actually gain one in a day you would have to eat 7700 kcal above maintenance in that day. The number on the scale can move that far by lunchtime, which tells you what it is mostly made of.

Why the average is the real reading#

Take a run of weigh-ins and draw a line through them and the noise cancels. Take two and subtract, and you have kept all the noise and thrown away the signal.

SmartCrunch draws a 7-day trailing moving average through your measurements, and everything downstream reads that line rather than the individual dots: the trend, the forecast date, and the maintenance figure the whole budget hangs off.

The same reasoning shows up as a hard threshold elsewhere in the app. It will not solve your maintenance from your own data until your weigh-ins span at least a fortnight, because a shorter series has no slope that can be told apart from ordinary water movement — and it fits that slope by least-squares across every reading rather than by subtracting the ends, precisely because two points inherit their own noise.

How to weigh yourself so the number means something#

Consistency beats precision. The scale does not need to be accurate in absolute terms — it needs to be wrong in the same direction every time, so the differences are real.

Morning, after the loo, before eating or drinking. This is the most repeatable state a body offers.
Same amount of clothing, or none.
Same scale, same spot on the floor. Carpet gives different readings than tile. Do not compare gym scales to home scales.
Log it and move on. Do not weigh again later “to check”. The second reading is not a correction, it is a different point in a noisy day.

How to read what you get#

Give it a fortnight before drawing any conclusion. Then look at the line, not the last dot.

The line is going down at roughly the rate you intended. Nothing to do.
The line is flat for two full weeks and you are logging honestly. Your maintenance is probably not where you think it is. This is the situation the app’s adaptive method exists for: it can solve your actual maintenance backwards from the same two series you already have.
The line jumped and stayed. Look for the ordinary explanation first — a salty weekend, a new training block, a cycle. Two weeks will separate a step change from a trend.
Individual dots are jagged but the line is steady. That is what normal looks like. Nothing is wrong.

When to weigh less, or not at all#

This is worth saying plainly: for some people a daily number is not a neutral measurement, and the honest advice is to stop. If weighing sets the tone of your day, if you find yourself weighing repeatedly, or if you have a history of disordered eating, weigh weekly or not at all and judge by how clothes fit and how training goes. An app cannot tell the difference between a useful habit and a harmful one, and this one does not try. The health disclaimer covers the rest.