How to weigh yourself consistently and read the trend

Set up once, then leave it alone

• Use one scale. If you weigh on one scale this week and another next week, you can't tell whether a difference came from you or from the scale.
• Put it on a hard, flat floor. The WHO STEPS manual puts the scale on a firm, flat surface and says not to use carpet, a sloping surface or a rough, uneven one. Pick a spot and leave the scale there.
• Zero it first. In the WHO procedure the scale is switched on and the person waits until the display shows 0.0 before stepping on.
• Pick your unit. Pounds or kilograms, and stay with it, so you never compare a number in one unit with a number in the other.

Stand and read it the same way

Both survey manuals treat the weigh-in as a procedure with fixed steps:

• WHO STEPS: footwear and socks off, heavy belts off, pockets emptied of phones, wallets and coins. Step on with one foot on each side of the scale, stand still, face forward, keep your arms at your sides, and wait until you're told to step off.
• CDC NHANES: stand in the center of the platform with your hands at your sides, looking straight ahead, and take the reading once the display has become stable. NHANES treats a weight taken with shoes on as invalid.

At home, that means shoes off, pockets empty, standing still in the middle of the platform, and waiting for the number to settle before you read it.

Read a trend, not a reading

Here's how to read a series of weigh-ins so that no single day decides anything.

1. Collect several weeks of readings. One reading a day, or a few a week at the same time, for several weeks. A gap is fine. Leave it empty rather than filling it with a guess.
2. Look at all of them at once. A chart of the readings in date order shows the direction better than any single number.
3. Average each week. Add up that week's readings and divide by how many there are. Weekly averages smooth out the day-to-day spread, so you can compare one week with the next.
4. Notice the spread. How far do single readings sit from their weekly average? A change smaller than your usual spread is within the noise of your own readings, so it's not a new direction by itself.

Here's the arithmetic with made-up numbers. Week 1 readings: 180.2, 181.0 and 179.8 lb, which average 180.3 lb. Week 2 readings: 180.6, 179.9 and 180.8 lb, which average 180.4 lb. Within a week the readings are up to 1.2 lb apart, while the two weekly averages are 0.1 lb apart. The weekly averages show a flat series, while two single days picked from different weeks could be more than a pound apart.

A trend shows you what your numbers did. It doesn't say why, and whether a change matters is a question for your clinician.

If a reading looks unexpected, check your conditions first: the floor, the clothing, the time, the scale. Log what you saw and note what was different. Don't throw a reading away because you didn't like it.