Why your weight goes up and down every day

Step on the scale two mornings running and it can easily differ by a kilogram or more, without any real change in body fat. Here's what moves the number, and how a trend shows the direction you're really heading.

What makes the scale jump

• Water and salt. A salty meal makes your body hold on to more water for a day or two.
• Carbohydrate. Your body stores carbohydrate with water, so a carb-heavy day can show up on the scale the next morning, and a low-carb day the opposite.
• Food and drink still on board. What you ate and drank yesterday weighs something until it's digested.
• Timing and clothes. You're lighter first thing in the morning than in the evening.
• Hormones. Many women retain water at certain points in their menstrual cycle.
• Exercise. Hard training can make muscles hold extra water while they recover.

None of these is fat gained or lost. But they're big enough to hide a real change of half a kilogram a week, or to make a good week look like a bad one.

What Trend Weight is

Trend Weight is a smoothed version of your Weigh-ins. Each day it moves 10% of the way from yesterday's trend toward today's Weigh-in: one surprising reading barely moves it, but a real change, day after day, steadily does. It's the method John Walker popularised in The Hacker's Diet.

Here are two weeks of Weigh-ins. Day to day they jump by up to 1.2 kg and look like they're going nowhere; the trend shows a steady drift down.

Over the fortnight the trend falls from 85.0 to 84.6 kg. Because it moves only part of the way each day, it lags a little behind a real change, so it's best read over weeks, not days.

How to weigh in for the clearest trend

• Weigh in first thing in the morning, after the loo and before eating or drinking.
• Use the same scale, on a hard, level floor, in similar clothes (or none).
• Weigh in often. Every day gives the steadiest trend, but missing a day is fine: the trend simply moves further when you're back.
• Expect the odd high reading after a salty meal or a big night, and let the trend absorb it.

How Progressaurus uses Trend Weight

Weigh-ins and Trend Weight are free in Progressaurus, and Trend Weight is the only weight figure it uses for goals and guidance, so one surprising reading stays in perspective.

• Weight Goals (Premium). To lose or gain, choose a Pace of 0.25%, 0.5% or 0.75% of your body weight a week, and see a Projected Goal Date. To maintain, choose a Tolerance either side of your weight, ±2 kg unless you change it.
• Private by design. If you share your weight with an Accountability Partner, they see only the direction of your Trend Weight against your Pace, never a figure.
• Steps, food and habits alongside. Track only weight, or add whatever else helps.