Why your scale weight goes up and down

Day-to-day changes on the scale are mostly water and food, not fat. Muscle stores about 3 g of water with every gram of glycogen (stored carbohydrate), salty meals can make you hold extra fluid, and what's still in your gut adds weight until it's digested. Weight also follows a weekly rhythm: one study of 1,421 adults found it peaks on Monday and is lowest on Friday. The useful number is the trend over a few weeks.

Carbs and glycogen

Your muscles store carbohydrate as glycogen, and they store water with it. In a 2015 study, muscle held about 3 g of water for each gram of glycogen it rebuilt after exercise when fluid was limited. So a day of more carbs than usual (pasta night, a birthday, a carb-up before a comp) refills glycogen and the water that goes with it. A low-carb week does the opposite, which is part of why the first week of a low-carb diet can show a quick drop that then slows.

Fernández-Elías VE et al. Relationship between muscle water and glycogen recovery after prolonged exercise in the heat. Eur J Appl Physiol 2015;115(9):1919-26. PMID 25911631.

Salt

Sodium affects how much fluid your body holds. The Better Health Channel lists oedema (fluid retention) among the problems linked to too much sodium, and controlled feeding studies have measured more extracellular fluid when people eat more of it. The amount varies a lot between people. For reference, healthdirect suggests adults keep sodium under 2,000 mg a day, about 5 g of salt. Takeaway, soy sauce, stock and processed meat are the usual reasons a day goes well over that.

Better Health Channel, Salt (betterhealth.vic.gov.au); healthdirect, Salt (healthdirect.gov.au); Heer M et al. Br J Nutr 2009;101(9):1286-94. PMID 19173770.

The weekly pattern

Most people eat and drink differently on weekends, and it shows. In a 2020 study of 1,421 adults across Europe, weight moved about 0.35% across the week: highest on Monday, then Sunday, falling through the week to a low on Friday. For an 80 kg person that's about 0.3 kg between Monday and Friday with no real change underneath. A Finnish study that tracked 4,657 daily weigh-ins found weight starts rising on Saturday and starts falling on Tuesday, and the people losing or holding weight were the ones who brought it back down during the week.

Turicchi J et al. Weekly, seasonal and holiday body weight fluctuation patterns among individuals engaged in a European multi-centre behavioural weight loss maintenance intervention. PLoS One 2020;15(4):e0232152. Orsama AL et al. Weight rhythms: weight increases during weekends and decreases during weekdays. Obes Facts 2014;7(1):36-47.

The menstrual cycle

Fluid retention changes across the cycle. A year-long Canadian study of 62 women and 765 cycles found self-rated fluid retention was lowest in the middle of the follicular phase, rose around ovulation, and peaked on the first day of the period. If your weight jumps at the same point each month, compare it with the same point last month rather than with last week.

White CP et al. Fluid retention over the menstrual cycle: 1-year data from the prospective ovulation cohort. Obstet Gynecol Int 2011;2011:138451. PMID 21845193.

Creatine and new training

Starting creatine usually adds a little weight. The International Society of Sports Nutrition says loading causes short-term fluid retention of about 0.5 to 1.0 L, in line with the weight gain people see. That's water in the muscle, and it's expected.

Kreider RB et al. International Society of Sports Nutrition position stand: safety and efficacy of creatine supplementation in exercise, sport, and medicine. J Int Soc Sports Nutr 2017;14:18. PMID 28615996.

How to read the scale

Weigh at the same time and in the same way: first thing in the morning, after the toilet, before food or drink. Then look at the direction over two to four weeks, not at one reading. A weekly average smooths out the Monday bump and the salty-dinner days. If the average is moving the way you want, a heavy morning doesn't matter. If it's been flat for a few weeks, that's the time to look at your intake, with your maintenance kJ as a starting point.

KiloMate charts every weigh-in on its body screen and shows your change over the period on the dashboard, next to the kJ you ate and burned. That makes it easier to see a salty weekend for what it is.

If the scale is making you anxious, or you find yourself eating much less after a heavy reading, weigh less often and talk to your GP. The Butterfly Foundation also runs a free national helpline for anyone worried about eating or body image.

Questions

Why do I weigh more the day after a big meal?

Mostly food and fluid that's still in your gut, plus the water your muscles store along with carbohydrate. Muscle holds about 3 g of water with each gram of glycogen it stores, so a high-carb, salty meal can show up on the scale the next morning without much change in body fat.

Why am I heavier on Mondays?

That's a common pattern. A 2020 study of 1,421 adults found weight was highest on Monday and lowest on Friday, moving about 0.35% across the week. A smaller Finnish study saw the same thing: weight starts rising on Saturday and starts falling again on Tuesday.

Does salt make you retain water?

It can. Australia's Better Health Channel lists fluid retention among the effects of too much sodium, and controlled studies show the body holds more extracellular fluid when sodium intake goes up. How much varies between people, so there's no reliable kg figure to put on it.

Does creatine make you gain weight?

Usually a little, at first. The International Society of Sports Nutrition's position stand says creatine loading causes short-term fluid retention of about 0.5 to 1.0 L, roughly in line with the weight gain people see, and that weight gain is the only side effect consistently reported.

How often should I weigh myself?

There's no single right answer. A review of 17 studies found regular self-weighing went with more weight loss and wasn't linked to worse psychological outcomes. If you do weigh daily, compare weekly averages rather than single days. If weighing makes you anxious, weigh less often or not at all.

Sources

All read on 2 Nov 2026: Fernández-Elías et al. 2015 (Eur J Appl Physiol, PMID 25911631); Turicchi et al. 2020 (PLoS One, PMID 32353079); Orsama et al. 2014 (Obes Facts, PMID 24504358); White et al. 2011 (Obstet Gynecol Int, PMID 21845193); Heer et al. 2009 (Br J Nutr, PMID 19173770); Kreider et al. 2017 (J Int Soc Sports Nutr, PMID 28615996); Zheng et al. 2015, Self-weighing in weight management: a systematic literature review (Obesity, PMID 25521523); Better Health Channel and healthdirect pages on salt. The 80 kg example is our own arithmetic from the 0.35% figure.

General information only, not medical advice. See your GP about sudden or unexplained weight change, or swelling.