Weight gain: How long does it take to put on weight?

Whether you've been trying to put on the pounds or are looking to lose weight, you've probably asked this question

A collage of a woman thinking about food
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How long does it take to gain weight? Whether you've been trying to put on pounds or your new years resolution is to reach a certain weight, the experts have the answer below.

How long it takes to gain weight differs from person to person and is dependant on multiple factors, in fact it's possible to put on weight as quickly as overnight - especially for women during their menstrual cycle. For some people, their goal is to gain weight in a healthy way, which is why they may choose to try reverse dieting (opens in new tab). But whether you're looking to bulk up or slim down, there are things you need to know about weight gain.

"Body weight is always fluctuating and it's important to know there are many different factors that contribute to the number we see on the scale. For example, it's common to see daily weight change by up to 5 to 6lb (2-3kg)," explains Daniel Carpenter (opens in new tab), senior technical trainer and researcher at Common Purpose Personal Training.

How long does it take to put on weight?

It is possible to put on weight in just one day, but it's more likely to be something else other than fat or muscle. In this case, looking to the scales is not the best way to find out whether you've gained weight or not.

"It is 100% possible to increase your weight in just 24 hours and put on 5-6 pounds." Sophie Wedlock-Smith (opens in new tab) from SW Nutrition says, "However, I would question whether this is fluid retention or inflammation as you would need to eat 3500 calories in food to put on just one pound."

If you're wondering why you can't lose weight (opens in new tab), for instance, it's unlikely to be anything to do with what you've eaten in the last week. The same applies for those looking to gain weight, our experts say, it's not going to happen overnight. A short period of eating more calories than your maintenance level may register as more of a slight glitch in your body, rather than having a major effect on your weight.

For example, in one study (opens in new tab) published in the American Journal of Physiology, eight men were put on a high-calorie diet for five days and then for 28 days. The five day period didn't have much of an effect on their weight at all and made no difference to their overall fat mass. However, after 28 days of eating more than they normally would, the men's fat mass increased by 3 pounds and their overall weight rose by 3.5 pounds.

What causes fast weight gain?

  • Water retention
  • The contents of your stomach and bladder
  • An undiagnosed autoimmune condition, such as hypothyroidism or a dysregulation of hormones
  • Prescription medication, such as steroids or antidepressants
  • Regular bodily processes, such as being on your period, falling pregnant (opens in new tab) or heightened cortisol production as a result of stress
  • Tiredness

So, if you feel like you've suddenly gained weight very quickly and the scales appear to confirm your suspicions, take a step back. There are "a multitude of possible factors that can contribute to weight gain", says nutritionist Sophie.

"One of the biggest contributors to daily weight fluctuations is water retention." Personal trainer Daniel adds, "Increased carbohydrate and salt intake can increase water retention. If you eat a carbohydrate rich meal (opens in new tab), for example, your body will store these carbohydrates as glycogen in your muscle and liver. Interestingly, with every 1g of glycogen stored, 3-4g of water will be bound up with it."

He adds, "Water retention also occurs when we eat a lot of salt, the body retains extra sodium, which increases the amount of fluid in the body."

"Another contributing factor to daily weight fluctuations is your stomach and bladder contents." Daniel says, "Try this; hop on a scale, then eat a big meal and drink a lot, then hop back on the scale then you’ll see what happens."

There are also various serious conditions that can cause weight gain, including an undiagnosed autoimmune condition, endometriosis, polycystic ovary syndrome (PCOS) and diabetes. "Women who suffer from PCOS are also much more likely to suffer from insulin resistance, and this can lead to issues such as type 2 diabetes," Personal trainer Kimberly Mitchell (opens in new tab) from Ori Gym says.

A close up of women's feet stood on scales

(Image credit: Getty Images)

But sometimes, it's the simplest thing that causes an apparent quick change in weight. For instance, the NHS lists tiredness as a reason why you could have gained weight seemingly overnight.

"Some studies (opens in new tab) have shown that people who sleep less are more likely to be overweight than those who sleep more." They explain, "It's not clear why, but one theory is that sleep-deprived people have reduced levels of leptin, the chemical that makes you feel full, and higher levels of ghrelin, the hunger-stimulating hormone."

How much do you need to eat to gain weight?

The general rule is that you need to eat more calories that you burn to gain weight, but it's a little bit more complicated than that.

"Rate of fat gain / loss will depend on your energy balance." Daniel says, "A common equation nutritionist statement is that 1lb of fat “costs” 3500kcal. So if you’re eating 500kcal a day more than your body is using, you will theoretically gain 1lb of fat a week (and visa versa for fat loss)."

"We often view fat loss as a % of body weight. Here’s some different rates of fat loss amongst men and women," He adds, offering the chart below as a representation.

Swipe to scroll horizontally
Progress% Body WeightMenWomen
Extreme1-1.5% body weight~2-3 lb~1.65-2.5 lb
Reasonable0.5-1% body weight~1-2 lb~0.8-1.65 lb
Comfortable<0.5% body weight~<1 lb~<0.8 lb”

"Now, this equation is highly simplified and it’s a bit more complicated than this in real life, but it gives you a good indication of the disconnect between weight gain (up to 5-6lb a day) and fat gain (around 1lb a week)."

Can you gain weight in a day?

Yes, it's very possible to gain weight in just one day. However, this is likely to be water retention, the contents of your bladder or stomach, or the consequence of another influencing factor changing the scales, rather than actual fat gain.

As Sophie and Daniel have described, for example, a woman's daily recommended calorie intake is 2000 calories. In order to gain any weight just overnight, you'd have to eat an additional 3500 calories on top of this and not burn any of it off.

So while the scales might change from one day to the next, you actually haven't gained pounds of fat overnight.

If you put on weight quickly, can you lose it quickly?

Unfortunately, there's not a definitive answer to how quickly you can lose weight if you're wondering how to lose weight (opens in new tab) fast. Much like the time it takes to gain weight, weight loss timeframes will differ for everyone. 

"Each body is different, and if you gain weight rapidly you may find it harder to lose it at the same rate, due to biological impacts such as insulin resistance or hyperthyroidism," Kimberly Mitchell says.

"A factor that may however affect the rate in which somebody loses or gains weight is the speed of which the initial weight was lost. It is widely known that losing weight through crash diets or at a rapid pace can promote faster weight gain once the dieting has come to an end."

PT Daniel adds, "As a rule of thumb weight can fluctuate rapidly (hours and days) but fat gain/loss happens relatively slowly (weeks and months)."

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Grace Walsh
Features Writer

Grace Walsh is a Features Writer for Goodto.com, covering breaking news health stories during the Covid-19 pandemic as well as lifestyle and entertainment topics.  She has worked in media since graduating from the University of Warwick in 2019 with a degree in Classical Civilisation and a year spent abroad in Italy. It was here that Grace caught the bug for journalism, after becoming involved in the university’s student newspaper and radio station.