How much weight do you lose the first month after gastric bypass? Your Questions Answered.

How much weight do you lose the first month after gastric bypass? Your Questions Answered. Bari Life

Gastric Bypass Weight Loss Calculator

How much weight will you lose?

Your Gender
How tall are you? (ft/in)
How much do you weigh? (lb)

Excess Weight (how much weight you could lose after gastric bypass surgery)

When anyone talks about losing weight they talk about how many pounds they want to lose, how many pounds they lost…etc. it’s in absolute numbers. Losing weight is great, but without context, the amount of weight loss doesn’t matter as much…

For instance: if someone loses 50lbs that sounds amazing, right? I mean, 50lbs is a lot of weight. But what if the person weighs 1,000lbs? In this case, 50lbs is a good start, but they still have a long way to go…

All this is to say that, in bariatrics, we want more context.

That’s why with bariatric weight loss the term that is used the most is Excess Weight Loss.

EWL is defined as your current body weight minus your “Ideal Body Weight.”

EWL provides context to the relationship between current weight and “ideal weight,” or your goal weight.

“Ideal Weight”

The widely accepted reference for “Ideal Weight” is the ideal weight chart from the Met Life Insurance Company. It was first created in 1943 and revised in 1983.

How much weight do you lose the first month after gastric bypass? Your Questions Answered. Bari Life

The chart provides an “Ideal Weight” for men and women at any given height. You’ll notice that there is a “small frame,” “medium frame” and “large frame.”

What this chart represents for bariatric patients is a clear body weight for your long-term goal weight post-op. If you start working towards this number you’ll be on the right track.

So now that we know what “Ideal Weight” means we can arrive at an accurate Excess Weight number.

Determining your excess weight

Let’s say for example that we have a female patient who is 5’3” and weighs 230lbs. According to the Met Life Ideal Body Weight Chart, her ideal weight is 128 lbs.

How much weight do you lose the first month after gastric bypass? Your Questions Answered. Bari Life

Excess Weight = Current Weight – Ideal Weight
Excess Weight = 230 lbs – 128lbs
TOTAL Excess Weight = 102lbs


If this patient lost the number for Excess Weight (102 lbs) then they have lost 100% of their excess weight. (EWL = 100%)

While bariatric surgery is extremely effective at reducing a patient’s Excess Weight, no procedure results in 100% EWL…

Expected excess weight loss after gastric bypass

Since each bariatric procedure is unique it stands to reason that the amount of Excess Weight Loss would differ across the procedures that are available.


As a guideline, here is the average % of EWL by procedure:

  • Roux-en-Y Gastric Bypass (RNY): 70% of excess weight will be lost
  • Vertical Sleeve Gastrectomy (VSG): 60% of excess weight will be lost
  • Duodenal Switch (DS): 85% of excess weight will be lost

What does this mean, exactly?

If we take the same example from above, the female bariatric patient who is 5’3” and 230lbs pre-gastric bypass surgery, she has Excess Weight of 102lbs.

Now we take that Excess Weight number and multiply it by the % of EWL for the procedure she is having.

So if she’s having a Roux-en-Y Gastric Bypass it works out like this:


102lbs Excess Weight X 70% EWL = 71.4lbs.

This patient can expect to lose 71-72lbs by having the gastric bypass surgery.


Gastric bypass weight loss chart

Most patients will see the majority of their EWL happen over the first 12 months after their bariatric surgery. After that point the focus shifts from weight loss to weight maintenance.

How much weight do you lose the first month after gastric bypass? Your Questions Answered. Bari Life
** Data points taken from Dr. Boyce’s practice **

As you can see the majority of EWL happens during these first 12 months and then levels out.

Just remember that this graph “smooths out” the ups and downs. In reality, weight loss is rarely perfectly linear. You’ll have periods of stalls and plateaus, but this is normal on your path to your goal weight and ONEDERLAND.

Keep your eye on the weight loss trend over the 12 months.

How much weight do you lose the first month after gastric bypass?

During the first month after gastric bypass surgery, most patients can expect to lose ~17% of their Excess Weight. (this is taken directly from Dr. Boyce’s practice and working with thousands of patients).

So let’s pull that same example from earlier:

Female gastric bypass patient who is 5’3” tall and 230 lbs pre-surgery.

If you recall her Excess Weight was calculated at 102 lbs.

By losing 17% of her excess weight during the first month she can expect to lose 17.34 lbs during the first month after her gastric bypass surgery!

How much weight do you lose the first month after gastric bypass? Your Questions Answered. Bari Life

The reason why you will lose so much weight during the first month is due to a few key factors:

  • You’re not hungry – you won’t feel like eating so your calories will naturally be low
  • You’re on a modified 4-Phase diet – The first phase is liquids only so you are in the most calorie-restricted phase of your post-op diet
  • Your motivation is high – Right after surgery, you’re excited about your new life!

We call this the “Honeymoon Phase”

Losing all of this weight immediately after surgery is awesome and it can give a real boost to morale and motivation; however, I urge you to stay focused on the big goal:

Sustained, lasting weight loss so you can live the healthy life you imagined for yourself.

It’s totally doable and it all comes down to committing to every phase of your gastric bypass diet.

Your gastric bypass diet (all phases)

You’ll find that during the bariatric journey your diet changes radically. Starting with your preparation for surgery all the way to when you reach your goal weight your diet is in a constant state of flux.

It’s important that you get each phase right so you can heal after surgery, lose weight and stay healthy in the long-term. Here’s the 10,000 foot view of the main phases:

  • Pre-op diet – During this phase, your goal is to reduce your liver and lose weight before surgery. It helps to reduce complications and set you up for what’s to come post-op
  • 4 Phase diet – immediately after surgery you’ll get started on the 4 Phase diet. This diet is designed to help you heal from your surgery while delivering the nutrition you need. You’ll start with liquids only and progress to real food.
  • Long-term stabilization diet – this is the diet you’ll be on after you’re healed. You’ll follow this diet until you reach your goal weight (whether that’s 3 months or 1 year, you’ll stay on it until you reach your goal weight).

We’ve covered these specific diets in EXTREME detail in the links above so whatever stage you’re in we can help guide you through to success!

Key Takeaways

If you have gastric bypass surgery you will lose weight and you’ll lose a lot during the first month after surgery.

Successful bariatric patients view this time as a kickstart – the major boost they needed to get the weight loss started. Then they take the reins from there and use the surgery to help carry them to their goal weight.

This article gives you good context about the short-term weight loss you can expect after gastric bypass surgery, but you need to keep the momentum going and hit your goal weight

This is an amazing journey and transformation you’re going through. Enjoy every phase and give it your all. If you need help along the way, you know where to find us!


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