Before you begin

A few quick notes will make these guides easier to use.

• Use a desktop or laptop when possible: These guides often link out to calculators and supporting references. A larger screen makes it much easier to move between tabs and compare information.
• Use the linked tools instead of doing manual math: Interactive calculators are linked wherever they can save time. The equations are sometimes shown for reference, but you usually do not need to calculate them yourself.
• Treat the guide as a framework, not a rulebook: Use the guidance as a strong starting point, then adjust based on your body, recovery, and training response instead of following it rigidly.

1. Understanding Underweight

Whether you’re aiming for a lean, athletic physique or the sheer size of a pro bodybuilder, your success ultimately hinges on a few fundamental principles. Mastering them is the difference between feeling lost and making consistent, tangible progress. It ensures every ounce of effort you pour into your training actually yields the results you’re after.

It all boils down to deliberately managing three key factors: the fuel you consume (calories and macros), your overall mass (body weight), and what that mass is made of (body fat percentage).

What the Scale Doesn't Tell You

It’s easy to get fixated on the number on the scale, but that figure doesn't tell the whole story. Your body isn't one solid mass; it’s a composition of fat mass and lean mass. Fat mass is exactly what it sounds like, while lean mass is everything else—muscle, bones, water, and organs. Knowing both your body weight and your body fat percentage allows you to understand this ratio, giving you a crystal-clear picture of your starting point and the quality of the progress you're making.

What's the Goal?

Your objective will dictate your entire approach, so it’s crucial to know exactly what you’re aiming for.

• Cutting is all about lowering your body fat while fighting to preserve as much hard-earned muscle as possible.
• Body Recomposition is the process of building muscle and losing fat at the same time.
• A Lean Bulk focuses on maximizing muscle gain while accepting a minimal, controlled increase in body fat.
• An Underweight Bulk is a more aggressive approach for those who need to gain weight to reach a healthy range.

The Problem with Most Body Fat Measurements

Since your body fat percentage is such a critical metric, you need an accurate way to measure it. The truth is, most methods are wildly inconsistent.

First, you need an accurate body fat measurement.

The easiest way is to find a place with a Multi Segment Body Composition Analysis machine, like an InBody 970, 700,580, 380s, 270 or 260. It's accessible, cheaper than a DEXA scan, and accurate enough for tracking.

Don't worry it doesn't cost much (few dollars), many gyms offer it for free. Make sure it's a professional grade inbody machine, not consumer grade BCA anaylzers which tend to be highly inaccurate. (unfortunately some gyms install them)

A word of warning: don't trust consumer grade smartwatches and scales. While they're convenient, their body fat estimations are often unreliable and can lead you to make the wrong decisions. The professional equipment costs thousands of dollars for a reason that's a level of technology you simply won't find in a sub $1000 gadget. Finding a place for a real scan is easier than you think; a quick Google search for "body composition analysis near me" will likely point you to a local gym or clinic that offers InBody scans for a small fee.

For body fat % measurement, in terms of accuracy: MRI > DEXA > Hydrostatic Weighing > Bod Pod > InBody Multi Freq Device (specially the ones which also use Mhz freq, eg, InBody 970, 700) > InBody Multi Freq Device (no Mhz freq, eg, InBody 260,270, 280) > Other Multi-Frequency BIA Devices> Calipers (Skinfold, highly depends on operator skill) > Ultrasound > US Navy Formula > Single-Frequency BIA Devices.

Visual estimates, even by professionals, are significantly less accurate than DEXA scans, no matter what some forum discussions might claim. That said, for contest or photoshoot preparation, visual assessments for conditioning are a practical method. In such cases, it's beneficial to seek guidance from a coach's trained eye for a more reliable evaluation.

For most people, not pro bodybuilders/atheletes, even inbody 260,270,280 are decent place to start and widely available but yes if you've access to a better device use that.

Fueling the Machine: Calories and Macros

Your body's transformation is fueled by what you eat. Calories are simply energy—eat more than you burn, and you’ll gain weight; eat less, and you’ll lose it. But the type of calories you eat determines the quality of that change. These are your macros:

• Protein is the brick and mortar for building muscle. Without enough of it, your body simply can't repair and grow, no matter how hard you train.
• Fats are non-negotiable. Healthy fats are essential for regulating the hormones that drive muscle growth and recovery, not to mention keeping your joints healthy. Skimping on them will sabotage your progress.
• Carbohydrates are your body's go-to energy source, especially for powering you through high-intensity workouts.

Who is Underweight?

You are classified as underweight if your BMI < 18.5. Calculate your BMI here

Long Term Risks of Being Underweight

Maintaining a BMI below 18.5 over the long term can carry several health risks, especially if nutritional intake is inadequate. These risks are well documented in scientific literature:

Bone Health Issues

• Higher risk of osteoporosis and fractures due to reduced bone mineral density.
• Low body fat can disrupt hormones like estrogen, especially in women.
• Source: Feskanich et al., "Calcium, vitamin D, milk consumption, and hip fractures", JAMA, 1997

Mental Health Concerns

• Greater risk of depression, anxiety, and in some cases, cognitive decline.
• Could be linked to nutritional deficits and psychological stress.
• Source: McCrea et al., "Underweight and mental health disorders in the general population", Psychological Medicine, 2012

Weakened Immunity

• Chronic undernutrition impairs immune function, increasing susceptibility to infections.
• Source: Chandra RK, "Nutrition and the immune system", American Journal of Clinical Nutrition, 1996

Muscle Wasting

• Loss of lean muscle mass and reduced strength, especially in aging adults.
• May lead to fatigue, physical weakness, and frailty.

Hormonal and Fertility Issues

• Women may experience amenorrhea (loss of periods), and men may have lower testosterone.
• Both sexes can face fertility challenges.
• Source: Loucks et al., "Energy availability and reproductive function in women", Medicine & Science in Sports & Exercise, 2003

Increased Mortality Risk

• Epidemiological studies show a U shaped curve: both low and high BMI are associated with higher death risk.
• Source: Prospective Studies Collaboration, "Body mass index and cause specific mortality in 900,000 adults", The Lancet, 2009

Rule Out Medical Causes First

Before proceeding, it's important to understand why you may be underweight. In many cases, addressing the root cause can significantly improve your progress.

Some common medical reasons for being underweight include:

• Hyperthyroidism
• Celiac disease or other malabsorption issues
• Chronic infections or inflammatory diseases
• Eating disorders
• Depression or anxiety

If any of these apply to you, consult a healthcare provider before starting this guide. Treating the underlying issue first often leads to faster and more sustainable results.

Who Should Follow This Guide?

This guide is meant only for individuals who are underweight.

Use this guide only if you are underweight. If you are not underweight, refer to this tool instead:

Bulk or Cut Decision tool: Fitness Strategy Planner

Important: Following the underweight guide when you are not actually underweight can lead to excessive fat gain, poor body composition, and slower long term progress. Make sure you’ve correctly identified your current status before proceeding.

2. Calorie Needs and Surplus Calculation

Your BMR (Basal Metabolic Rate) is the amount of energy your body burns just to stay alive at rest. Your TDEE (Total Daily Energy Expenditure) includes your BMR plus all the calories you burn from daily life and exercise. While most online calculators give you an estimate, the only way to find your true TDEE is by tracking your food intake against your weight changes over a few weeks. For a deeper dive, see this accurate TDEE calculator.

After step2, your TDEE will be dead on center, not adjustment needed

TDEE is the maintenance calories, if you eat at matenance your weight will not change if you average it over 3 weeks which should take out water weight fluctations.

If you eat more than maintenance, you gain weight

if you eat less than maintenance, you lose weight.

Weight gain or lost will be some part fat, some muscle, depending on training/diet

Just use this calculator (it's the most accurate): TDEE Calculator

It will give you your Total Daily Energy Expenditure (TDEE).

Select a Surplus

Use Energy Phase Selector to figure out what Surplus % you need.

Info given below is for reference only, you can skip to next step.

Recommended Calorie Surplus by BMI:

Your ideal calorie surplus depends on your BMI and activity level. The Energy Phase Selector will calculate the right surplus based on your height, weight, lean mass, BMI, and activity level.

⚠️ Increase your surplus gradually to avoid digestive or metabolic distress. Don’t jump to your target surplus on day 1. For example, if your target surplus is 30%, start at 10% for a few days, then move to 20%, and finally reach 30% over a couple of weeks.

Write it down: Example: If Alex has a BMI of 16, he should eat at a surplus of 35% or more based on the table above. Now write down your target surplus from the table.

We’ll use this in the next step in the Energy Macro Planner.

3. Macronutrient Setup

If you have a low appetite, try drinking your calories through nutrient-dense shakes. Focus on incorporating healthy fats, which provide 9 kcal per gram—more than double the energy from carbs and proteins, which offer just 4 kcal per gram.

There’s no need to do all this math manually. The Energy Macro Planner will do the heavy lifting for you—just select the right surplus based on the info above, plug it in, and it'll calculate your exact protein, fat, and carb targets. If you're curious about the numbers behind that calculation, the details are below for you to read. Otherwise, feel free to skip ahead to the next step.

Protein

• 1.6-2.2 grams of protein per kilogram of lean body mass (LBM) per day This is overidden if protein falls below 15% of total calories consumed. Why? underweight people have low Body Weight so they might not get enough protein if we base protein intake on that alone.
• Most accurate estimate for underweight or recovering individuals
• Prevents underestimation caused by low fat mass when using total body weight
• Supported by: Morton et al., 2018 (Br J Sports Med); Schoenfeld et al., 2018 (J Int Soc Sports Nutr) - This range is optimal for maximizing muscle protein synthesis and hypertrophy, which is critical for underweight individuals aiming to gain muscle and overall mass.

Fat

• 1.0-1.5 grams of fat per kilogram of total body weight per day
• Provides energy dense calories to support weight gain and hormone function
• Recommended for underweight or recovering individuals to promote hormonal balance and overall health
• Supported by: ESPEN guidelines (2017) and general nutritional recommendations for weight gain emphasize the importance of energy dense foods. Higher fat intake (e.g., 1.0-1.5 g/kg total body weight, falling within broad bulking recommendations like Helms et al., 2019 [0.5-1.5 g/kg]) effectively contributes to the significant caloric surplus needed for overall weight restoration, ensuring essential fatty acid intake and supporting hormonal balance in underweight individuals. This approach prioritizes total mass gain over lean body composition optimization.

Carbohydrates

• Fill the rest of your calories with carbs
• Helps fuel workouts and recovery

4. Meal Plan Generation

Once you’ve got your calories and macros from the Energy Macro Planner, use the Meal Plan Planner to generate your personalized 7-day meal plan.

5. Training Guidelines

Before you start any program

Hypertrophy just means muscle growth. You'll find plenty of programs but first you must learn what really matters in a program. Read Hypertrophy Blueprint, it contains all the knowledge you need to grow muscles. After reading this, you'll be able to make changes to any program.

Are you a beginner?

When you're new to fitness, the best thing you can do is follow a program built by an experienced coach. It's tempting to try and design your own routine, but you're unlikely to come up with something more effective. A solid plan is much more than just a list of exercises; it’s a careful balance of volume, intensity, and smart progression that takes expertise to get right.

If you’re working out from home or have limited equipment, this free beginner program list is an excellent place to start. It’s flexible and offers options based on the gear you have, including a full bodyweight program if you have no equipment at all.

So, what kind of schedule works best for beginners? A full-body routine three times a week or an upper/lower split four times a week are your best bets. As a novice, you get better results by training each muscle group at least twice a week, which helps maximize your body's muscle-building response. This is supported by the study "Effects of resistance training frequency on measures of muscle hypertrophy, a systematic review and meta analysis" by Schoenfeld et al., published in Sports Medicine, 2019. This approach also lets you practice the main lifts more often to build a solid foundation. If you can train four days a week, an upper/lower split is a fantastic choice because it hits every muscle twice while giving you plenty of time to recover and grow.

Ready to hit a fully-equipped gym? Here are a few great starting points:

Beginner Optimal Hypetrophy Program

If you're a late beginner or intermediate, proceed to Intermediate Programs

Resistance Training (Essential)

Underweight individuals should train with a moderate level of effort to build muscle without overtaxing their limited energy reserves. Avoid training to absolute failure or high volume in the beginning, as recovery capacity and nutritional reserves are typically lower.

• Frequency: 3 sessions per week to allow for full-body recovery
• Intensity: Keep 2–3 reps in reserve (RIR) on most sets. That means you stop 2–3 reps before failure

Research shows training 2–3 RIR still results in significant hypertrophy while reducing recovery burden (Schoenfeld, 2016; Helms et al., 2018). This makes it especially suitable for underweight individuals who may have reduced energy availability.

Zone 2 Cardio (Optional)

• 1-2x/week, 20-30 minutes per session
• Improves cardiovascular fitness, recovery, and metabolic health
• Does not interfere with strength or appetite when kept low in intensity and duration
• Avoid high intensity or long duration cardio, which can increase energy needs and suppress appetite

For a practical explanation of Zone 2 and how to implement it, see: Zone 2 Cardio

(Sources: Venables et al., 2005; Schumann et al., 2018; Wilson et al., 2012)

6. Troubleshooting

Not gaining weight

• Increase intake by another 100-200 kcal (or raise surplus to +30-35%)
• Add calorie dense, low volume foods like oils, nut butters, trail mix
• Use liquid meals/shakes to reduce appetite burden

Low appetite

• Avoid excess fiber and raw vegetables
• Split food into 4–6 smaller meals/day
• Add calories to meals with toppings (cheese, ghee, nuts)

Poor recovery

• Ensure 7-9 hrs of sleep per night
• Reduce training volume if excessively sore
• Verify protein and fat intake are sufficient

7. Progress Tracking

How do I know if it's working?

See: Progress Tracking

8. When to Stop Following This Guide

You should stop following this guide once your BMI reaches or exceeds 18.5, which means you are no longer classified as underweight.

What to Do Next:

• Once BMI ≥ 18.5, do not continue with the same calorie surplus and diet plan, as it may lead to unnecessary fat gain and poor body composition. You can use this calculator to calculate your BMI
• Instead, switch to this guide to decide your next step: Bulk or Cut or Body Recomposition Decision tool: Fitness Strategy Planner

Warning: Continuing this guide after reaching a healthy BMI can lead to excessive fat gain, which will make future cutting phases longer and harder. Transition appropriately.

9. Where to go from where?

Once you are Normal BMI, follow Body Recomposition guide. This will lean you out slowly, while adding more muscules to your frame. So, you'll look lean and muscular. Once you hit 10% bodyfat, you can always go for Lean Bulk to add more muscles to your frame slowly or just switch to matenance if you are happy with the results.