Can you lose 40kg in a year?

Can you lose 40kg in a year? Yes, and here’s exactly what the math and research say about making it happen.

Losing 40kg in 12 months means losing about 3.3kg per month, or just under 800g per week. That sits at the upper edge of what research considers safe and sustainable, but it’s achievable for people who start with a significant amount of body fat and apply the right strategy from day one.

Here’s what you need to know.

Is Losing 40kg in a Year Actually Possible?

Yes. Losing 40kg in a year requires a consistent daily calorie deficit of around 800 to 1,000 calories. One kilogram of body fat holds roughly 7,700 calories of stored energy, so losing 40kg means burning through approximately 308,000 calories more than you eat across the year.

That breaks down to about 845 calories per day in deficit.

For someone starting at 120kg or more, this is very realistic. The heavier you are, the more calories your body burns just to function, which means the deficit required is easier to reach without starving yourself.

A 2010 study found that dieters who got a full night’s sleep lost more than twice as much fat as sleep-deprived dieters. Research also confirms that a combination of dieting and exercise produces the best results, consistently outperforming diet or exercise alone.

How Many Calories Do You Need to Cut?

The formula is straightforward. To lose 1kg of fat, you need a deficit of 7,700 calories. To lose 40kg, you need 308,000 calories of deficit across the year.

Here’s how that looks week by week:

1. Losing 0.5kg per week requires a daily deficit of roughly 550 calories
2. Losing 0.75kg per week requires a daily deficit of roughly 820 calories
3. Losing 1kg per week requires a daily deficit of roughly 1,100 calories

Most people targeting 40kg in a year sit somewhere between option 2 and option 3. The key word is consistency, not perfection. Research shows that people who log their food about three times per day and around 20 times per month are more likely to hit clinically significant weight loss results.

Use a calorie tracking app and enter your age, weight, height, and activity level to find your personal number. Aim for a deficit of 500 to 800 calories per day from your maintenance calories.

What Should You Actually Eat?

Protein is the most important part of your diet when losing large amounts of weight.

Studies show that going from a low-protein to a high-protein diet can raise your daily calorie burn by 4 to 5%, because protein costs more energy to digest than carbohydrates or fat. Around 20 to 30% of protein calories get burned during digestion alone, compared to less than 10% for other macronutrients.

Protein also keeps you full longer and protects your muscle while you lose fat.

Target your protein intake by multiplying your body weight in kilograms by 1.8. So if you weigh 110kg, aim for around 198g of protein per day.

Good protein sources include:

1. Chicken breast, turkey, and lean beef
2. Greek yogurt and cottage cheese
3. Eggs and egg whites
4. Canned tuna and salmon
5. Protein bars and protein powder for convenience

Research on low-carb versus low-fat diets consistently shows no meaningful difference in fat loss when protein and calories are matched. Pick the eating style you can maintain for the next 12 months. Adherence matters far more than the specific diet you choose. A meta-analysis on popular diets found that adherence was the single strongest predictor of weight loss results, regardless of the diet type.

Does Exercise Speed Up 40kg of Weight Loss?

Yes, significantly.

Exercise contributes in two ways: it burns extra calories and it preserves muscle while you lose fat. Losing muscle slows your metabolism, so protecting it keeps your calorie burn higher throughout the year.

Weight training has direct research support for preserving muscle during a calorie deficit, and it increases your resting metabolism by building lean tissue.

For fat burning, moderate to high-intensity cardio and interval training top the research. A large 2023 study on cardio and visceral fat found that moderate to high-intensity cardio and interval training reduced belly fat more than other exercise types. The threshold was getting above 75% of your maximum heart rate.

A 30-minute walk burns 100 to 200 calories, and adds up to roughly 1kg of extra fat loss per month. Daily walking alone isn’t the whole strategy, but it makes a real difference over 12 months when stacked on top of diet changes.

A practical weekly exercise plan for someone targeting 40kg in a year:

1. 3 to 4 sessions of weight training per week
2. 2 to 3 sessions of cardio above moderate intensity
3. 7,000 to 12,000 steps per day through normal movement

Why Do Most People Fail to Lose 40kg?

Six out of every seven obese people lose a significant amount of weight at some point in their life. The problem is keeping it off. Research shows they regain it because they treat weight loss as a temporary event rather than a permanent change in how they eat and live.

The most common reasons people don’t reach or keep 40kg of weight loss:

1. They pick a diet they can’t maintain for longer than 8 weeks
2. They cut calories too hard, lose muscle, and see their metabolism drop
3. They make no plan for what happens after the weight comes off
4. They rely on willpower instead of building habits and structure

For weight loss to last, the changes you make have to last too. Research is direct on this point. People who approach weight loss as a permanent lifestyle shift maintain results. People who approach it as a temporary diet don’t.

How Fast Is Too Fast?

Losing more than 1kg per week consistently starts to cause muscle loss, nutritional deficiencies, and metabolic adaptation. When you lose muscle, your body burns fewer calories at rest, making the deficit harder to maintain.

Aim for 0.5 to 1kg per week as your target range. Some weeks you’ll lose more, some weeks less. The average over the month is what matters.

If you’re starting at a very high body weight, like 130kg or above, your body naturally drops faster in the first 4 to 8 weeks. Some of this is water weight and inflammation reduction, not pure fat. Don’t try to match that initial pace forever.

What Tools Actually Help?

A calorie tracking app is the single most useful tool for anyone trying to lose 40kg. Apps that adjust your calorie target based on your weekly weigh-ins are more accurate than static calculators, because they adapt to your actual metabolism rather than relying on averages.

Self-monitoring of diet, exercise, and weight consistently shows up in research as one of the most reliable strategies for weight loss, particularly in the early months.

A food scale, cheap at around $20 to $30 AUD, removes the guesswork from tracking. Portion estimates made by eye are notoriously inaccurate, and even experienced dieters underestimate intake by 20 to 40%.

Increasing water intake also supports weight loss. Research shows that higher water intake increases thermogenesis, reduces food intake, and improves fat oxidation.

FAQ

Can an overweight person lose 40kg in less than a year?

Some people do. Those starting at very high body weights, like 140kg or above, can lose faster in the early months because their daily calorie burn is high and the initial deficit is large. However, pushing past 1kg per week consistently raises the risk of muscle loss and nutritional problems. A year is a realistic and safe timeframe for most people.

Do you need surgery to lose 40kg?

No. Diet and exercise produce 40kg of weight loss regularly without surgery. Bariatric surgery, like gastric bypass or gastric banding, reduces the stomach size and limits food intake, but carries surgical risks and requires medical consultation. Surgery works as a tool for severe obesity when lifestyle changes haven’t succeeded.

What about weight loss drugs like semaglutide?

Semaglutide reduces appetite and has shown meaningful weight loss results, particularly for people with obesity and type 2 diabetes. It works by suppressing hunger signals, making the calorie deficit easier to maintain. It doesn’t replace diet and exercise, it makes the deficit more achievable. Discuss with a doctor whether it’s appropriate for your situation.

Does your metabolism slow down when you lose 40kg?

Yes, to some degree. A lighter body burns fewer calories at rest. This is why calorie targets should adjust as weight drops, and why building muscle through weight training matters throughout the process. Research shows that metabolic rate is not as predictive of long-term weight loss success as many people think. Regular physical activity, consistent weigh-ins, and weight training show up more reliably as success factors.

What happens after you lose 40kg?

This is where most people fail. The habits that got you to 40kg of fat lost have to continue. Researchers consistently find that people who maintain weight loss do so by staying physically active, continuing to self-monitor, and making permanent changes to eating patterns rather than returning to old habits. Plan for maintenance from the start, not as an afterthought.

How much does it cost to lose 40kg?

You don’t need expensive programs. A food scale ($25 AUD), a calorie tracking app (free or $10 to $20 AUD per month), and gym access ($30 to $80 AUD per month depending on where you go) cover the essentials. Protein powder ($40 to $80 AUD per month) helps hit daily protein targets if whole food alone falls short. The total cost is around $100 to $200 AUD per month, far less than most commercial diet programs.