Your internal organs make up a surprisingly small fraction of your total body weight, yet they account for the bulk of your energy use. For a typical adult, the liver weighs roughly 1.4 to 1.8 kilograms, the brain about 1.3 to 1.4 kg, and the heart somewhere around 300 grams. But those textbook figures are averages that shift depending on your sex, height, body composition, and age. The actual range is wide enough that forensic pathologists and transplant surgeons rely on detailed reference tables rather than single numbers, and even those tables have been getting revised upward in recent decades.
A Tour of the Major Internal Organs
The liver is the heaviest internal organ. In adult men it typically falls in the range of 1.5 to 1.8 kg, with women running somewhat lighter. It sits in the upper right abdomen and performs hundreds of metabolic tasks, from filtering blood to producing bile. Its weight can fluctuate more than almost any other organ depending on fat storage, blood supply, and dietary status.
The brain comes next. In adults it generally weighs between 1.3 and 1.5 kg, with men averaging slightly heavier brains than women. Unlike most other organs, the brain’s weight is remarkably stable across healthy adults of the same sex and remains relatively unchanged by body mass or fitness level. It does, however, shrink measurably with advanced age.
Each lung weighs roughly 400 to 600 grams, with the right lung slightly heavier than the left because of its extra lobe. Lung weight is tricky to pin down because it depends heavily on how much blood and fluid the tissue contains at the moment of measurement.
The heart in a healthy adult man weighs about 300 to 350 grams, and in women roughly 250 to 300 grams. It is one of the organs most sensitive to both body size and disease. Chronic high blood pressure, for instance, can push heart weight well past 500 grams through thickening of the muscle walls.
The kidneys each weigh around 120 to 170 grams. The spleen, tucked behind the left rib cage, typically comes in at 100 to 170 grams. The pancreas, nestled behind the stomach, averages about 70 to 100 grams. These numbers come largely from autopsy studies, which remain the gold standard for organ-weight data. A large study examining this autopsy data found that median organ weights in modern populations have actually increased compared to the older reference tables that many textbooks still cite.
1PubMed. Updating Normal Organ Weights Using a Large Current Sample DatabaseThe Body’s Biggest Contributors by Mass
When people ask how much their organs weigh, they usually picture the liver, brain, and heart. But those internal organs combined make up only a modest share of body weight. The real heavyweights are skeletal muscle, bone, and skin.
Skeletal muscle is by far the largest tissue system. In men, it averages about 33 kg, which works out to roughly 38 percent of body mass. In women, it is closer to 21 kg, or about 31 percent of body mass.2PubMed. Skeletal muscle mass and distribution in 468 men and women aged 18-88 yr That single tissue type outweighs every internal organ put together, several times over.
The skeleton in an adult weighs roughly 3 to 5 kg of dry bone mineral, though the functional skeletal system including marrow and cartilage adds considerably more. Skin, often overlooked in organ-weight conversations, is the body’s largest organ by surface area. It covers about 1.5 to 2 square meters and accounts for roughly 15 percent of total body weight, which in an average adult works out to around 10 to 11 kg.3PubMed. Understanding the structure and function of the skin That makes skin heavier than any internal organ by a wide margin.
Why Men’s and Women’s Organs Differ in Weight
Across virtually every population studied, men’s internal organs are heavier than women’s. A study comparing autopsy data from eight different populations found that male organ weights could be expressed as a straightforward linear function of the corresponding female organ weights, with the relationship holding tightly across populations.4PubMed. Male/female weight of internal organs: examination across multiple populations In other words, the gap between male and female organ weight is not random. It scales predictably and mirrors the overall sex difference in height and body mass.
Data from autopsies in southern Iran confirmed the pattern: all organs were heavier in men than in women, with the heart, brain, spleen, and right kidney showing the largest statistically significant gaps.5PubMed Central. Statistical Analysis of Organ Morphometric Parameters and Weights in South Iranian Adult Autopsies Interestingly, for some organs like the liver and pancreas, the sex difference was present but small enough that it did not always reach statistical significance in individual studies. The takeaway is that if you are comparing your organ weights to a reference table, the table needs to match your sex, not just your age.
How Body Size and BMI Shift the Numbers
Organ weight does not just track with sex. It tracks powerfully with overall body size. A study using medical autopsy data found statistically significant increases in the weights of the liver, spleen, kidneys, and heart as body mass index went up, in both men and women.6PubMed. Organ weight changes associated with body mass index determined from a medical autopsy population This makes intuitive sense: a larger body demands more metabolic work, more blood filtration, and more detoxification, so the organs responsible for those jobs grow to match.
The liver is especially responsive to body size. In people with obesity, the liver can become substantially heavier, partly because of increased metabolic demand and partly because of fat accumulation within liver cells. The heart also enlarges, and in men specifically, higher BMI has been linked to increased heart weight while advancing age was associated with thickening of the left ventricular wall.6PubMed. Organ weight changes associated with body mass index determined from a medical autopsy population This means an overweight 60-year-old man may have both a heavier heart and thicker heart walls than a lean 30-year-old, for partly different reasons.
Autopsy reference ranges from Malaysian populations confirmed that organ weights were affected by sex, ethnicity, and BMI, underscoring that a single global reference table cannot do the job for all populations.7PubMed. Establishment of Reference Ranges for Normal Organ Weights in Malaysian Adults at Autopsy This is why forensic pathologists sometimes need region-specific data rather than textbook numbers derived from a single ethnic group decades ago.
What Happens to Organ Weight As You Age
Most organs lose mass as you get older. The general pattern, documented across many populations, is that organ weights decrease with age, with the notable exception of the heart and the thyroid gland, which tend to hold steady or even increase.8PubMed. Organ weight in 684 adult autopsies: new tables for a Caucasoid population The same autopsy data showed that organ weights correlated with at least one external parameter such as height or BMI for every organ, except thyroid weight in women, which appeared essentially independent of body size.
The brain is a conspicuous example. Brain volume peaks in early adulthood and then declines steadily, losing mass through a combination of neuronal shrinkage, reduced water content, and thinning of cortical layers. The liver and kidneys also shrink meaningfully with age. As the same BMI-focused autopsy study noted, the liver, spleen, and kidneys all showed weight decreases with advancing age.6PubMed. Organ weight changes associated with body mass index determined from a medical autopsy population So when organ-weight tables give a single number for “adult liver weight,” they are blurring together a 25-year-old’s liver and an 80-year-old’s liver, which can be meaningfully different.
This age-related decline also affects overall body composition. The reduction in body weight seen during later stages of aging is accompanied by a coordinated loss of internal organ weight and volume, not just loss of muscle or fat. Researchers have described this as a kind of coordinated involution, where the body’s organs shrink somewhat in proportion to one another rather than in isolation.
When Disease Changes the Equation
Organ weight in pathology is not just a curiosity. It is a diagnostic tool. When a pathologist weighs a heart at autopsy and finds it substantially above the reference range, that is a red flag for cardiac hypertrophy, the diagnostic workup for which involves searching for pressure overload, storage diseases, and cardiomyopathies. Chronic high blood pressure is the most common culprit, forcing the heart to pump harder and thickening its walls in response.
The liver is just as revealing. A study examining the effect of fatty liver disease on liver weight found that steatosis, the accumulation of fat in liver cells, clearly increases liver weight and size, while fibrosis alone (scarring without fat) does not, at least not until steatosis accompanies it. The pattern of enlargement is also distinctive: steatosis causes the liver to expand disproportionately in the front-to-back direction rather than growing uniformly in all dimensions.9PubMed. The effect of hepatic steatosis and fibrosis on liver weight and dimensions
Kidneys can change size in the opposite direction. When one kidney is removed, as in living kidney donation, or when nephrons are gradually lost to chronic kidney disease, the remaining kidney tissue undergoes compensatory hypertrophy. The surviving nephrons enlarge, driven primarily by cell growth rather than cell division, to pick up the slack. This adaptation works well initially but can eventually accelerate kidney decline through overwork of the remaining tissue.
The spleen is perhaps the most dramatically variable organ of all. It can double, triple, or even reach ten times its normal weight in response to infections, blood disorders, or chronic liver disease that backs up blood flow into the splenic vein. In malaria-endemic regions, for example, splenic enlargement is so common that spleen size has historically been used as an epidemiological marker for malaria transmission intensity.
Organs Burn Calories Out of Proportion to Their Weight
One of the most striking things about organ weights is how little your internal organs weigh relative to how much energy they consume. Your brain makes up about 2 percent of your body mass but uses roughly 20 percent of your resting energy. The heart and kidneys, despite their small size, burn energy at even higher rates per gram of tissue.
The classic estimates for organ-specific metabolic rates, widely used in research, put the liver at about 200 kilocalories per kilogram per day, the brain at 240, and the heart and kidneys at 440 each. Skeletal muscle, by contrast, burns only about 13 kilocalories per kilogram per day at rest, and adipose tissue a mere 4.5.10PubMed Central. Evaluation of Specific Metabolic Rates of Major Organs and Tissues: Comparison Between Men and Women These rates held up across both men and women, meaning the same metabolic constants can be used regardless of sex.11PubMed Central. Specific metabolic rates of major organs and tissues across adulthood: evaluation by mechanistic model of resting energy expenditure
This uneven distribution of energy use helps explain why small changes in organ mass can have outsized effects on your resting metabolic rate. Lose a little liver mass, and your calorie burn drops more than if you lost the same amount of muscle. It also explains why crash dieting and severe caloric restriction can slow your metabolism more than you would expect from the amount of weight lost.
What Happens to Organs During Severe Caloric Restriction
When the body is starved or placed under aggressive caloric restriction, it does not simply burn fat. It also cannibalizes its own organs. Research revisiting the classic Minnesota Starvation Experiment found that caloric restriction caused significant losses in liver and kidney mass. The liver alone accounted for the majority of the organ-mass decline, with liver mass dropping by roughly 150 grams within just three weeks of caloric restriction.12The American Journal of Clinical Nutrition. Metabolic adaptation to caloric restriction and subsequent refeeding: the Minnesota Starvation Experiment revisited
A separate study examining the effects of two years of caloric restriction in healthy adults found reductions in the mass of skeletal muscle, heart, liver, and kidneys, with the high-metabolic-rate organs shrinking proportionally more than low-metabolic-rate tissues like muscle and bone. Kidney mass dropped by about 6 percent, the heart by about 5 percent, and the liver by about 4 percent, while brain mass stayed essentially unchanged.13PubMed Central. Impact of calorie restriction on energy metabolism in humans The preferential shrinkage of energy-hungry organs accounted for about 30 percent of the observed decline in resting energy expenditure during caloric restriction, above and beyond what simple weight loss would predict.
This organ-level shrinkage is one of the mechanisms behind “metabolic adaptation,” the frustrating plateau dieters experience when their metabolic rate drops faster than their weight does. Your body is not just burning less because it is smaller. It is also shrinking the organs that burn the most calories per gram, making the slowdown even more pronounced.
How Organ Weights Are Actually Measured
Nearly everything we know about organ weights comes from autopsies. A pathologist removes each organ, dissects away surrounding fat and connective tissue, drains excess blood, and places it on a scale. Autopsy data from forensic cases, where death results from external causes like accidents rather than disease, provides the cleanest reference values because the organs are presumably healthy.
One of the earliest comprehensive modern autopsy studies compiled data from 684 forensic autopsy cases between 1987 and 1991, covering the heart, lungs, liver, spleen, pancreas, kidneys, and thyroid in adults who died from external causes and showed no pathological changes.8PubMed. Organ weight in 684 adult autopsies: new tables for a Caucasoid population These kinds of studies form the backbone of reference tables used in forensic pathology worldwide.
But autopsy data has inherent limitations. People who die suddenly of external causes are not a perfect random sample of the living population. They skew younger and more male. They may have had undiagnosed conditions. And the process of dying itself changes organ weight. Blood loss at the time of death, for instance, dramatically affects what you find on the autopsy table. A study examining the effects of blood loss on organ weight found that organs weighed significantly less in cases with blood loss, with the lungs weighing about 30 percent less, the spleen 28 percent less, the liver 18 percent less, and the kidneys 14 percent less compared to controls.14PubMed Central. Effects of blood loss on organ attenuation on postmortem CT and organ weight at autopsy Those are enormous differences, meaning the cause of death can substantially skew what a pathologist records as an organ’s weight.
Researchers are now developing imaging-based alternatives to bypass these problems. An AI-based approach using postmortem CT scans recently showed it could estimate organ weights with relatively close agreement to autopsy measurements, with the best accuracy for the brain and liver and the worst for the heart.15PubMed Central. AI-based organ weight estimation from postmortem computed tomography If these methods mature, they could eventually allow organ-weight estimation in living patients through routine medical imaging, which would be a significant advance over relying on data from the dead.
Why Organ Size Matters for Transplant Matching
Organ weight is not just an academic measurement. In transplant medicine, the size of a donor organ relative to the recipient’s body matters for outcomes. A liver that is too large for the recipient’s abdominal cavity creates surgical complications, while one that is too small may not provide enough metabolic capacity. Better matching of the donor organ to the recipient’s body size improves transplant outcomes and reduces the risk of graft failure and the need for re-transplantation.16PubMed Central. Matching donor to recipient in liver transplantation: Relevance in clinical practice
Surgeons typically estimate donor organ size using body weight, height, and imaging, then compare it to the estimated standard liver volume for the recipient. When the mismatch is too large in either direction, a donor organ may be declined even if it is otherwise healthy. In pediatric transplant, this issue is even more acute, because the pool of appropriately sized donor organs is much smaller. Understanding normal organ-weight ranges, stratified by age, sex, and body size, is what makes these clinical decisions possible.
How Animals Reshape Their Organs Seasonally
Humans carry roughly the same organ weights throughout the year, barring disease or dramatic changes in diet. Many animals do not. Red deer, for example, deliberately reduce the mass of their gut and internal organs during winter, when food is scarce. Alongside reduced appetite, their visceral organs shrink significantly, and the animals rely on stored body fat for energy instead. Researchers have concluded that this seasonal organ downsizing is an energy-saving strategy: maintaining a large digestive tract costs calories, and when there is not enough food around to justify those calories, the body lets the gut atrophy.17PubMed Central. Contrary seasonal changes of rates of nutrient uptake, organ mass, and voluntary food intake in red deer (Cervus elaphus)
Birds take this flexibility even further. House sparrows exposed to fluctuating cold temperatures increase their metabolic rates and bulk up their flight muscles to generate more heat through shivering.18PubMed. Responses of Body Mass, Organ Masses, and Metabolic Rates in Winter-Phenotype House Sparrows to Fluctuating Cold Temperatures Migratory birds show an even more dramatic version of this: they can shrink their digestive organs before a long migration to reduce body weight, then rebuild those organs at stopover sites when they need to refuel. This phenotypic flexibility in body composition and digestive system structure is considered a key adaptation that allows migratory birds to meet the conflicting demands of flight efficiency and caloric intake.19PubMed. Phenotypic flexibility in digestive system structure and function in migratory birds and its ecological significance
Compared to these animals, human organs are relatively fixed in size over the short term. But the caloric-restriction research described earlier suggests that our bodies retain a mild version of this flexibility. Under prolonged energy stress, humans do shrink their metabolically expensive organs, just more slowly and less dramatically than a deer shedding gut mass for winter. The biology of organ size, it turns out, is not purely structural. It is a dynamic negotiation between energy supply and metabolic demand, one that plays out differently across species but follows broadly similar rules.