How Many Calories Is a Pound of Body Fat? Why 3,500 Misleads

A pound of body fat is worth about 3,500 kcal, and a kilogram about 7,700 kcal. That is where the advice to cut 500 calories a day to lose a pound a week comes from. I’d take 3,500 as a fair first guess and a poor forecast for months, because your body changes as it loses weight. Below is where 3,500 comes from and where it stops working.

How many calories is a pound of body fat?

About 3,500 kcal, and the number rests on one assumption about what a pound of fat tissue contains. Kevin Hall’s 2008 paper traces the rule to an old calculation, cited to Wishnofsky (1958) and McArdle (1996), that assumes the weight lost is adipose tissue made of 87% fat. A pound is 453.6 g. Fat carries 9 kcal per gram on the general Atwater factors. Here are the sums.

What a pound holds	Grams of fat	Energy (kcal)
Pure fat, 453.6 g × 9 kcal/g	453.6	4,082
Adipose tissue at 87% fat, 394.6 g × 9 kcal/g	394.6	3,552

Round the second row down and you get 3,500. In metric, Hall gives 32.2 MJ per kilogram as the same rule, which is 7,700 kcal when you divide by 4.184 kJ per kcal.

Two things sit under that figure. Fat tissue isn’t pure fat, and not everything you lose is fat tissue. Both matter below.

Does cutting 500 calories a day lose a pound a week?

For a few weeks it is a reasonable first guess, because 500 × 7 = 3,500. After that it drifts. The 2013 AHA/ACC/TOS guideline says lifestyle programs typically prescribe a deficit of at least 500 kcal a day, so the figure is no outlier. The trouble is how the rule treats time.

The 3,500 rule draws a straight line: the same deficit, the same loss, every week, forever. Hall and colleagues tested it in their 2011 Lancet paper. Their man is sedentary, weighs 100 kg (220 lb), and cuts 480 kcal a day. By the rule he loses 22 kg in the first year, about 100% more than their model predicts. In the model he levels off at about 75 kg (165 lb) and gets 95% of the way there after about 3 years.

Hall also gives a rule of thumb for the average adult who is carrying extra weight. Each permanent change of 10 kcal a day leads to an eventual change of about 1 lb. Half of it shows after about 1 year and 95% after about 3. I applied that to a 500 kcal cut held constant.

Cut held for	3,500 rule (500 kcal a day)	Hall’s rule of thumb
1 year	52 lb	about 25 lb
3 years	156 lb	about 48 lb

The second column is my own arithmetic: 500 ÷ 10 = 50 lb in the end, half after a year, 95% after three. It describes an average person, not you. Even so, the gap between the columns shows why a straight-line forecast fails. A 500 kcal cut doesn’t produce 156 lb. The model says about 48.

If a weight forecast runs in a straight line for months, be wary of it.

Why does the 3,500 rule overstate long-term loss?

Because a smaller body uses less energy, so the gap you started with closes. The rule keeps the deficit fixed. Hall’s model lets energy expenditure change as weight changes, since the body adapts to its new weight and resting metabolic rate shifts with it. At the plateau, the man who lost 25 kg uses about as much energy as he now eats, and the weight stops falling.

Pace slows for the same reason. The Lancet paper’s abstract says the response of body weight to a change of intake is slow, with half-times of about 1 year. When I plot my own weigh-ins against a straight 3,500 line, the line pulls away from my log within the first couple of months.

A second flaw pulls the other way. The rule treats every pound lost as fat tissue, yet some of it is lean mass. Hall’s energy density for lean mass is 7.6 MJ per kg, about a quarter of the 32.2 MJ per kg the rule assumes. A loss that includes some lean mass needs a smaller deficit per kilogram. The paper says it is now generally acknowledged that the rule is an oversimplification.

The two flaws don’t cancel. Over a year or more the shrinking gap wins and the 3,500 line overshoots. So a scale that slows later on is following the model, not you slacking.

How many calories do I need to cut to lose a pound?

As a starting estimate, 500 a day for a pound a week, then check the number against your own weight trend. The NHS says to aim for 1 to 2 lb (0.5 to 1 kg) a week, and the CDC says about the same. The rule turns the pace into a daily gap.

Pace	Daily gap by the 3,500 rule
0.5 lb a week	250 kcal
1 lb a week	500 kcal

I would start at the lower end of the NHS and CDC range. The bigger gap leaves less room for error, because it has to fit under a floor. The AHA/ACC/TOS guideline describes diets of 1,200 to 1,500 kcal a day for women and 1,500 to 1,800 kcal for men. A 500 kcal gap therefore needs a maintenance of at least 1,700 kcal for a woman and 2,000 kcal for a man. A 1,000 kcal gap needs 2,200 and 2,500. Eating below those floors is not a plan this page supports.

Here is what I do instead of trusting a calculator’s date. I log my food for two to four weeks, weighing homemade dishes the way the recipe calorie counter does, weigh myself the same way each morning, and compare the trend with the rule. Say I averaged 1,750 kcal a day and the trend fell 1 lb in 14 days. The gap was 3,500 ÷ 14 = 250 kcal a day, so my maintenance is about 2,000 kcal. That is a measured number, and it moves as your weight does, so I redo the check in a month. Size is the other quiet error in a log: how many calories is a banana and calories per egg show how far small and large sit apart.

What should I do if the scale stalls?

Wait a few weeks before changing anything, then change one thing. A stall is what the model expects: the response is slow, and a smaller body needs less energy than the one you started with. Read the trend over two or three weeks, never one morning. If it is flat while you log honestly (oil poured by eye is the usual leak, see the calories in a tablespoon of olive oil), your maintenance has moved down and the gap you planned has shrunk. Trim a small amount, or hold where you are, which is a fine answer too. For planning the next few weeks 3,500 still works, and your own log can correct it from there. Cutting hard in response risks sending you below the floors above.

Pregnant, ill, on medication that affects your weight, or under 18: a calorie target is then a conversation for your doctor, not a rule from a page like this one.

If counting calories has started to feel hard to stop, or eating feels out of control, talk to someone: ANAD in the US (1-888-375-7767), Beat in England (0808 801 0677), NEDIC in Canada (1-866-633-4220) or the Butterfly Foundation in Australia (1800 33 4673). If you’re in danger right now, call your local emergency number. Other UK numbers and crisis lines are on our crisis support page.