Calories and energy balance, explained simply

Your weight goes down over time when you eat fewer calories than you burn, and up when you eat more.^[1] A calorie is a unit of food energy. Nobody can look up how many you need. You start from an estimate, eat to it for a few weeks, and adjust from what the scale does.

Eating less than you burn is a calorie deficit. Eating more is a calorie surplus. The amount that keeps your weight steady is your maintenance calories. Energy balance is the whole picture: energy in from food, set against energy out from living and moving.

The idea is simple. The catch is that you can't measure either side well, and the scale bounces around from day to day. Most of this article is about working around those two facts.

The last article, on choosing exercises, closed out the training basics. Food comes next, starting with the total.

Why a rough number is good enough, and an exact one is impossible

Say your estimate is 300 calories too high. You can eat to it carefully for a month and see the scale barely move. It's easy to decide at that point that your body is broken, or that calories don't count for you. The next section shows the likelier problem: the numbers on both sides are fuzzy.

The opposite mistake costs something too. Weight loss slows as you go, even when you stick to the plan.^[1] If you expect the same drop every week, a normal slowdown looks like failure. Then it's tempting to quit, or to cut food harder than you need to.

You don't need a perfect number. You need a starting guess and a way to correct it.

What the research says

Weight follows energy in and energy out

A 2012 expert panel described it plainly: body weight changes when the energy in the food you eat doesn't match the energy your body uses to stay alive and to move.^[1] Strong evidence. The panel grounds it in the law of conservation of energy, and closely supervised feeding studies bear it out.^[1]^[2]

Your daily burn has three parts.^[3]

Resting burn. The energy your body uses at rest, just to keep running. You may see it called BMR or RMR. It's usually the biggest part, about two-thirds of the total.^[1]^[3]
Digesting food. Breaking down a meal costs energy too. This is the thermic effect of food (TEF). It's about 10% of what you eat, and higher for protein than for carbs or fat.^[3]
Activity. Planned exercise, plus NEAT (non-exercise activity thermogenesis): walking, standing, fidgeting, and every other move that isn't a workout.^[1]

Expert consensus. This is the authors' view, not a tested finding.

Add the three together and you get your total daily burn, sometimes called TDEE (total daily energy expenditure). Protein gets its own numbers in how much protein you need.

The two sides of the balance aren't separate. When you eat less, your body burns less, and that pushes back against weight loss.^[3] Exercise also tends to take off less weight than its calories would predict. The authors think the likely reason is that people make up part of it by eating a bit more or moving less at other times, by different amounts.^[3] Expert consensus. This is the authors' view, not a tested finding.

Your daily burn can only be estimated

A lab can measure resting burn from the oxygen you breathe in and the carbon dioxide you breathe out. Outside a lab, people use an equation instead.

The Mifflin-St Jeor equation comes from a 1990 study of 498 healthy adults aged 19 to 78. Just under half had obesity.^[4] It uses your weight, height, age and sex:^[4]

Men: 10 × weight in kg + 6.25 × height in cm − 5 × age + 5
Women: 10 × weight in kg + 6.25 × height in cm − 5 × age − 161

A systematic review searches for every study on a question and weighs them in a set way. A 2005 one compared the common equations against measured resting burn. Mifflin-St Jeor was the most reliable. It landed within 10% of the measured number in more people than any other equation, and its errors had the narrowest range.^[5] The reviewers also warned that it can still be well off for one person. Older adults and ethnic minorities in the US were underrepresented in the studies.^[5] Moderate evidence.

In athletes, it did worse. A meta-analysis pools the results of many studies into one estimate. A 2023 one covered 29 studies and 1,430 athletes. Mifflin-St Jeor's estimates differed significantly from measured resting burn, and it was among the equations that kept underperforming in athletes.^[6] Moderate evidence, in athletes. Most beginners aren't athletes. But if you carry more muscle than most people, the equation may fit you less well.

That 10% is about resting burn only. To get a full daily number, the equation's result is multiplied by an activity factor: a number for how much you move. Picking your level is a rough guess, and we found no good study on how much error that step adds.

Food logs and labels tend to run low

The other side of the balance is fuzzy too. A 2019 systematic review compared what people said they ate with doubly labelled water. That's a lab method that measures how much energy a person actually burns. Across 59 studies and 6,298 adults, most studies found that people reported eating significantly less than they burned.^[7] In the food-diary studies that found a shortfall, it ranged from 11% to 41%.^[7] Moderate evidence that self-reports run low: one large review, and most of its 59 studies agree. How low varies a lot from study to study.

One small 1992 study shows how big the gap can get. Ten adults with obesity said they couldn't lose weight even on under 1,200 calories a day. Their measured burn was within 5% of what was predicted for them. They underreported what they ate by about 47% on average, and overstated their exercise by about 51%.^[8] Limited evidence: ten people, chosen because dieting hadn't worked for them. Undercounting showed up in most of the 59 studies above.^[7] It's a measuring problem, not a character flaw.

Labels are close, but not exact. In a US test of 24 packaged snack foods, calories came out about 4% above the label (the middle value), well within US legal limits.^[9] Limited evidence: a small sample of US snacks.

Even with the best lab method for measuring burn, the 2012 panel says the combined error in measuring energy in and energy out can easily reach 1,000 calories a day.^[1] That's mostly because self-reported intake is so imprecise.^[1] Expert consensus. This is the panel's view, not a tested finding. So neither your log nor your equation can tell you your maintenance on its own. Your weight over several weeks can.

The scale swings from day to day

Day-to-day swings of about 0.5–1 kg (1–2 lb) are normal. They come mainly from water and from what's in your gut, not from fat.^[1] Expert consensus. This is the panel's view, not a tested finding.

Carbs add to the swing. Your body stores carbohydrate as glycogen, in the liver and muscles, and holds three to four parts water with it.^[10] When a diet starts, glycogen and its water drop, so the scale can fall fast in the first days. A high-carb day can bring some of it back.^[10] Limited evidence: a small, older study. The 2012 panel also notes that water moves with glycogen.^[1]

Take a 70 kg person. They can weigh 69.6 kg one morning and 70.4 kg two days later with no change in fat at all.

Why the "3,500 calories = 1 lb" rule over-predicts over months

The old rule says that cutting 3,500 calories takes off 1 lb (about 0.45 kg) of body weight.^[2] Used as a forecast, it suggests a steady deficit takes off the same amount every week, for as long as you keep it up.^[1]

It doesn't work that way over months. The rule ignores that as you get lighter, you burn less, both at rest and when you move.^[11] So the same deficit shrinks as you go.

Moderate evidence. A model and measured studies point the same way. The 2012 panel concluded that the 3,500-calorie rule "should no longer be used".^[1] Expert consensus.

The same model gives a rule of thumb for an average adult with overweight: a lasting change of about 10 calories a day moves your eventual weight by about 1 lb (0.45 kg). It takes years to get all the way there.^[11]^[1] Even with perfect adherence, people in the model ended up about 4 kg either side of the average after several years.^[11]

Notice the order. In the first days, the scale can drop faster than the rule says, because of glycogen and water.^[10] Over months, it drops slower.

Your body adapts, but less than the measuring error

When you lose weight, your burn can drop by more than the change in your body explains. This is called metabolic adaptation. How long it lasts is still debated.^[3]

In a 2-year trial, 34 non-obese adults cut their calories by about 15% and lost 8.7 kg on average. Their daily burn ended up about 80–120 calories lower than their weight loss explained.^[12] Moderate evidence that metabolic adaptation happens. Limited evidence on its exact size.

In that trial it was real but small, next to a measuring error that can easily reach 1,000 calories a day.^[1] When a diet stalls, the numbers are a likelier suspect than a broken metabolism.

How to set your calories and adjust them

These steps are common coaching practice. They haven't been tested head-to-head.

Start with an estimate. Use Mifflin-St Jeor, then multiply by an activity factor. Treat the result as a first guess, not your maintenance.
Log what you eat for two to three weeks. Include drinks, snacks and cooking oil. Aim for honest, not perfect. How to track food covers a method that doesn't take over your day.
Weigh yourself several mornings a week. Same conditions each time: after the toilet, before food. Compare weekly averages, not single days.
Compare the trend with your goal. If the average isn't doing what you wanted after two to three weeks, change your target by a small step. Try 100–200 calories a day. Then wait another two to three weeks.
Re-check every few weeks when losing. You burn less as you get lighter, so a target that worked at the start will do less later.^[11]

Here's the sum for two people.

A 55 kg woman, 160 cm tall, aged 28.

Resting burn: 10 × 55 + 6.25 × 160 − 5 × 28 − 161 = about 1,250 calories.
A 10% miss on that is 125 calories either way, before the activity guess.
Say her activity factor is 1.375. Her starting estimate is about 1,720 calories a day.
She wants to hold her weight. After three weeks at 1,720, her weekly average is slowly drifting down. She adds 150 calories a day and checks again in three weeks.

A 90 kg man, 180 cm tall, aged 35.

Resting burn: 10 × 90 + 6.25 × 180 − 5 × 35 + 5 = about 1,855 calories.
Say his activity factor is 1.55. His starting estimate is about 2,875 calories a day.
He wants to lose fat and eats below that estimate. After three weeks, his weekly average hasn't moved. He's eating closer to his real maintenance than the estimate suggested, so he drops another 150 calories a day.

How big a deficit or surplus to pick depends on your goal. Cut, bulk or recomp covers that.

Losing faster than about 1% of your body weight a week, or eating far below your needs, carries health risks. Don't try it without medical supervision. If you have a health condition, talk to your doctor before changing how much you eat. If counting food makes you anxious, or you have a history of an eating disorder, skip the numbers and speak to a doctor or a registered dietitian.

Common mistakes

Treating the calculator's number as fact. An equation gives a precise-looking number, so it feels exact. It's a starting point. Your weight trend over a few weeks is the real answer.
Reading one weigh-in as fat. A jump of 1 kg overnight feels like a disaster. It's mostly water and food. Compare weekly averages instead.
Expecting the same loss every week. The 3,500-calorie rule makes a straight line look normal. Loss slows as you get lighter. Re-check your target every few weeks instead of panicking.
Eating back every workout. It's tempting to treat a session as calories in the bank. Exercise usually takes off less weight than its calories suggest. Keep your target and let the trend decide.
Blaming your metabolism when the log is the problem. Undercounting is common. Before you cut harder, check your portions and the foods you forgot to log.

How Gymbers fits

A calorie target is only useful if you treat it as an estimate and check it against the trend. The Gymbers app estimates your daily burn with the Mifflin-St Jeor equation multiplied by an activity factor (how we calculate targets). Then it adjusts that number up or down for your goal to set your calorie target. You log what you eat each day against it, along with protein, carbs and fat.