Calorie Balance Calculator

Last updated: 2026-05-22

What is calorie balance?

Calorie balance is the difference between the energy a person consumes as food and drink and the energy their body expends over the same period. When intake exceeds expenditure, the surplus is stored, mostly as body fat. When expenditure exceeds intake, the body draws on stored energy and weight falls. This calculator estimates both sides of that equation and reports the resulting daily balance.

How energy expenditure is estimated

Total daily energy expenditure (TDEE) is built up from two pieces. The first is the basal metabolic rate (BMR), the energy the body uses at complete rest to keep basic functions running — circulation, breathing, cell maintenance, and temperature regulation. BMR accounts for the majority of daily expenditure for most people. The second piece is everything added on top of rest: digestion, daily movement, and exercise. That additional energy is captured by an activity multiplier applied to BMR.

The calculator estimates BMR with the Mifflin-St Jeor equation, a regression fit to indirect-calorimetry data that has become the standard population estimate:

where $w$ is weight in kilograms, $h$ is height in centimeters, and $a$ is age in years. BMR is then multiplied by an activity factor to give TDEE:

The daily calorie balance is the difference between intake and TDEE:

A positive number is a surplus; a negative number is a deficit.

Projecting weekly weight change

A daily balance is converted into an estimated weekly weight change using the approximation that 7,700 kcal of accumulated surplus or deficit corresponds to roughly 1 kg of body fat:

At a daily deficit of 500 kcal, this projects to about −0.45 kg per week. At a daily surplus of 300 kcal, it projects to roughly +0.27 kg per week.

Worked example

Consider a 35-year-old woman who is 165 cm tall and weighs 68 kg, with a lightly active routine (multiplier 1.375). Her BMR is:

Her TDEE is $1{,}375.25 \times 1.375 \approx 1{,}891$ kcal. If she consumes 1,600 kcal per day, her balance is $1{,}600 - 1{,}891 = -291$ kcal, a deficit. Over a week, that projects to $(-291 \times 7) / 7{,}700 \approx -0.26$ kg.

Two directions

The calculator solves the same balance equation for either unknown:

• Track intake — given a daily intake, it returns the balance (surplus or deficit) and the weekly weight change that balance projects. This direction suits a known eating pattern whose outcome is in question.
• Hit a target — given a desired weekly weight change (for example −0.5 kg per week), it works backwards to the daily calorie intake that would produce it. This direction suits a fixed goal that needs an intake figure.

Both directions share the same TDEE estimate.

Choosing an activity level

The activity multiplier is the largest source of estimation error in the model. The five levels are:

Most people in white-collar jobs, including those who exercise regularly, fall in the 1.375–1.55 range. Selecting "very active" for a 45-minute gym session three times a week typically overestimates TDEE by 200–400 kcal per day — enough to erase a meaningful deficit.

Measuring intake accurately

Underestimating food intake is the most frequent reason a calorie balance comes out wrong. Studies consistently show that intake is undercounted by 20–40% on average. Weighing food with a kitchen scale and logging it in a tracking app removes most of this error, and even an approximate log is more actionable than a guess.

On the expenditure side, wearable fitness trackers and gym machines tend to overestimate calories burned during exercise, sometimes by 50% or more. The activity-multiplier approach used here is more conservative because it averages over a typical week rather than crediting individual workout sessions.

Limitations

• Individual variation: Mifflin-St Jeor was derived from population averages. Actual BMR can differ by ±15% between individuals — thyroid function, genetics, and body composition all play a role.
• Metabolic adaptation: sustained calorie restriction reduces TDEE over time. If weight loss stalls after several weeks at the same deficit, true maintenance calories have likely shifted downward.
• The 7,700 kcal rule: it assumes all weight change comes from fat. In practice, both fat and lean mass shift together, especially early in a deficit or surplus.
• Medical considerations: conditions such as hypothyroidism, PCOS, or insulin resistance, along with certain medications, can substantially affect metabolism. The calculator does not account for any of these factors.

Applying the result

For fat loss, a deficit of 300–500 kcal per day is a common target. Larger deficits accelerate weight loss initially but increase muscle loss and trigger stronger hunger responses, and research consistently supports moderate deficits for sustainable outcomes. For muscle gain, a modest surplus of 200–400 kcal per day supplies material to build new tissue without excessive fat accumulation; larger surpluses add weight faster, but most of the extra mass is fat. At maintenance, intake and TDEE are approximately equal, which is a useful baseline to establish before deliberately shifting toward a surplus or deficit.

The 7,700 kcal projection is a planning approximation, not a guarantee. The body is not a simple combustion engine — adaptive thermogenesis, changes in lean mass, and hormonal shifts all affect actual outcomes over time. For a plan tailored to a specific health situation, consult a registered dietitian or sports nutritionist.