Calorie Intake Calculator

Online calorie calculators all work on the same two-step principle: First they estimate your resting energy expenditure, then they adjust that figure according to lifestyle. However, none of these tools explain that there are five reliable methods and that results can vary by up to 500 calories per day for the same person depending on which method is used. This page runs all five methods simultaneously showing you the differences in the results so you can choose the one that works best for you.

The structure of the calculation:

Two multiplications and one subtraction - that's all. So overall, it is very simple.

The basal metabolic rate (BMR) is the amount of energy expended while at rest in a neutrally temperate environment, so it is the value that varies between the five equations. It accounts for 60-75% of total energy expenditure in most people.

The activity factor is used to apply the basal metabolism to the whole daily life. It is the ratio of total energy expenditure to the basal metabolism and already takes into account the energy required for commuting, unconscious movements, sports and digestion of food.

A more detailed look at each of these five methods is below.

Unless otherwise stated, weight is measured in kilograms, height in centimeters and age in years.

The Mifflin-St Jeor Equation (1990).

This is the standard method used on this site and almost everywhere. A review in the Journal of the American Dietetic Association found it to be among the most reliable general formulas for individuals without body composition data.

Revised Harris-Benedict Formula (1984)

The original Harris-Benedict formula dates from 1919. Roza and Shizgal revised it in 1984, and the version used on this page is the revised one. Beware though: some popular sites use the 1919 coefficients but call the formula "revised". If the constants are 66.47 and 655.1 then you have the original version.

The Katch-McArdle Formula

Of the five methods, this is the only one that does not take size, age or gender into account. The calculation is based on lean body mass. This is because it is actually the lean body mass that burns calories. So this method is the most accurate of the five if actual body fat percentage has been measured but the least accurate if only estimated body fat percentage is used.

Formula for the Institute of Medicine's Dietary Guidelines.

This is the method used by nutrition scientists with special software. It's based on measuring using double-marked water and it's oriented to "nutrition recommendations" published by the medical research institute in 2005. This measurement doesn't measure metabolism at rest in a laboratory, but rather the total energy consumption of people in their normal daily life. Unlike the other four methods, no multiplication with a factor is performed here because the calculated energy requirement estimate already represents the overall turnover. The activity factors are indicated in square brackets. The unit for body size is meters.

Schofield - The method used by Australia and the WHO.

Schofield's 1985 analysis forms the basis for energy requirements recommendations of WHO, FAO and UNU as well as Australian dietary guidelines. The Australian government's calculator is also based on these criteria. To calculate it only requires body weight and age group, no other information. For people with the same weight, whether their height is 1.60 m or 1.90 m makes no difference to the result. The output is in megajoules per day which is why kilojoules are used in Australian dietary guidelines.

Show reasons for differences in answers, sorted by size of difference.

There are four independent factors that affect this value. Many sites change several of these factors at the same time but don't mention it.

1) The unit of measurement for activity quantity is not uniform.

Let's take a look at how different publishers refer to office workers who sit down.

In the first measurement sedentary office workers are referred to as 1.2 while in the third measurement 1.4 is used. This choice alone changes the result by about 17% before any discussion of the equation begins. The IOM value appears lower because a direct comparison is not possible. These factors are part of a term within the equation, and this equation already calculates an overall amount and does not represent a resting metabolic rate.

2. The equations were developed for different population groups.

Harris and Benedict measured mostly young, lean American men in the 1910s. Mifflin and St Jeor used a more modern American population including overweight individuals from the 1990s. Schofield compiled international data with a large proportion of Italian data which generally results in higher estimates. For most body sizes, Schofield's results are several percent higher than Mifflin's but for older people the order can be reversed.

3. Some calculation tools double count heat produced by food intake.

Digestion of food requires energy, which is about 10% of the amount consumed. This is a real process but some common calculators from one particular publisher multiply the basal rate by an activity factor and then add on another 10%. The problem with this is that the activity factor is the ratio of total daily energy expenditure to the basal rate, and digestion is already included in the total daily energy expenditure. Adding it again leads to double counting, which makes any result about a factor of 0.1 higher. We have provided an option on this page to switch this off so you can check the effect and compare results with other sources. The default setting is turned off.

4. Calories, kilocalories, kilojoules

The "Calories" units used on food packaging (with a capital C) are kilocalories, which is 1000 times the calorie used in physics. Kilojoules are used in many regions outside of North America.

There are currently two conversion factors in use. The thermochemical calorie is exactly 4.184 joules and is used in European food labeling regulations as well as American nutrition labels. The international steam table calorie is 4.1868 joules, and at least one of the major calculators uses this value for energy conversions. The difference between them is 0.07%, so that it will not make a significant difference to the results when calculating a meal. It's important to know that this explains why the last digits in the results from two different tools will never always match.

Turn goals into plans.

Once you know your baseline metabolism there are three ways to calculate the adjustment. Even for the same person different values can result. This page will introduce these three methods.

These three methods are connected by a constant: one kilogram of body fat contains about 7,700 kcal, so one pound is about 3,500 kcal. This is the basis for the well-known recommendation of 500 kcal per day. If you multiply 500 by 7, you get 3500, which corresponds to the idea of losing a pound per week. For someone with a basal metabolic rate of 2,000 kcal, the values determined by the three methods are 1,500, 1,600 and 1,450 respectively. For someone with a BMR of 3,000 kcal, the values are accordingly 2,500, 2,400 and 2,450. Only the method based on the ratio is adapted to body size. This is the main reason why this method is preferred.

When the rule for 7,700 no longer applies:

This rule ignores the fact that the body can be thought of as a fuel tank with a certain volume and lighter bodies require less energy to move around. Within four to six weeks, the deviations are still small. After one year, the linear rule predicts about twice the weight loss compared to what is actually achieved. This is because the basal metabolism gradually decreases in line with calorie intake. All predictions on this page are based on this linear rule and are therefore optimistic. Consider the results for the first month as reliable.

Lower limit:

For women, the calculator will not output goals below 1200 kcal per day. For men, it will not output goals below 1500 kcal per day. These values are from Harvard Health and are supported by various guidelines. If these values are dropped below, it becomes difficult to meet protein, calcium, iron, and micronutrient requirements regardless of diet choice. If the pace chosen results in a goal below this minimum, then the goal will be raised to the minimum, and the adjustment shown in the result section will also be recalculated based on the minimum rather than the desired value.

Adapt your diet to your target weight.

The calculator developed by the Australian government has some suggestions that are not in the American tools. If your goal is weight loss, enter your ideal weight rather than your current weight. That's a clever idea. You don't have to pick a particular calorie deficit. When you eat the amount of calories necessary to maintain your ideal weight, the calculator will automatically give you the deficit.

This also has properties that are not explained on either side. As your base rate goes down when you lose weight, the difference between how many calories you take in and how many you need to maintain will gradually decrease as you get closer to your goal. The progress is not linear but slows down gradually.

All five formulae are linear in weight so the mathematics is correct. If Basal Metabolic Rate is a sum of a value and product of b and weight then the process of approaching the target is an exponential process with time constant being number of days obtained by dividing 7,700 by b.

For a sedentary man using the Mifflin formula b is about 12 kcal per kg so the time constant is about 640 days. It takes just over four years to achieve a difference of 90%. For this reason, the results panel shows the number of weeks until you reach 90% of your target weight, not the total number of weeks to reach your goal. If you want to achieve your goal this year, switch to "Pace" mode.

Alternate phases of calorie surplus and deficit.

The calorie cycling method involves eating more calories on some days and fewer on others while keeping the total amount per week the same. A common argument for this method is that otherwise your body will tend to adapt when food intake stays constant, another practical benefit being that it allows you to enjoy a more generous evening meal on Saturday without feeling guilty.

The two plans on this page are designed so that the arithmetic results are exact and not approximate. Plan A involves eating more calories for two days (the target amount is increased by a certain value), while eating fewer calories for five days (the target amount is reduced by two-fifths of the value). These two values balance out structurally. Plan B is a symmetric plan with incremental changes, where the sum of deviations is zero.

You can check this yourself in the table. The row for total amount per week and the row for evenly distributed over seven days are always identical. This is an honest summary of the calorie cycle: It only changes when you take in calories, but not the total amount.

Cross-validation self-assessment

The calorie chart in the "Dietary Guidelines for Americans" was created using IOM formulas for normal build adults and rounded to nearest 200 kcal. This can be used as a free test to check if the formulas on this page are correct.

Let's take the typical male according to the guidelines: he is 1.78m tall and weighs 69.9kg. At a low activity factor of 1.11 this formula gives 2,675kcal (for an age of 30) or 2,532kcal (for an age of 45). In the published table for all ages between 26 and 45 it says 2,600kcal. If you do the same calculation for a typical female with a height of 1.63m and a weight of 57.2kg you get values between 2,095 and 1,964kcal while the published value is 2,000kcal. In each age group in the table below these results are reproduced in exactly the same way so that you can check whether the formulae used on this page match those used in the guidelines.

This exercise also provides some valuable insight. The "moderate activity" category in the guidelines does not correspond to an activity factor but rather the IOM's low activity factor. If you calculate standard values for men and women using an activity factor, these values exceed published values in every age group. The terms used in the two systems are not comparable. This reflects the main point of this page. The designation of the amount of activity is more important than the equation itself, but no one presents it in the same way.

How was food energy originally calculated?

The calorie count on the label is not measured by burning the food; it's calculated using fixed conversion factors for the components in the food such as protein, fat and carbohydrates.

These are the factors used by European food information regulations and also the American system uses the same labels for the three main nutrients (4, 4, 9). It is important to note that these two columns do not represent exact conversions. 37 kilojoules divided by 4.184 equals 8.84, not 9. Since both columns are rounded separately for labelling purposes, a calculator tool converting kilojoules into calories will show the fat value as 8.8 and the protein value as 4.1. At least one of the result pages uses this method.

What this calculator cannot do is:

It is designed for adults 18 and older. There are separate DRI equations for children and adolescents that include growth factors, and the adult equation does not calculate need correctly. It also doesn't take into account pregnancy or breastfeeding. In these situations daily caloric needs increase by hundreds of calories, and there are specific guidelines for those. It can't recognize illnesses, thyroid conditions, medications, recent significant weight loss, or other conditions that alter BMR to a degree beyond the differences mentioned above. If any of those apply to you, the numbers on this page should only be used as a starting point for discussion with your doctor and are not definitive answers.