A calorie calculator prints four numbers and looks authoritative. The useful
question is where each number comes from, because two of the four are pure
arithmetic and two depend on a multiplier you pick by feel. Our tool runs the
Mifflin-St Jeor equation, and this article walks the exact path from your
inputs to each printed figure, including the parts the tool simplifies away.
The two core numbers
The tool prints BMR first. Basal metabolic rate is the estimated energy a
body uses at complete rest. The tool then prints maintenance calories, which
is BMR scaled by the activity multiplier you select.
BMR male = 10 x weight(kg) + 6.25 x height(cm) - 5 x age + 5
BMR female = 10 x weight(kg) + 6.25 x height(cm) - 5 x age - 161
maintenance = BMR x activity multiplier
Mifflin-St Jeor is an estimation equation for healthy adults, and our tool
presents it as an estimate. It does not know your body composition, and it
was not validated for every population. Treat its output as a starting point
for a two-week experiment with your own weight trend, not as a prescription.
The five multipliers, and how much they move the target
The activity selector offers five levels with fixed multipliers taken from
the standard TDEE convention.
| Level | Description in the tool | Multiplier |
| Sedentary | Little or no exercise | 1.2 |
| Lightly active | Exercise 1-3 days per week | 1.375 |
| Moderately active | Exercise 3-5 days per week | 1.55 |
| Active | Hard exercise 6-7 days per week | 1.725 |
| Very active | Very hard exercise, physical job | 1.9 |
The default selection is moderately active at 1.55. The step size between
adjacent levels matters more than most people expect. For a BMR of 1648.75,
moving from lightly active to moderately active adds 288.53 kcal per day.
That single selector decision swings your target by more than many people
manage to change through diet discipline in a first week.
Use a concrete rule to pick the level. Count only planned exercise sessions
from the last two weeks and trust the lower option when you sit between two
descriptions.
A worked example, checked against the code
Enter male, age 30, weight 70 kg, height 175 cm, moderately active. The BMR
computation gives 1648.75, and the tool rounds the display to 1649.
Maintenance is 1648.75 x 1.55 = 2555.5625, displayed as 2556.
Enter female with the same three measurements. BMR is 1648.75 minus 166,
which is 1482.75, displayed as 1483. Maintenance is 1482.75 x 1.55 =
2298.2625, displayed as 2298.
Note the rounding behavior. The tool rounds each printed figure from the
unrounded chain, so your own recomputation can land one kilocalorie away if
you start from a rounded number. Recompute maintenance from the printed
female BMR of 1483 and you get 1483 x 1.55 = 2299, while the tool prints
2298 because it chains from the unrounded 1482.75.
The flat 500 and its honest limits
The tool prints two extra lines, maintenance minus 500 and maintenance plus
500. The 500 kcal figure is a fixed heuristic for gradual weight change, and
the tool applies it to everyone regardless of body size.
That flatness has a cost. At a maintenance of 2300, a 500 kcal cut is about
22 percent of intake. At a maintenance of 1500, the same cut is a third of
intake. The smaller your maintenance number, the more aggressive the same
500 becomes. Apply one condition to catch this. If the printed deficit falls
below the point where you can eat normally and feel well, treat that as a
signal to review the plan with a qualified professional rather than a target
to hit. We state that as a caution, because our tool is not a medical device
and draws no line for any individual.
The equation also has known blind spots. It carries age as a straight
deduction, it ignores body composition, and it does not apply to pregnancy,
illness, or adolescent growth. None of those conditions belong inside this
calculator.
Metric inputs, and one silent failure
The tool accepts kilograms and centimeters only. If you measure in imperial
units, convert first using the same constants our unit converter stores. One
pound is 0.453592 kg, so 154 lb is 69.85 kg. One inch is 2.54 cm, so 68 in
is 172.72 cm.
Validation is silent. If age, weight, or height is empty, zero, or negative,
clicking Calculate does nothing and shows no message. When the button seems
dead, one of the three fields failed that check.
Steps to use the output responsibly
1. Convert your weight and height to kg and cm before entering them.
2. Pick the activity level by counting real sessions from the last two
weeks.
3. Record BMR, maintenance, deficit, and surplus.
4. Eat near your chosen target for two weeks while logging morning weight.
5. If weight trend disagrees with the prediction by a wide margin, adjust
the activity level and rerun before changing anything else.
Checklist before you act on the numbers
- Inputs are in kg and cm, converted with the constants above.
- Activity level reflects an honest two-week count.
- You know which printed lines are arithmetic and which depend on your
multiplier choice.
- The deficit line stays in a range you can sustain, and you flagged anything
extreme for professional review.
- You plan to validate with your own weight trend within two weeks.
Run your own figures through our calorie calculator at
/calorie-calculator. If your two-week weight trend contradicts the printed
maintenance number, send us the inputs and the trend, because that mismatch
is real measurement data and it beats any equation on this page.
