I am a developer, not a dietitian, so when I decided to get my own nutrition in order a couple of years ago I did what developers do: I tried to find the underlying number. Not a fad, not an app that shames you, just the one figure everything else hangs off. That figure is TDEE - Total Daily Energy Expenditure - and once I understood how it is actually calculated, the whole subject stopped feeling like guesswork and started feeling like arithmetic I could trust. That is the same instinct that led me to build the calculators on toolz.dev: I would rather see the formula than be handed a magic answer.
This guide walks through what TDEE and BMR mean, the equation the good calculators use, how activity level turns a resting number into a daily one, and how to turn all of it into a calorie target you can eat to. You can run your own numbers in the free TDEE Calculator as you read - like everything on the site, it computes entirely in your browser, so your age and weight never touch a server.
TL;DR: Your TDEE is roughly your BMR (the calories you burn at rest) multiplied by an activity factor between 1.2 and 1.9. Eat around your TDEE to maintain weight, below it to lose, above it to gain. The best predictive formula for BMR is Mifflin-St Jeor. Any calculator gives an estimate - expect a margin of error of around ten percent, and adjust based on how your weight actually moves over two to three weeks rather than trusting the number blindly.
What is TDEE, and how is it different from BMR?
Two acronyms do most of the work here, and keeping them straight is half the battle. The equation underneath is Mifflin-St Jeor (1990), which replaced Harris-Benedict because it predicted resting energy expenditure more accurately in modern populations.
BMR is your Basal Metabolic Rate - the number of calories your body would burn in twenty-four hours if you did absolutely nothing but lie still. It is not zero, and it is not small. Keeping your heart beating, your lungs moving, your brain running, and your body temperature stable is metabolically expensive; for most adults BMR sits somewhere between 1,300 and 1,900 calories a day. It is the floor of your energy needs, the cost of simply being alive.
TDEE is your Total Daily Energy Expenditure - BMR plus everything else you do. Walking to the kitchen, typing, fidgeting, exercising, even digesting food all burn calories on top of the basal cost. Because nobody actually lies still all day, TDEE is always higher than BMR, and it is the number that matters for planning what to eat. When people say "maintenance calories," TDEE is what they mean: eat that amount and your weight holds steady.
The relationship is simply TDEE = BMR × an activity factor. The TDEE Calculator shows you both numbers side by side, which is genuinely useful - seeing that your BMR is 1,650 but your TDEE is 2,550 makes it obvious how much of your daily burn comes from movement, and how much room a more active week actually buys you.
Which formula should a TDEE calculator use?
Not all BMR formulas are equal, and the one a calculator picks changes the answer.
The oldest is Harris-Benedict, published in 1919 and revised in 1984. It was the standard for decades and still shows up in older tools. The problem is that it was derived from a small, non-representative sample and tends to overestimate BMR, especially for people carrying more body fat.
The modern default is Mifflin-St Jeor, published in 1990. When researchers compared the common predictive equations against measured resting energy expenditure, Mifflin-St Jeor came out as the most accurate for the general adult population, and it is now the formula most dietitians reach for first. The TDEE Calculator uses it. The equation is worth seeing in the open:
- Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
- Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161
The only difference between the two is the final constant, +5 versus −161, which reflects the average difference in lean body mass. Plug in 70 kg, 175 cm, and age 30 for a man and you get 1,649 calories - the calculator shows you exactly this working so the number is never a black box.
There is a third formula, Katch-McArdle, which bases BMR on lean body mass rather than total weight. It can be more accurate if you know your body fat percentage precisely, but most people are guessing at that figure, and a wrong body-fat input makes Katch-McArdle worse than Mifflin-St Jeor, not better. For the vast majority of people, Mifflin-St Jeor with honest height and weight is the right default.
How do activity levels turn BMR into TDEE?
The activity multiplier is where estimates most often go wrong, because it is the one input that is subjective. BMR is arithmetic on hard numbers; the activity factor is your honest assessment of how much you move, and honesty is harder than it sounds.
The standard multipliers, drawn from the Physical Activity Level (PAL) research, run like this:
| Activity level | Multiplier | Roughly means |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no deliberate exercise |
| Lightly active | 1.375 | Light exercise 1–3 days a week |
| Moderately active | 1.55 | Moderate exercise 3–5 days a week |
| Very active | 1.725 | Hard exercise 6–7 days a week |
| Extra active | 1.9 | Physical job, or training twice a day |
The trap is almost universal: people overestimate. A few gym sessions a week feels active, but if the other twenty-three hours of each day are spent sitting, "lightly active" is often closer to the truth than "very active." And the error compounds - the difference between a 1.375 and a 1.55 multiplier on a 1,650 BMR is nearly 300 calories a day, which is the difference between slowly losing weight and slowly gaining it. My advice, and the advice most coaches give, is that when you are torn between two levels, pick the lower one. You can always adjust upward if the scale is not moving.
This is also why the number is a starting point rather than a verdict. The multiplier is a population average applied to your individual life, and your real activity varies week to week. Treat the TDEE Calculator's output as hypothesis one, then let reality refine it.
How many calories should I eat to lose or gain weight?
Once you have your TDEE, turning it into a goal is straightforward arithmetic built on one well-worn approximation: roughly 3,500 calories equals about 0.45 kg (one pound) of body weight. Spread over a week, a daily deficit of 500 calories works out to about a pound of loss a week, and 250 a day to about half that.
The calculator lays this out as a table so you do not have to do the sums:
| Goal | Daily calories | Expected change |
|---|---|---|
| Weight loss | TDEE − 500 | ≈ 0.45 kg (1 lb) per week |
| Mild weight loss | TDEE − 250 | ≈ 0.25 kg (0.5 lb) per week |
| Maintain | TDEE | Weight holds steady |
| Mild weight gain | TDEE + 250 | ≈ 0.25 kg (0.5 lb) per week |
| Weight gain | TDEE + 500 | ≈ 0.45 kg (1 lb) per week |
Two cautions matter here. First, aggressive deficits backfire. Cutting far below your TDEE costs you muscle alongside fat, tanks your energy, and is miserable to sustain, which is why most guidance warns against dropping below roughly 1,200 to 1,500 calories a day without professional supervision. Slower is more durable. Second, the 3,500-calorie rule is a useful approximation, not a law of physics - real metabolism adapts as you lose weight, so a plan that worked at the start needs re-checking every few weeks. If you want to reason about the percentages involved - say, what a 15% deficit actually is in calories - the percentage calculator does that arithmetic cleanly.
What about macros - how should I split protein, carbs, and fat?
Calories decide whether you lose or gain weight; macronutrients decide how you feel and, to a degree, what that weight is made of. The TDEE Calculator suggests a balanced 30% protein, 40% carbohydrate, 30% fat split of your maintenance calories as a starting point, and converts each percentage into grams so you can actually plan meals.
The conversion rests on the energy content of each macronutrient: protein and carbohydrate provide about 4 calories per gram, and fat about 9. So on a 2,550-calorie TDEE, 30% protein is 765 calories, which is about 191 grams; 40% carbohydrate is 1,020 calories, about 255 grams; and 30% fat is 765 calories, about 85 grams. Seeing it in grams rather than percentages is what makes it usable at the supermarket.
That 30/40/30 split is a reasonable default, not a commandment. If you are losing fat or building muscle, many people push protein higher - 1.6 to 2.2 grams per kilogram of body weight is a common target - and adjust carbs and fat around it to preference. Endurance athletes lean more heavily on carbohydrate. The right split is the one you can stick to while hitting your protein floor, so treat the calculator's numbers as a sensible middle you can tune. Converting between metric and imperial units as you plan is easy with the unit converter if your recipes and your scale disagree.
How does TDEE relate to BMI and other body metrics?
People often calculate their TDEE and their BMI in the same sitting, so it is worth being clear on what each tells you, because they answer different questions.
TDEE is about energy - how many calories you burn, and therefore how many to eat. It is a forward-looking planning number. BMI, which you can work out with the BMI Calculator, is about proportion - a rough screen of weight relative to height that sorts you into categories like underweight, healthy, or overweight. BMI says nothing about calories, and TDEE says nothing about whether your current weight is in a healthy range. They are complementary, not interchangeable.
Both share the same honest limitation, though, and it is worth stating plainly: they are population-level estimates applied to an individual. BMI famously mislabels muscular athletes as overweight because it cannot tell muscle from fat. TDEE cannot see your genetics, your hormones, your medical history, or the day-to-day noise in your activity. Neither is a diagnosis, and neither replaces a conversation with a doctor or a registered dietitian - especially if you have a health condition, are pregnant or breastfeeding, or are calculating for a child, for whom these adult formulas simply do not apply. Use them the way I do: as a rational starting point that turns a vague goal into a concrete number, then let real-world feedback do the fine-tuning. I keep this and the rest of my everyday utilities close at hand, and rounded them up in the web developer toolkit; the reason they all run in the browser rather than on a server is the same privacy stance I laid out in the guide to data privacy with online tools - your body metrics are nobody's business but yours.
Frequently asked questions
What is TDEE and how is it calculated?
TDEE (Total Daily Energy Expenditure) is the total number of calories you burn in a day. It is calculated by first estimating your Basal Metabolic Rate (BMR) - the energy your body uses at rest - and then multiplying it by an activity factor between 1.2 and 1.9 that reflects how active you are. The result is your daily maintenance calorie level.
What is the difference between BMR and TDEE?
BMR is the number of calories your body would burn if you did nothing but rest for 24 hours - it covers breathing, circulation, and keeping your organs running. TDEE is your BMR plus the energy spent on everything else: walking, exercise, digestion, and daily movement. TDEE is always higher than BMR, and it is the number you plan your diet around.
Which formula does this TDEE calculator use?
It uses the Mifflin-St Jeor equation for BMR: for men, BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5; for women the final term is −161 instead of +5. Studies comparing predictive equations found Mifflin-St Jeor to be the most reliable for most people, which is why it is the modern default over the older Harris-Benedict formula.
How accurate is a TDEE estimate?
It is a good starting estimate, typically within about 10% of measured values for most people, but it is not exact. Predictive equations cannot account for individual differences in body composition, genetics, hormones, or medical conditions. Treat the number as a hypothesis: track your weight and intake for two to three weeks and adjust your calories up or down based on how your body actually responds.
How many calories should I eat to lose weight?
To lose weight you eat below your TDEE. A deficit of about 500 calories per day corresponds to roughly 0.45 kg (1 pound) of weight loss per week, and 250 per day to about half that. Avoid very aggressive deficits, which cost muscle and are hard to sustain, and most guidance advises not going below roughly 1,200 to 1,500 calories a day without professional supervision.
What activity level should I choose?
Pick the level that matches a typical week, counting deliberate exercise, not just how you feel. "Sedentary" fits a desk job with little exercise; "moderately active" fits training 3–5 days a week; "extra active" is for physically demanding jobs or twice-daily training. Most people overestimate this, which inflates the result, so when in doubt choose the lower option.
How should I split my macros?
A balanced 30% protein, 40% carbohydrate, 30% fat split of your maintenance calories is a good starting point, and the calculator converts each into grams. Protein and carbohydrate provide about 4 calories per gram and fat about 9. Your ideal split depends on your goals and preferences - higher protein is common when losing fat or building muscle - so adjust the ratios to suit you.
Is this calculator suitable for everyone?
It uses an adult equation and is intended for generally healthy adults. It is not designed for children, and pregnancy, breastfeeding, and certain medical conditions change energy needs in ways a predictive formula cannot capture. It provides general information, not medical or dietary advice - consult a doctor or registered dietitian before making significant changes, especially if you have a health condition.
Written by Liton - builder of toolz.dev, WP Adminify, and a long list of Laravel and React projects. This guide is general information, not medical advice. Every tool mentioned runs free and fully in your browser at toolz.dev.
