Key takeaways
- Your target is your TDEE minus 15–20% — not a number you read somewhere.
- Size the deficit as a percentage, not a flat 500 kcal. The same cut is mild for a 2,800 kcal body and severe for a 1,700 kcal one.
- 0.5–1% of body weight per week is the rate that reliably protects muscle.
- The deficit shrinks on its own as you get lighter, so recalculate every 5% of body weight lost.
- Do not eat back exercise calories if your target already used an activity multiplier.
The honest answer is a formula rather than a figure: eat 15–20% fewer calories than you burn. For most adults that lands between 1,500 and 2,400 calories a day, because daily expenditure itself ranges from roughly 1,800 to 3,000 depending on size, sex, age and how much you move. Anyone who gives you a single number without knowing those four things is guessing.
Start with what you burn, not with a diet number
Every workable calorie target is built the same way: estimate maintenance, then subtract. Maintenance is your Total Daily Energy Expenditure, and it comes from two pieces — the calories your body spends simply existing, and a multiplier for everything you do on top of that.
Resting expenditure is estimated with the Mifflin-St Jeor equation, which has the lowest average error across general populations and is the formula the 2BIB TDEE calculator uses. Then it gets multiplied:
| Activity level | Multiplier | Looks like |
|---|---|---|
| Sedentary | 1.2 | Desk job, little or no 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 |
| Extremely active | 1.9 | Hard exercise plus a physical job |
The multiplier is where most estimates go wrong. Most people who train three times a week are 'lightly active', not 'very active' — the other 165 hours of the week count too.
Pick the level below the one you want to pick
Over-rating your activity level is the single most common reason a calculated target does not produce weight loss. Three gym sessions on top of a desk job is a 1.375 week, not a 1.725 one. If you are unsure, take the lower multiplier — you can always adjust upward after three weeks of real weight data.
How big should the deficit be?
Between 15% and 20% of your TDEE for most people. That range is large enough to produce visible progress within a month and small enough that you can hold it while training, working and sleeping normally.
| Deficit | At 2,000 kcal TDEE | At 2,800 kcal TDEE | Rough rate | Suits |
|---|---|---|---|---|
| 10% | 1,800 kcal | 2,520 kcal | 0.2–0.35 kg/wk | Lean, or prioritising training |
| 15–20% | 1,600–1,700 kcal | 2,240–2,380 kcal | 0.4–0.7 kg/wk | Most people, most of the time |
| 25% | 1,500 kcal | 2,100 kcal | 0.7–1.0 kg/wk | Higher starting body fat, short blocks |
| 30%+ | 1,400 kcal | 1,960 kcal | Fast, then stalls | Rarely worth it outside supervision |
Rates are approximate and assume consistent logging. Larger deficits take a bigger share of their weight from lean tissue and get harder to hold each week.
Notice what the percentage framing fixes. A flat 500 kcal cut is a quarter of a 2,000 kcal TDEE and under a fifth of a 2,800 kcal one — the same advice producing a moderate diet for one person and an aggressive one for another. The percentage scales itself.
The 0.5–1% rule as a sanity check
Whatever number the arithmetic gives you, check the resulting rate against your body weight. Losing 0.5–1% of body weight per week is the band where muscle is well protected: 0.45–0.9 kg a week at 90 kg, 0.3–0.6 kg a week at 60 kg. If your target predicts faster than that, the deficit is too large for your size.
15–20%
Deficit as a share of TDEE
The default for most people starting out
0.5–1%
Body weight lost per week
The rate that preserves lean mass
5%
Body weight lost before recalculating
Roughly every 4–5 kg for a 90 kg adult
Why the 3,500-calorie rule overpromises
A pound of body fat stores roughly 3,500 calories, so a 500 kcal daily deficit should cost you a pound a week — forever. It does not, and the reason is not willpower. The rule assumes your expenditure stays fixed while your body shrinks.
It does not stay fixed. A lighter body has less tissue to maintain and less mass to move, so TDEE falls as you lose. Hold intake constant and the deficit quietly narrows month by month until it reaches zero — which is what most people are actually experiencing when they say a diet “stopped working”. The plateau post works through that arithmetic in detail, including the two other losses that compound with it.
The recalculation is the part people skip
2BIB recalculates your TDEE as your weight changes and compares real progress against your weekly milestones. When the two diverge, your calorie and macro targets move with them — so the number you are eating to is never six weeks out of date.
How low is too low?
The commonly cited floors are about 1,200 calories a day for women and 1,500 for men. They are not magic thresholds; they are the point below which it becomes genuinely difficult to hit protein and micronutrient needs from food, and below which most people cannot sustain training or concentration.
If your TDEE minus 20% lands under those numbers, the answer is not to eat less. It is to raise the other side of the equation — more daily movement, resistance training to hold onto the muscle that keeps expenditure up — and to accept a slower rate for a while.
When to involve a clinician
Very-low-calorie diets, and any calorie target set while you are pregnant or breastfeeding, managing diabetes or thyroid disease, taking a GLP-1 or other weight medication, or recovering from an eating disorder, belong with a doctor or registered dietitian rather than a calculator. This article is general education, not medical advice.
Do not add exercise calories on top
If you set your target using an activity multiplier, your training is already in the number. Eating back the calories a watch says you burned counts the same workout twice, and the second count is the unreliable one — wrist wearables have shown energy-expenditure errors from around 27% to over 90% against laboratory measurement, even while reading heart rate almost perfectly. Why calorie burn estimates miss so badly covers the mechanism.
Do this
Not this
What goes inside the target matters too
Calories decide whether you lose weight; protein and training decide how much of that loss is fat rather than muscle. In a deficit, aim for 1.6–2.2 g of protein per kg of body weight — see the protein guide for how to distribute it — and keep lifting something heavy twice a week. The macro calculator splits a calorie target into daily protein, carb and fat grams if you want the numbers done for you.
Protein also does the most work per calorie on satiety, which makes the deficit itself easier to hold. That is the practical argument for it, ahead of any argument about metabolism.
Setting your number, in order
- 1Calculate your TDEE with today’s weight and an honest activity level.
- 2Subtract 15–20%. Round to something memorable — the precision is false anyway.
- 3Check the predicted rate against 0.5–1% of your body weight per week, and against the 1,200/1,500 floors.
- 4Hold it for three weeks and log everything, including weekends. Two weeks is not enough signal to judge.
- 5Compare weekly averages, not daily weights. If the trend has not moved, the log is the first thing to audit — not the target.
- 6Recalculate every 5% of body weight lost, or any time progress stalls for two to three weeks.
The number is a starting hypothesis
Every calorie formula is a population average applied to one person, and individual expenditure varies by several hundred calories around any estimate. So treat the calculated target as a hypothesis and let three weeks of weight data test it: losing faster than 1% a week, eat a little more; nothing moving after three honest weeks, take 150–200 calories off — not 500.
- Trend, not readings. Weigh at the same time daily, compare week against week.
- Audit the log before cutting further. Untracked oils, drinks and weekends erase deficits more often than a wrong target does.
- Plan maintenance breaks. Eight to twelve weeks in a deficit, then a week or two at maintenance, by design.
For the underlying mechanics see what TDEE is and why it moves, and the full fat loss guide for how the target fits into everything else.