What is a calorie deficit?
A calorie deficit means you eat fewer calories than your body uses—your total daily energy expenditure (TDEE). Over time, that gap can draw on stored energy, often body fat. Many plans use about 500 kcal/day below maintenance as a starting point; that is not a promise of one pound lost per week and is not right for everyone.
Key numbers
- At weight stability, energy in from food and drink is roughly balanced with energy out (TDEE)—the sum of resting metabolism, activity and the thermic effect of food.[1]
- NHLBI weight-loss guidance cites reducing daily intake by 500 to 1,000 kcal per day from usual intake; actual loss varies with adherence, water shifts and metabolism.[2]
- NIH guidance for adults with overweight or obesity often cites about 5–10% body weight over six months as a reasonable gradual target—not a fixed weekly scale number.[3]
- Build’s “lose” goal subtracts 500 kcal/day from its weight-based maintenance estimate (body weight in kg × 30–35 kcal), then sets protein at 1 g per lb.(MFM methodology)
- If you are starting a very low-calorie diet, talk with your doctor; large aggressive deficits can increase health risks and are not what MFM recommends.[4]
What is TDEE?
Total daily energy expenditure (TDEE) is the calories your body uses in a day—chiefly resting metabolism, physical activity (including non-exercise movement) and the thermic effect of digesting food. When intake matches TDEE, weight tends to stay fairly stable; when intake is lower, you are in a deficit.
MFM does not run a clinical TDEE lab test. Build estimates maintenance from body weight (kg × 30–35 kcal/day) as a quick planning number. That shortcut can differ from formula-based calculators; see How MFM works for how those estimates are built.
TDEE = Total Daily Energy Expenditure
Basal Metabolic Rate
– energy at rest
Non-Exercise Activity Thermogenesis
– everyday movement
Exercise Activity Thermogenesis
– planned exercise
Thermic Effect of Food
– digesting food
Example: a moderately active day
- BMR 65% — energy at rest
- NEAT 15% — everyday movement
- EAT 10% — planned exercise
- TEF 10% — digesting food
Eating below TDEE
Weight often trends down
1,500 kcal example
Eating at TDEE
Weight stays about steady
2,000 kcal example
Eating above TDEE
Weight often trends up
2,500 kcal example
Show data
Example: a moderately active day
BMR/REE about 60–70%, physical activity (NEAT + exercise) about 15–50%, TEF about 10%
| Component | Example share | Typical range | Example kcal | Notes |
|---|---|---|---|---|
| BMR — Basal Metabolic Rate | 65% | about 60–70% (BMR/REE) | 1,300 | Basal or resting energy expenditure—the largest share of TDEE for most adults; varies with body size, age and lean mass. |
| NEAT — Non-Exercise Activity Thermogenesis | 15% | physical activity (NEAT + exercise) about 15–50% | 300 | Non-exercise movement—posture, walking, chores and other daily activity outside planned workouts. |
| EAT — Exercise Activity Thermogenesis | 10% | physical activity (NEAT + exercise) about 15–50% | 200 | Planned exercise and sport—the structured part of activity thermogenesis. |
| TEF — Thermic Effect of Food | 10% | about 10% | 200 | Energy used to digest, absorb and store what you eat. |
Source: NCBI Bookshelf — Dietary Reference Intakes for Energy
| Scenario | kcal/day | vs example TDEE | Notes |
|---|---|---|---|
| Below TDEE | 1,500 | −500 kcal/day | A calorie deficit for gradual fat-loss planning. MFM Build’s “lose” goal uses this same 500 kcal offset from its maintenance estimate—not a guarantee of results. |
| At TDEE | 2,000 | 0 kcal/day | Estimated maintenance—energy in roughly matches energy out. Build’s “maintain” goal keeps this offset at zero. |
| Above TDEE | 2,500 | +500 kcal/day | A surplus for weight gain or muscle-building phases. Build’s “gain” goal adds 500 kcal/day to its maintenance estimate. |
How big should a deficit be?
Smaller deficits are easier to sustain and are less likely to feel punishing. NIH materials for adults with overweight or obesity often discuss gradual loss on the order of about 5–10% of body weight over six months rather than chasing the fastest possible scale drop.
MFM does not recommend very large deficits. If you are considering an aggressive cut, a medically supervised program, or you have a history of disordered eating, talk to a doctor or registered dietitian instead of DIY calorie math on this site.
Why doesn’t the scale match a neat calorie formula?
Water, glycogen, lean tissue and day-to-day intake all change what the scale shows, even when you are in a deficit.
Fat loss slows and adapts over time; plateaus are common. Use deficits as a meal-planning tool, not a contract with the scale.
How do I plan meals in a deficit?
Pick a calorie and macro target you can hit with real food, keep protein adequate (MFM defaults to 1 g per lb and explains why on Build), and adjust based on hunger, performance and clinician advice—not guilt.
After you eat part of the day, Macros Left subtracts what you logged from the same targets. For a full day of meals at your numbers, open Build or generate a deterministic day on Plan.
How MFM uses this
- Build (lose goal) Sets maintenance from weight, subtracts 500 kcal/day for “lose,” and fills protein, carbs and fat targets you can edit.
- Macros Left See what fits after partial-day intake using the same estimate philosophy.
- Plan Build a full day of recipes at your kcal and macro query—useful once your deficit targets are set.
- How MFM works Read how maintenance kcal and diet filters are estimated—not lab-verified TDEE.
What we can't do (yet)
- Build maintenance is a weight-based estimate, not Mifflin–St Jeor TDEE and not a metabolic test.
- Recipe and food kcal are editorial estimates; logging errors and restaurant meals add uncertainty.
- No rate-of-loss predictions, date goals or before/after promises on MFM.
- Not for allergy screening, clinical diets, or starting a very low-calorie diet without talking to your doctor first.
Questions
What is a calorie deficit in plain terms?
It is eating fewer calories than your body burns in a day (your TDEE). Over time that gap may come partly from stored fat, but the scale also reflects water and other tissue—not just fat.
How many calories should I eat to lose weight?
There is no single number for everyone. A common starting point is maintenance minus a modest deficit—MFM Build uses 500 kcal/day below its maintenance estimate for “lose.” Adjust with a clinician if you have health conditions, are pregnant or have an eating-disorder history.
Is a 1,000-calorie deficit safe?
Large deficits can increase risks and are hard to sustain. If you are starting a very low-calorie diet, talk with your doctor. MFM does not promote aggressive deficits; ask a doctor or dietitian what is appropriate for you.
Why doesn’t MFM guarantee weight loss?
Real life includes mis-estimated portions, adaptive metabolism, stress, sleep and medical factors. We show planning math and meals—not outcomes. Nutrition values on the site are estimates and not medical advice.
How does MFM’s lose goal relate to TDEE?
Build estimates maintenance as body weight (kg) × 30–35 kcal, then subtracts 500 kcal for the lose goal—the same offset shown in the chart on this page. That is a meal-planning shortcut, not a measured TDEE.
General education, not medical advice. Nutrition values on MFM are estimates. For kidney disease, pregnancy or breastfeeding, work with a clinician on calorie and protein targets. If you have an eating-disorder history, skip DIY calorie math and talk to a doctor or registered dietitian; in the U.S., call or text 988 (Suicide & Crisis Lifeline) and the National Alliance for Eating Disorders helpline at 866-662-1235 (weekdays, Eastern time).
Try it
Open Build with a fat-loss goal and a 500 kcal deficit offset applied to your maintenance estimate.