How Big Should a Calorie Deficit Be?
Subtract 500 calories from a 1,700 kcal maintenance estimate and you have cut just over 29%, while the same 500 taken off 3,000 is a cut of about 17%. One preset, two completely different diets. The calorie calculator will lay out scenarios for you, and whether losing weight suits you at all is a question for you and your doctor.
Why 500 calories does not guarantee a pound a week
The familiar 3,500 kcal-per-pound rule is a simplified energy sum. Your body weight carries water and lean tissue as well as fat, and your expenditure falls as you get smaller and as your activity changes. Straight-line predictions drift further from reality the longer you run them, and the NIDDK Body Weight Planner accounts for changing energy requirements instead of promising you a fixed weekly result.
Compare the three cuts
Say your estimated maintenance is 2,400 kcal a day, which is what the TDEE calculator prints for a moderately active adult of average size. A 10% reduction takes off 240, giving 2,160. A 15% reduction takes 360, giving 2,040. A flat 500 gives 1,900. Same starting estimate, three different plans.
Change the starting point and the flat preset changes character completely: it is 29% of a small maintenance and 17% of a large one, which is why a fixed preset needs your own context before it means anything. A worked example also sets no safe target, no deadline and no predictable amount of fat loss.
How low is too low
The 2013 AHA/ACC/TOS obesity guideline puts the lowest intake worth running without clinical supervision at 1,200 to 1,500 calories a day for women and 1,500 to 1,800 for men. Those are the floors most clinicians work from, and they exist because below them your plate runs out of room before protein, calcium, iron and the B vitamins are covered. The NHLBI evidence report is blunter about the next step down: anything under 800 calories a day belongs under medical supervision, and the extra restriction buys no additional weight loss a year later.
Read 1,200 and 1,500 as a stop sign. A 25% cut off the 1,700 kcal maintenance estimate at the top of this page lands at 1,275, which is already at the floor for a woman and under it for a man. When the only way to balance the numbers is down there, add activity or take a smaller cut over a longer stretch. Eating below those figures is a decision for your dietitian or your doctor.
What to consider before using a target
Food quality, adequate nutrition, health conditions, medication, activity and your relationship with food all sit alongside the calorie total. A low number leaves too little room to eat well. Your BMR is a prediction and not a line you have to clear, as the BMR and TDEE guide explains.
Resistance exercise and adequate protein help you hold onto muscle. An aggressive deficit still costs you something, and your training volume often needs adjusting for recovery. Run one body weight through the macro calculator at a 10% deficit and again at 25%, and the pattern is plain: protein and fat keep their per-kilogram figures while your carbohydrate absorbs the whole cut. A dietitian can fit the plan to your circumstances, which is more than any preset can do.
Interpreting progress
Weigh under comparable conditions and read the trend across several weeks. A single gain is mostly water, and a single fast drop is mostly water too. Read about water and scale changes before you touch your intake.
When progress slows, look at the estimate, the routine and the measurement error before you cut again. Persistent fatigue, dizziness, missed periods, worsening training or distress around food are reasons to seek professional advice.
These examples are general education for healthy adults. Children, pregnancy, breastfeeding, eating-disorder treatment and medically prescribed diets all need an assessment this page cannot give. Diet breaks help some people stick with a plan, and the schedule is yours to pick, because no universal one exists and no metabolic reset comes with them. See NHLBI information on individualized weight-management treatment.
Scope and correction note, 13 September 2026: This guide provides general adult education. It has been revised to distinguish estimates from measurements and to remove unsupported guarantees. It has not received independent clinical review. See the editorial policy and medical disclaimer.