Lose Weight
Calorie Deficit Calculator
To lose weight, eat 250 to 750 calories below your maintenance calories each day. This calculator finds your maintenance with the Mifflin-St Jeor formula, subtracts your chosen deficit, and shows your daily calorie target, weekly loss rate, and a projected goal date.
Your plan
Learn how this works
How to Calculate a Calorie Deficit
A step-by-step guide to what a calorie deficit is, how the maintenance-minus-deficit math works, and how to pick a safe daily target.
Read the guideKey facts about calorie deficits
- A 500 kcal daily deficit produces roughly 0.45 kg (1 lb) of weight loss per week.
- One kilogram of body mass stores approximately 7700 calories of energy.
- SmartDietChoices never sets a calorie target below 1200 kcal per day for women or 1500 kcal per day for men.
- Losing 5 to 10 percent of starting body weight is linked to measurable improvements in blood pressure, blood sugar, and blood lipids.
How does a calorie deficit make you lose weight?
Weight loss comes down to energy balance. When you eat fewer calories than you burn, your body makes up the difference from stored energy, mostly fat, and you lose weight. That gap between what you burn and what you eat is your calorie deficit. This tool estimates your total daily energy expenditure with the Mifflin-St Jeor equation (1990), the same math as our TDEE calculator, subtracts the deficit you choose, and projects how your weight falls over time. Every formula we use is documented on the methodology page.
The formula in plain language
Your maintenance calories are your basal metabolic rate multiplied by an activity factor. To turn calories into weight, we use the well established estimate that one kilogram of body mass holds about 7700 calories. A 500 calorie daily deficit is 3500 calories a week, which is close to half a kilogram of fat, or a little over one pound. The table below shows how each preset translates into weekly loss.
| Preset | Daily deficit | Weekly loss (kg) | Weekly loss (lb) |
|---|---|---|---|
| Gentle | 250 kcal | 0.23 | 0.5 |
| Steady | 500 kcal | 0.45 | 1.0 |
| Fast | 750 kcal | 0.68 | 1.5 |
Worked example
A 30 year old woman weighs 85 kg, is 168 cm tall, exercises lightly, and wants to reach 70 kg. Her maintenance is about 2185 calories. Choosing the Steady 500 preset sets her target at roughly 1685 calories a day. That produces about 0.45 kg of loss per week, so her 15 kg goal takes around 33 weeks, a little over seven months. As she loses weight her maintenance drops, so recalculating every 5 kg keeps the plan accurate.
How do you choose the right deficit?
The Gentle 250 and Steady 500 presets suit most people and are easy to sustain. The Fast 750 preset loses weight quicker but is harder to stick to and can cost more muscle, especially if protein and training are not on point. To protect lean mass while cutting, keep protein high with the protein calculator and set your full macro split with the macro calculator. If a flat deficit every day feels restrictive, the calorie cycling calculator lets you eat more on some days while keeping the same weekly total.
When should you not use this calculator?
This tool is built for healthy adults. Do not use it if you are under 18, pregnant or breastfeeding, recovering from an eating disorder, or managing a medical condition that affects appetite or metabolism; in those cases calorie targets belong in a conversation with a professional. It is also the wrong tool if your goal is gaining: the surplus math lives in the weight gain calculator.
Progress is rarely a perfectly straight line. Water weight, sodium, hormones, and digestion all move the scale day to day, so weigh under consistent conditions and watch the weekly average. Track your percentage lost with the weight loss percentage calculator to see milestones like 5 and 10 percent, which research links to real health improvements.
Activity multipliers behind your maintenance calories
Before subtracting a deficit, the tool estimates the calories you burn in a day by multiplying your basal metabolic rate by an activity factor. Picking the honest level matters more than any other input, because it moves your whole target. If in doubt, choose the lower option and adjust from real progress.
| Activity level | Multiplier | Typical week |
|---|---|---|
| Sedentary | 1.20 | Desk job, little planned exercise |
| Light | 1.375 | Light exercise 1 to 3 days |
| Moderate | 1.55 | Exercise 3 to 5 days |
| Very active | 1.725 | Hard exercise 6 to 7 days |
| Extra active | 1.90 | Physical job or two-a-day training |
How much weight will I lose, and how do I use the target?
Your projected goal date assumes an average weekly loss from your chosen deficit, so treat it as a plan, not a guarantee. To use this as a practical calories-to-lose-weight calculator, hold your target for two to four weeks, weigh under consistent conditions, and compare your real weekly average to the prediction. If the scale is not moving after three weeks of consistent eating, trim another 100 to 150 calories or add daily steps rather than making a single large cut. Very aggressive gaps, such as a 1000 calorie deficit, speed early loss but are hard to sustain and cost more muscle, so most people do better staying near the Gentle or Steady presets and recalculating every 4 to 6 kg.
Limitations of this calculator
This is an estimate built on population averages. The Mifflin-St Jeor equation is accurate to within roughly 10 percent for most adults, but genetics, daily movement, thyroid and other medical conditions, medications, and measurement error all shift real needs. The 7700 kcal per kg conversion is an average and early water-weight changes can mask fat loss for a week or two. The calculator cannot diagnose anything or account for your full medical picture, and it is not designed for people under 18, during pregnancy or breastfeeding, or while recovering from an eating disorder. Use it as a starting point and see a doctor or registered dietitian for individual advice. Every equation is documented on the methodology page, and our health disclaimer explains how to read these results.
Sources and references
- Mifflin MD, St Jeor ST, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990. PubMed 2305711
- National Institute of Diabetes and Digestive and Kidney Diseases. Body Weight Planner. niddk.nih.gov/bwp
- Centers for Disease Control and Prevention. Steps for losing weight. cdc.gov
- Hall KD, et al. Energy balance and its components: implications for body weight regulation. Am J Clin Nutr. 2012. PubMed 22357723
Formulas and constants used across the site are listed on the methodology page.
Frequently asked questions
How big should my calorie deficit be to lose weight?
A deficit of 250 to 500 calories per day suits most people, producing 0.25 to 0.5 kg of loss per week. Larger deficits work faster but are harder to sustain and cost more muscle. This tool never sets targets below 1200 calories for women or 1500 for men.
How is the projected goal date calculated?
One kilogram of body mass holds about 7700 calories. Your daily deficit times seven gives a weekly energy shortfall, which converts to weekly weight change and projects forward to your goal weight. Treat the date as a planning estimate, not a promise.
Why does weight loss slow down over time?
As you lose weight your body needs fewer calories, so the same intake becomes a smaller deficit. Daily movement often drops slightly too. Recalculating your target every 4 to 6 kg keeps the plan accurate.
Will I lose muscle in a calorie deficit?
Some lean mass loss is normal, but keeping protein at 1.6 to 2.2 grams per kilogram of body weight and doing resistance training protects most of it. Aggressive deficits increase muscle loss, so stay near the moderate presets.
Should I eat back exercise calories?
No. Your activity level multiplier already accounts for regular exercise, so adding exercise calories on top double counts them. If one day is unusually hard, a small increase is fine, but weekly tracking smooths this out.