Key takeaways
- Daily deficit = pace per week × 7,700 ÷ 7, so 0.5 kg (1 lb) a week needs about 550 kcal (500) a day.
- A percentage-based deficit fits small and large bodies more fairly than a fixed 500 kcal.
- Predicted loss assumes your TDEE stays the same; in reality it falls as you lose weight.
- A four-week review of your weight trend, food log and steps keeps the plan on track.
On this page
- What a calorie deficit calculator does
- The maths under the hood
- Deficit calculator or TDEE calculator?
- Fixed deficit or percentage?
- Choosing a pace
- Why the predicted date is only a guess
- A four-week review checklist
- It works for gaining weight too
- The warnings in our calculator
- Common mistakes
- Calorie deficit questions
What a calorie deficit calculator does
You give it your current details, a goal weight and how fast you want to get there. It gives you back a daily calorie target, the number of weeks the goal should take and an estimated finish date. Our calorie deficit calculator also compares four paces side by side and warns you if a plan is too aggressive.
The maths under the hood
TDEE = BMR (Mifflin-St Jeor) × activity factor Daily deficit = pace (kg a week) × 7,700 ÷ 7 Daily target = TDEE − daily deficit Weeks = kg to lose ÷ pace
The 7,700 figure is a rough rule for the energy in a kilo of body weight (3,500 kcal per pound). So a pace of 0.5 kg a week needs a daily deficit of about 550 kcal; 0.25 kg needs about 275; 1 kg needs about 1,100.
Example: a 30-year-old man, 175 cm and 80 kg, moderately active, has a TDEE of about 2,711 kcal. To lose 10 kg at 0.5 kg a week, the calculator sets his target at about 2,161 kcal and estimates 20 weeks.
Deficit calculator or TDEE calculator?
They answer different questions. A TDEE calculator asks how much you burn and shows targets for several goals at once, usually as percentages: lose fast, lose, lose slowly, maintain and so on. A deficit calculator starts from where you want to end up and how quickly, and tells you the daily calories and the date. Use the TDEE calculator to understand your numbers and the deficit calculator to plan a specific goal. Both rest on the same TDEE estimate, so if that estimate is off, both will be off by the same amount until you correct it with real data.
Fixed deficit or percentage?
Many calculators default to a fixed 500 kcal deficit. It is simple, but it treats everyone the same, which they aren’t:
| TDEE | 500 kcal is… | A 20% deficit is… |
|---|---|---|
| 1,700 kcal (smaller, less active) | 29% of TDEE | 340 kcal |
| 2,600 kcal (average) | 19% of TDEE | 520 kcal |
| 3,300 kcal (larger, active) | 15% of TDEE | 660 kcal |
For a smaller, less active person, 500 kcal is a steep cut that often pushes intake below 1,200 kcal. For a large, active person it is gentle. A percentage, usually 15 to 25% of TDEE, makes the effort similar for everyone. Choosing a pace as a share of body weight, such as 0.5 to 1% a week, does the same thing from the other direction.
Choosing a pace
| Pace a week | Daily deficit | Good for |
|---|---|---|
| 0.25 kg (½ lb) | ≈ 275 kcal | The last few kilos, smaller people, anyone who wants it to feel easy |
| 0.5 kg (1 lb) | ≈ 550 kcal | Most people; a good default |
| 0.75 kg (1½ lb) | ≈ 825 kcal | Larger bodies with more to lose |
| 1 kg (2 lb) | ≈ 1,100 kcal | Only people with a lot to lose and a high TDEE; often too fast |
As a rule, keep the pace at or below about 1% of your body weight a week. For a 70 kg person that is 0.7 kg; for 100 kg, 1 kg. Faster loss tends to cost muscle, increases hunger and is harder to keep up.
Why the predicted date is only a guess
The calculator’s timeline assumes your TDEE stays the same for the whole diet. It doesn’t. As you lose weight, your body burns less, so the same daily target gives a shrinking deficit. Here is a simple model for the man above, eating 2,161 kcal every day without recalculating:
| Week | His TDEE | His deficit | Lost so far |
|---|---|---|---|
| Start | 2,711 | 550 | 0 kg |
| Week 4 | 2,688 | 527 | 2.0 kg |
| Week 8 | 2,659 | 498 | 3.8 kg |
| Week 12 | 2,631 | 471 | 5.6 kg |
| Week 20 | 2,581 | 420 | 8.8 kg |
Instead of 10 kg in 20 weeks, the model gives about 8.8 kg. Real life adds more noise: a fast drop of water in week one, possible adaptation and less daily movement as he diets, and holidays. That is why the date is best treated as “around this time, if you keep adjusting”. Recalculating every 4 to 5 kg brings the deficit back up.
A four-week review checklist
Every four weeks, sit down for five minutes and check:
- Weight trend. Compare this week’s average with four weeks ago. Is the loss roughly on plan, ignoring the first week?
- Pace. Is it under about 1% of your body weight a week?
- Logging. Have you been recording everything, including oils, drinks and weekends?
- Steps. Has your daily average fallen since you started?
- Training. Are your lifts holding steady?
- How you feel. Sleep, hunger, mood, energy and, for women, your cycle.
- Weight change since your last recalculation. More than 4 to 5 kg? Recalculate your TDEE.
Then make at most one change: 100 to 200 kcal less if progress has stalled for three weeks with good logging, or 100 to 200 kcal more if you are losing too fast or feel run down.
It works for gaining weight too
Enter a goal weight above your current weight and the same maths runs in reverse: the calculator adds a daily surplus instead of subtracting a deficit and estimates how long the gain will take. For muscle gain, slow paces of about 0.25 kg (½ lb) a week or less usually work best; the lean bulk calculator sets the surplus from your training experience instead.
The warnings in our calculator
- Below 1,200 kcal (women) or 1,500 kcal (men) a day: a common minimum without medical supervision. Choose a slower pace.
- More than 1% of body weight a week: aggressive; slower is easier to keep and protects muscle.
- A goal below a BMI of 18.5: below the healthy range for your height.
These are general guides, not medical judgements. If any appear, adjust the plan or talk to a professional.
Common mistakes
- Choosing the fastest pace and burning out in week three.
- Setting the activity level too high, so the “deficit” is much smaller than it looks.
- Adding exercise calories from a watch on top of an activity level.
- Treating the goal date as a promise and giving up when it slips.
- Never recalculating as the kilos come off.
For what to do when the scale stops moving, read our plateau guide; for how big a deficit is too big, see is your calorie deficit too large?
Calorie deficit questions
How many calories should I subtract from my TDEE?
About 15 to 25%. For most people that is 300 to 700 kcal a day. Smaller and less active people should stay near the lower end.
Do I need a 500-calorie deficit to lose weight?
No. Any consistent deficit works; 500 is just a round number. A smaller deficit is slower but often easier to keep, and for smaller people 500 can be too much.
Should I eat back my workout calories?
Not if your target came from an activity level, because your usual training is already included. Only eat extra on unusually long or hard days.
I’m eating at my deficit target but not losing weight. Why?
Usually water is hiding fat loss, or the TDEE estimate was too high, or some food isn’t being logged. Wait three weeks, check your logging and steps, then reduce by 100 to 150 kcal.
When should I recalculate my deficit?
After every 4 to 5 kg (10 lb) lost, after a three-week stall, or when your job or training changes.
Can I set a deadline, such as a wedding date?
Yes, but let the pace decide the date, not the other way round. If the deadline needs more than about 1% of your body weight a week, aim for less weight by that date instead.
Does the deficit calculator work for gaining weight?
Yes. Enter a goal above your current weight and it calculates a surplus and a timeline. For muscle gain, keep the pace slow, around 0.25 kg (½ lb) a week or less.
Why does my goal date move when I update my weight?
Because the calculator recalculates your TDEE at your new weight, and a lighter body burns less. Updating every few kilos keeps the plan realistic; the date usually moves a little later each time.
Is it safe to lose 1 kg a week?
For people with a lot of weight to lose and a high TDEE, sometimes, for a limited time. For most people it is more than 1% of body weight a week, which is faster than recommended.
Sources
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- Thomas DM, Martin CK, Lettieri S, et al. Can a weight loss of one pound a week be achieved with a 3500-kcal deficit? Commentary on a commonly accepted rule. Int J Obes (Lond). 2013;37(12):1611-1613. doi.org/10.1038/ijo.2013.51
- Wishnofsky M. Caloric equivalents of gained or lost weight. Am J Clin Nutr. 1958;6(5):542-546. doi.org/10.1093/ajcn/6.5.542
- Hall KD. What is the required energy deficit per unit weight loss? Int J Obes (Lond). 2008;32(3):573-576. doi.org/10.1038/sj.ijo.0803720
- Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. J Int Soc Sports Nutr. 2014;11:20. doi.org/10.1186/1550-2783-11-20
- Rosenbaum M, Leibel RL. Adaptive thermogenesis in humans. Int J Obes (Lond). 2010;34(Suppl 1):S47-S55. doi.org/10.1038/ijo.2010.184