LogmaA calorie deficit means you eat less energy than your body uses. That is the entire mechanism behind fat loss. Every diet, every plan, every app claim eventually comes back to this one number: calories in versus calories out.
The confusing part is not the concept. It is picking a deficit size that actually works for you: big enough to see progress, small enough that you can keep it up for months instead of days.
This post walks through the math, gives you a table to see how deficit size translates into weekly loss, and covers how to pick a number you can live with.
What a calorie deficit actually is
Your body burns a certain number of calories a day just to keep you alive and moving. That number is your total daily energy expenditure, or TDEE. It comes from your basal metabolic rate, the energy your body uses at rest, multiplied by an activity factor for how much you move.
If you eat below your TDEE, your body pulls the difference from stored energy, mostly fat. That gap between what you eat and what you burn is your calorie deficit. Eat above TDEE and you build a surplus instead, which is how weight is gained.
You can estimate your own TDEE with the Mifflin St Jeor equation: BMR is 10 times your weight in kg, plus 6.25 times your height in cm, minus 5 times your age, plus 5 for men or minus 161 for women. Multiply that by 1.2 if you are sedentary, up to 1.9 if you are extra active. Or skip the arithmetic and use the free calorie calculator, which does the same math for you.
Once you know your TDEE, the deficit itself is just subtraction. If your TDEE is 2200 kcal and you eat 1700 kcal a day, you are running a 500 kcal deficit. Nothing about the food choices themselves creates or removes the deficit. Only the gap between intake and expenditure does that.
The math: 7700 kcal per kg of fat
A useful rule of thumb is that about 7700 kcal of deficit equals roughly 1 kg of body fat lost. This is an approximation, not a lab measurement, but it holds up well enough to plan around.
That means a 500 kcal daily deficit, sustained for a week, adds up to 3500 kcal, which works out to about 0.5 kg lost that week. Smaller or larger deficits scale from there in a fairly linear way, at least within the ranges most people use.
How big should your deficit be?
This is where the math turns into a decision. A bigger daily deficit means faster loss on paper, but it also means more hunger, less energy, and a higher chance you give up. Here is how common deficit sizes translate into expected weekly loss, using the 7700 kcal per kg rule:
| Daily deficit | Weekly deficit | Expected weekly loss |
|---|---|---|
| 250 kcal | 1750 kcal | about 0.23 kg (roughly 0.5 lb) |
| 500 kcal | 3500 kcal | about 0.45 kg (roughly 1 lb) |
| 750 kcal | 5250 kcal | about 0.68 kg (roughly 1.5 lb) |
| 1000 kcal | 7000 kcal | about 0.91 kg (roughly 2 lb) |
These are averages, not guarantees. Water retention, sleep, and hormonal shifts can hide or exaggerate a week’s real fat loss on the scale. Look at the trend over three or four weeks rather than any single weigh in.
Why bigger is not always better
A 1000 kcal deficit sounds appealing if you want results fast, but it comes with real costs. A larger deficit makes it harder to hit your protein target, which matters because adequate protein, generally 1.6 to 2.2 g per kg of body weight for people trying to keep muscle while losing fat, protects lean mass during a deficit. Cut calories too hard and more of what you lose comes from muscle instead of fat.
A large deficit also tends to increase hunger and fatigue, which is exactly the condition under which people abandon a diet. Safe, sustainable fat loss generally falls in the range of about 0.5 to 1 percent of body weight per week. For most people that lands somewhere between the 250 and 750 kcal rows in the table above, not at the extreme end.
If you are unsure which end of that range fits your situation, the full breakdown of how many calories to eat for your goal walks through picking a target step by step.
How to actually hit your deficit consistently
None of this math matters if you cannot track your intake accurately enough to know whether you are actually in a deficit. Studies consistently show people underestimate intake substantially when self reporting, which is often the real reason a “500 kcal deficit” produces no visible change: the deficit never existed in the first place.
The fix is not more willpower. It is lowering the friction of logging so you actually do it every day, at every meal, including the ones you would normally skip logging because you are in a hurry. That is easier when logging a meal takes about 3 seconds instead of a minute of searching a database.
It also helps to stop treating a kitchen scale as mandatory. Most people can track calories reasonably well without weighing every ingredient, which removes another barrier that causes people to quit partway through.
Logma was built around this idea: you say what you ate, it estimates calories and macros from how you describe portions, standard plate, regular wrap, 330ml drink, and logs it in about 3 seconds. There is a free tier with 30 AI logs to try it, and you can find it on the App Store.
FAQ
What is a good calorie deficit to start with?
A 500 kcal daily deficit is a reasonable default for most people, working out to about 0.45 kg lost per week. It is large enough to see steady progress and small enough to sustain for months.
Is a 1000 kcal deficit safe?
It can be for some people, especially those with more weight to lose, but it raises the risk of losing muscle along with fat and makes hunger harder to manage. Check that it does not push you below a sensible calorie floor for your size and activity level.
Why am I not losing weight even though I am in a deficit?
The most common cause is that the deficit is smaller than believed, often because logged intake is lower than actual intake. Tightening how consistently and accurately you log for two to three weeks usually reveals the real picture.
How do I know my TDEE to calculate my deficit from?
Use the Mifflin St Jeor equation with an activity multiplier, or use the calorie calculator to get the same estimate without doing the math by hand. Subtract your chosen deficit from that number to get your daily calorie target.
Should my deficit change as I lose weight?
Yes, generally. As body weight drops, TDEE drops with it, since a smaller body burns somewhat less energy at rest and in daily movement. Recalculating your TDEE every few kilograms lost keeps the deficit accurate instead of slowly shrinking without you noticing.