Calorie deficit to lose weight: how to get the number right

Here's how to find your number, how big to make it, and one popular rule about calories and pounds that's more myth than math.

What a calorie deficit actually is

Your body burns a certain number of calories a day just existing — breathing, digesting, keeping your heart beating — plus whatever you burn moving around. That total is your maintenance calories, sometimes called TDEE (total daily energy expenditure). Eat exactly that number and your weight holds steady. Eat less than that number, consistently, and your body makes up the difference by burning stored fat. That gap between what you eat and what you burn is the deficit.

It sounds simple because it is simple — the hard part is estimating the two numbers accurately and sticking with it long enough to see the scale move.

How to calculate your calorie deficit

Step 1: find your maintenance calories

Formulas like Mifflin-St Jeor estimate maintenance calories from your age, sex, height, weight, and activity level — most calorie-tracking apps (Bocco included) will do this math for you. It's an estimate, not a lab measurement, so treat the number as a solid starting point you'll adjust based on what actually happens to your weight over a few weeks.

Step 2: subtract to set your deficit

Pick how much you want to cut below maintenance, and that's your daily calorie target. If maintenance is 2,200 calories and you want a 500-calorie deficit, your target is 1,700. That's the whole calculation — the nuance is in choosing the right deficit size, which is where most advice online gets vague or reckless.

How big should your deficit be?

The CDC recommends losing weight at a gradual, steady pace — about 1 to 2 pounds a week — because people who lose weight this way are more likely to keep it off long-term.¹ In calorie terms, that generally lines up with a deficit of roughly 500 to 750 calories a day, though the exact conversion isn't as fixed as it sounds (more on that below).

Bigger isn't better here. A very aggressive deficit — say, 1,000+ calories a day for someone who doesn't need one that large — tends to backfire: you're hungrier, your energy tanks, you lose muscle along with fat, and it's harder to sustain past a few weeks. A deficit you can actually live with for months beats a brutal one you abandon in two weeks.

Why "3,500 calories = 1 pound" isn't quite right

You've probably seen the rule: a pound of fat is about 3,500 calories, so cut 500 a day and you'll lose a pound a week, like clockwork. It's a useful rough estimate, and it's not wrong to use it as a starting point — but it's not the precise formula it gets treated as.

NIH researchers who built the agency's Body Weight Planner modeled how the body actually responds to a sustained deficit, and found that weight loss isn't linear the way the 3,500-calorie rule assumes.² As you lose weight, your maintenance calories drop too — a lighter body burns less just existing — so the same 500-calorie deficit produces less weight loss over time than it did in week one. That's why people often see faster results early on a diet and a frustrating plateau later, even while eating the same "deficit" number. It's not a sign something's broken; it's how the math actually behaves.

The practical takeaway: use the 3,500-calorie rule to get a starting deficit, then let a few weeks of real results — not the formula — tell you whether to adjust. We used to tell people the math was that clean, too, before we actually watched a few weeks of real logs go by; it isn't, and pretending otherwise just makes the plateau feel like failure instead of physiology.

How Bocco helps

Hitting a calorie deficit consistently is mostly a logging problem, not a willpower problem — you can't manage a number you're not tracking. Most people who quit don't quit because the deficit was wrong; they quit because logging every meal by hand got old around week two. Bocco reads your meals from a photo and returns calories and macros in seconds, so you can see your actual deficit each day instead of guessing. If an estimate looks off, you correct it in a sentence and the numbers recalculate — no re-typing, no spreadsheet.