How to Calculate Your Calorie Deficit: The Formula in Plain English

“Eat in a calorie deficit” is the most repeated advice in weight loss and the least often explained. People nod along, download a tracker, guess at a number, and then wonder why the scale stalls six weeks later. The deficit itself is simple arithmetic — two numbers and a subtraction. What trips everyone up is that one of those two numbers is an estimate, and estimates drift. Here’s how to calculate yours properly, what the formula is actually doing, and where it quietly stops working. This is general nutrition information, not medical advice.

What a calorie deficit actually is

A calorie deficit is the gap between the energy your body uses in a day and the energy you eat that day. Use 2,400 and eat 2,000, and you’re in a 400-calorie deficit. That’s the whole concept. It isn’t a diet, a food list or a protocol — keto, fasting, portion control and “clean eating” all work, when they work, because they land you on the lower side of that gap.

The word doing the heavy lifting is uses. Your body doesn’t only burn calories when you exercise. It burns them keeping you alive at rest, digesting your food, and moving you around in all the ways you never log — walking to the car, standing at the sink, fidgeting through a meeting. Add those together and you get the number the whole calculation hangs on.

The calorie deficit formula

The formula itself is a single line:

Calorie deficit = maintenance calories − calories eaten.

Your maintenance calories — usually called TDEE, total daily energy expenditure — are the calories that keep your weight where it is right now. So the real work isn’t the subtraction. It’s getting a usable TDEE, which takes two steps of its own.

How to calculate yours, step by step

Step 1 — estimate your BMR. Your basal metabolic rate is what you’d burn lying still all day. The Mifflin-St Jeor equation is the one most calculators use:

• Men: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) + 5
• Women: BMR = (10 × weight in kg) + (6.25 × height in cm) − (5 × age) − 161

Step 2 — multiply by how much you move. BMR × an activity factor gives TDEE. Common multipliers: 1.2 desk job, little exercise; 1.375 light activity 1–3 days a week; 1.55 moderate, 3–5 days; 1.725 hard training 6–7 days; 1.9 physical job plus training.

Step 3 — subtract. Take a percentage off your TDEE and that’s your target intake.

A worked example. A 34-year-old woman, 70 kg, 168 cm, at the gym three times a week:

• BMR = (10 × 70) + (6.25 × 168) − (5 × 34) − 161 = 1,419
• TDEE = 1,419 × 1.375 = 1,951
• A 20% deficit = 1,951 × 0.8 ≈ 1,560 calories a day

If you’d rather not do that by hand, our free Calorie Deficit Calculator runs all three steps and hands you the target; the TDEE Calculator stops at step two if you only want your maintenance number, and the Macro Calculator splits the result into protein, carbs and fat. No account, no card.

One thing to be honest about: every number above is an estimate. Two people with identical stats can differ by 200–300 calories a day. The formula gives you a starting point, not a verdict. What tells you whether it was right is two or three weeks of real data — if your weight hasn’t moved and you’ve tracked honestly, your true TDEE is lower than the equation guessed, and you adjust.

How big should the deficit be?

The usual range is 10–25% below maintenance. Roughly, 10–15% is a slow, comfortable cut that’s easy to sustain; 20% is the common middle; 25% and up gets hard to hold and starts costing you muscle and training quality alongside fat. A frequently quoted anchor is a 500-calorie daily deficit, which on paper works out near half a kilo a week.

Bigger is not better, and this is where “aggressive deficit” calculators mislead. A very large gap doesn’t just accelerate the same process — it changes it. Hunger climbs, unplanned movement drops, training suffers, and adherence collapses somewhere around week three. A deficit you actually keep for twelve weeks beats a steeper one you abandon in four.

Two floors are worth respecting: don’t drop below your BMR as a matter of routine, and treat very low intakes as something to discuss with a doctor or dietitian rather than something a calculator hands you. If you’re pregnant or breastfeeding, under 18, managing a health condition, taking medication that affects appetite or weight, or have any history of disordered eating, a target number from a formula is the wrong tool — talk to a professional who knows your situation.

Why the 3,500-calorie rule overshoots

You’ll see it everywhere: 3,500 calories equals a pound of fat, so a 500-calorie daily deficit equals a pound a week, so 20 weeks equals 20 pounds. The first step is roughly right. The extrapolation isn’t, and it’s the single biggest reason people conclude their deficit “stopped working.”

The rule treats your body as a fixed machine, and it isn’t. As you lose weight there is simply less of you to carry and maintain, so maintenance calories fall — the TDEE you calculated in January is not the TDEE you have in March. Meanwhile people in a deficit tend to move less without noticing: fewer steps, less fidgeting, more sitting. So the deficit narrows from both ends while the number on your app stays the same.

The practical takeaway isn’t that the arithmetic is wrong — it’s that it needs re-running. Recalculate your TDEE every 4–6 kg of loss, or whenever the scale has been flat for three weeks of honest tracking. Expect the curve to flatten over time rather than falling in a straight line, and don’t read that flattening as failure.

Where calorie deficits quietly close

When a deficit stops producing results, the arithmetic is rarely the culprit. Five things account for most of it:

• The untracked bites. Oil in the pan, a handful off someone else’s plate, the milk in three coffees. Two or three of those a day is 200–400 calories — often the entire deficit.
• Weekends. Five days at −500 and two days at +600 nets out to a deficit of about 250 a day, not 500. Deficits are weekly, not daily.
• Liquid calories. Juice, lattes, beer and smoothies land quickly and satisfy hunger poorly, which makes them the easiest thing to under-log.
• Portion creep. Eyeballed portions grow over weeks. The same “bowl of rice” in week eight is often meaningfully bigger than it was in week one.
• Eating back exercise calories. Wearables and machines tend to overstate burn, and the “earned” snack usually costs more than the workout returned. Our guide to whether steps count as calories burned unpacks that one.

None of these require more willpower to fix. They require seeing them, which is what tracking is for.

Tracking a deficit without weighing every meal

The reason most people stop tracking isn’t motivation — it’s friction. Weighing every ingredient and searching a database three times a day is a chore nobody keeps up for long, and a deficit you don’t measure is a guess.

Photographing the plate removes most of that friction. Swoodie Scan estimates calories and macros from a photo of the meal, so logging costs a few seconds instead of a few minutes, and packaged food can be scanned by barcode instead. Set your calorie and macro targets from the number you calculated above and you can see the gap between plan and reality day by day — which is the only way to find out whether your estimated TDEE was any good.

Two habits make the estimates behave. Log before you eat where you can, since retroactive logging is where the untracked bites disappear. And track the weekly average rather than judging any single day — one heavy dinner is noise, three heavy weeks is a signal.

Go deeper

Once your calorie target is set, the next question is what to build it out of: how to set your macros covers protein, carb and fat splits for different approaches, and how much protein to build muscle matters more in a deficit than out of one, because protein is what protects muscle while you lose fat.

Comparing tools before you commit to one? Our roundup of the best AI calorie counter apps covers what photo logging is and isn’t good at — including how to track without a kitchen scale — and the weight loss guide shows how people put tracking and meal planning together. Calculators, all free: Calorie Deficit, TDEE, Macro and Protein Intake. Losing weight alongside a partner? Swipe Together lets you both swipe on meal ideas and cook the matches, which is a good deal easier than two people running two different deficits out of the same kitchen.