What is a calorie deficit? It’s simply eating fewer calories than your body burns in a day — and it’s the one thing that has to be true for you to lose fat, no matter which diet you follow. Low-carb, high-protein, fasting, “clean eating”: they all only work if they get you into a deficit, whether you’re counting or not.

The good news is the concept is genuinely simple. The slightly harder part is knowing your numbers and staying consistent. Let’s walk through both.

What a calorie deficit actually is

Your body uses energy every single day — to breathe, pump blood, digest food, walk to the kitchen, think, and move. We measure that energy in calories. When the calories you eat are lower than the calories you burn, your body makes up the difference by tapping into stored energy, mostly body fat. That gap is your calorie deficit.

The mechanism is real physics, not a marketing claim. Roughly 3,500 calories is stored in about half a kilo (one pound) of body fat. So if you run a deficit of around 500 calories a day, that adds up to roughly 3,500 calories a week — and a fat loss of about half a kilo per week. The exact maths varies person to person (water weight, hormones, and how your body adapts all play a role), but the direction is reliable: sustained deficit, fat comes off.

The opposite is also true. Eat more than you burn — a calorie surplus — and the excess gets stored. Eat roughly the same amount you burn and your weight holds steady at maintenance. Everything in body composition orbits these three states.

What a deficit is not: it’s not starving yourself, and it’s not a specific diet you buy. It’s just an energy balance you create, and there are sensible and reckless ways to get there.

How to calculate your calorie deficit

To know how much to eat, you first need to estimate how much you burn. That figure is your TDEE — Total Daily Energy Expenditure. Here’s the beginner-friendly version, in three steps.

1. Estimate your BMR (Basal Metabolic Rate). This is the energy you’d burn lying in bed all day — keeping your organs running. A widely used formula is the Mifflin-St Jeor equation:

• 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

For example, a 35-year-old woman who is 165cm and 70kg has a BMR of about 1,440 calories.

2. Multiply by an activity factor to get TDEE. Your BMR only covers the basics, so scale it up for how active you are:

• Sedentary (desk job, little exercise): BMR × 1.2
• Lightly active (light exercise 1–3 days/week): BMR × 1.375
• Moderately active (3–5 days/week): BMR × 1.55
• Very active (6–7 days/week): BMR × 1.725

Our example woman, lightly active, would have a TDEE of about 1,440 × 1.375 = 1,980 calories. That’s roughly what she burns in a day, and what she’d eat to maintain her weight.

3. Subtract to create your deficit. Take a percentage off your TDEE rather than a fixed scary number. A 10–20% reduction is the sweet spot for most people. For our example, 15% off 1,980 is about 300 calories, leaving a target of roughly 1,680 calories a day for steady, sustainable fat loss.

A quick reality check: these formulas give an estimate, not gospel. Online calculators and wearables guess at the same thing. The smartest move is to use your number as a starting point, then adjust based on what actually happens on the scale and tape measure over two to three weeks.

What a safe, sustainable deficit looks like

Bigger is not better. A gentle deficit is far easier to stick to, protects your muscle and energy, and tends to win over months — which is the timescale that matters.

A sensible target for most people is:

• A deficit of about 10–20% below maintenance, or
• A rate of roughly 0.5kg (about 1lb) per week

If you’re carrying a lot of weight, you can often lose a little faster at first without much downside. The leaner you get, the more you’ll want to slow down to hold onto muscle and avoid feeling wrecked.

Two things make a deficit dramatically more comfortable. First, prioritise protein — aim for around 1.6 to 2.2g per kilo of body weight. Protein keeps you full, costs more calories to digest, and signals your body to burn fat rather than muscle. Second, keep moving and ideally do some resistance training, which protects lean tissue so the weight you lose is mostly fat.

A light safety note: this is general guidance, and individual needs vary. If you have a medical condition, take medication that affects appetite or blood sugar, or are considering a very low-calorie diet (typically under 1,200 calories a day), please do it under the supervision of a doctor or registered dietitian.

Common calorie deficit mistakes to avoid

Most people don’t fail at a deficit because the science is wrong — they fail because of a few predictable traps.

• Going too aggressive, too fast. Slashing to 1,000 calories feels productive for a week, then backfires: relentless hunger, low energy, muscle loss, and a binge that erases your progress. A smaller deficit you can actually live with beats a heroic one you quit.
• Under-eating protein. Cut calories without enough protein and a chunk of your weight loss comes from muscle. You end up lighter but softer, with a slower metabolism. Protein is the macro you should guard most fiercely in a deficit.
• Not actually tracking — or tracking sloppily. “I barely eat” is the most common reason the scale won’t budge. Oils, dressings, drinks, and generous “eyeballed” portions add up fast. Most people underestimate their intake by hundreds of calories without realising it.
• Forgetting that maintenance moves. As you lose weight, your TDEE drops because there’s less of you to fuel. A deficit that worked at the start may become maintenance later, so recalculate every few kilos.
• Obsessing over a single day. Weight bounces daily with water, salt, and hormones. Judge progress on weekly averages, not this morning’s number.

Seeing whether you’re really in a deficit

Here’s the gap between theory and results: you can calculate a perfect target and still stall, because the deficit only exists if your actual intake lands below your actual burn. The formula is an estimate — your logging is the evidence.

That’s where consistent tracking earns its keep, and where it usually falls apart for beginners. Weighing every ingredient and searching a database for each meal is tedious, so people quietly stop after a week. With Nutrify you snap a photo of your plate and it logs the calories, macros, and protein for you — which removes most of the friction and lets you actually see, day to day, whether you’re sitting in the deficit your plan calls for.

You don’t need to be perfect. You need to be roughly right, consistently, for long enough to notice the trend. If you’re new to this, our guide on how to track macros for beginners breaks down the whole system, and how to track calories without weighing food shows you how to estimate portions when you’re away from the kitchen scales.

Frequently asked questions

What is a calorie deficit? A calorie deficit is when you eat fewer calories than your body burns over a day. Because your body needs that energy from somewhere, it draws on stored fat to make up the gap — which is why a sustained deficit leads to fat loss regardless of the specific diet you use.

How do I calculate my calorie deficit? Estimate your maintenance calories (TDEE) by working out your BMR with the Mifflin-St Jeor equation, multiplying by an activity factor, then subtracting 10–20%. Use that number as a starting point and adjust based on your real-world results over two to three weeks.

How big should my calorie deficit be? For most people, a deficit of about 10–20% below maintenance — or roughly 0.5kg of weight loss per week — is sustainable and protects muscle. Larger deficits work short-term but get harder to maintain and risk muscle loss and burnout.

How do I know if I’m in a calorie deficit? The clearest sign is a downward trend in your weekly average body weight over a few weeks. To confirm it day to day, track what you actually eat against your target — logging (such as photo-logging your meals) shows whether your real intake is genuinely below your burn.

Will I lose only fat in a calorie deficit? Not automatically. Some loss can come from muscle, especially with an aggressive deficit and low protein. Eating around 1.6–2.2g of protein per kilo of body weight and doing resistance training tilts the loss heavily toward fat.

The bottom line

A calorie deficit is the simple, non-negotiable engine of fat loss: eat less than you burn and your body uses stored fat for the difference. Calculate your TDEE, take 10–20% off, lean on protein, and judge progress by weekly trends rather than daily noise. The hardest part is knowing whether your real intake matches your plan — photo-logging your meals with Nutrify makes that easy to see, so the deficit you designed is the deficit you’re actually living in.