What Is a Calorie Deficit?

A calorie deficit happens when you consume fewer calories than your body uses in a day. Your body needs energy for everything — breathing, digestion, walking, thinking, exercising. The total energy you burn in a day is called your TDEE (Total Daily Energy Expenditure).

When you eat less than your TDEE, your body makes up the difference by breaking down stored energy — primarily body fat. A deficit of 3,500 calories equals roughly 1 lb of fat loss. So a daily deficit of 500 calories = 1 lb per week = about 1 stone every 14 weeks.

What Is TDEE and How Is It Calculated?

TDEE has two components:

1. BMR (Basal Metabolic Rate) — the calories your body burns at complete rest, just to keep you alive. This accounts for 60–70% of most people’s TDEE.
2. Activity multiplier — additional calories burned through movement, exercise, and the thermic effect of food.

Our calculator uses the Mifflin-St Jeor equation, which the Academy of Nutrition and Dietetics considers the most accurate BMR formula for most adults:

• 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

The BMR is then multiplied by an activity factor to get your TDEE.

Which Calorie Deficit Should You Choose?

Activity Level — What Should I Choose?

Your activity level is the most common source of error in TDEE calculations. Most people overestimate their activity. Here’s an honest guide:

• Sedentary: You work a desk job AND don’t exercise regularly. Walking to the car park doesn’t count. This is most office workers.
• Lightly Active: Desk job + light exercise 1–3 times per week (30-min walks, light gym sessions, yoga).
• Moderately Active: You exercise at moderate intensity 3–5 times per week (running, cycling, gym classes, weight training), OR you have a job that keeps you on your feet (retail, nursing, teaching).
• Very Active: Intense exercise 6–7 times per week, or a physically demanding job (construction, warehouse work) plus regular exercise.

When in doubt, choose one level lower. You can always eat more if you’re losing too fast — but starting too high means you might not lose at all and won’t know why.

Why Am I Not Losing Weight in a Calorie Deficit?

If the scale isn’t moving despite eating in what you think is a deficit, here are the most common reasons:

1. You’re not actually in a deficit. Studies show people underestimate their calorie intake by 30–50% on average. Cooking oils, sauces, drinks, and “just a few bites” add up fast.
2. You’re overestimating your activity level. Fitness trackers can overestimate calories burned by 30–50%. Recalculate with a lower activity setting.
3. Water retention is masking fat loss. High-sodium meals, carb refeeds, menstrual cycles, and cortisol from stress all cause water retention. You might be losing fat but the scale doesn’t show it yet.
4. Weekend eating undoes weekday progress. A 500 cal/day deficit Monday–Friday is 2,500 calories. Two meals out plus drinks on Saturday can easily add 3,000+ calories back.
5. Metabolic adaptation. After weeks in a deficit, your body becomes more efficient, slightly lowering your TDEE. A planned “diet break” (eating at maintenance for a week) can help reset this.

10 Tips for Sticking to a Calorie Deficit

1. Prioritise protein — it’s the most filling macronutrient and preserves muscle. Aim for 1.6–2.0g per kg body weight. (Use our protein calculator →)
2. Eat whole, unprocessed foods — they’re more filling per calorie than processed alternatives.
3. Don’t drink your calories — swap sugary drinks, fruit juices, and lattes for water, black coffee, and herbal tea.
4. Weigh yourself daily, track weekly averages — daily fluctuations are noise; the weekly trend is signal.
5. Prep meals in advance — hunger + no plan = poor decisions. Batch cooking on Sundays solves most weekday temptations.
6. Sleep 7–9 hours — poor sleep increases ghrelin (hunger hormone) and decreases leptin (fullness hormone).
7. Walk more — 10,000 steps/day can burn an extra 300–500 calories without formal exercise.
8. Use smaller plates — it sounds simplistic, but research shows it genuinely reduces portion sizes.
9. Allow planned treats — rigid restriction leads to binge cycles. Budget 10–15% of calories for foods you enjoy.
10. Track for 2 weeks, then adjust — if you’re not losing after 2 weeks of accurate tracking, reduce by another 100–200 calories.

Frequently Asked Questions