Calorie deficit: what it is and how to calculate it
You're in a calorie deficit when you eat less energy than your body burns in a day. To find yours, estimate what you burn (basal metabolism × activity) and set a daily limit a little below it. For most adults, losing 1 to 2 lb (0.5 to 1 kg) a week is a safe, sustainable pace.
What a calorie deficit is
Your body spends energy all day long: breathing, keeping warm, digesting food and moving around. That energy is measured in calories (technically kilocalories, or kcal). When you eat a bit less than you burn for days and weeks, your body makes up the gap with stored energy, mostly body fat. That gap is your calorie deficit.
The opposite, eating more than you burn, is a surplus, and over time it shows up as weight gain. When intake and output roughly match, your weight stays about the same.
A deficit doesn't require "forbidden" foods or a magic eating window. You can get there with food you enjoy, as long as your daily total lands below what you burn. What you eat still matters, though: some meals keep you full on far fewer calories than others.
How to calculate what you burn
Your total daily energy expenditure (TDEE) is estimated in two steps.
1. Your basal metabolic rate
This is the energy you burn at rest. The most widely used way to estimate it is the Mifflin-St Jeor equation, published in 1990 from measurements of healthy adults:
Men: 10 × weight (kg) + 6.25 × height (cm) − 5 × age + 5
Women: 10 × weight (kg) + 6.25 × height (cm) − 5 × age − 161
Using pounds and inches? Multiply pounds by 0.4536 to get kilograms and inches by 2.54 to get centimeters. The calculator below does it for you.
2. Your activity level
Then you multiply by an activity factor. The commonly used factors, and the same ones SUMAAI uses, range from 1.2 to 1.9:
| Level | What your week looks like | Factor |
|---|---|---|
| Sedentary | Desk job, little or no exercise | 1.2 |
| Light | Light exercise 1–3 days a week | 1.375 |
| Moderate | Moderate exercise 3–5 days a week | 1.55 |
| Active | Hard exercise 6–7 days a week | 1.725 |
| Very active | Very hard training or a physical job | 1.9 |
A worked example
Take a 36-year-old woman who is 5'5" (165 cm), weighs 165 lb (75 kg) and exercises lightly. Her basal rate is 10 × 75 + 6.25 × 165 − 5 × 36 − 161 ≈ 1,440 kcal. Times 1.375, she burns about 1,980 kcal a day. If she aims to lose 0.5% of her body weight per week (about 0.8 lb, or 0.38 kg), her daily limit lands near 1,560 kcal, a deficit of roughly 420 calories a day.
Keep in mind this is an estimate. Two people with identical numbers can burn different amounts. That's why it pays to check the math against real life: what you actually eat and how your weight moves over several weeks.
Calorie deficit calculator
How big a deficit is safe
Health authorities broadly agree that slow and steady beats crash dieting:
- The US CDC recommends losing about 1 to 2 pounds a week, noting that people who lose weight at that pace are more likely to keep it off.
- The UK's NHS (NHS 111 Wales) describes 0.5 to 1 kg a week as a safe and sustainable rate, and says most people are advised to cut about 600 calories a day.
- Harvard Health advises that, unless you're supervised by a health professional, calories shouldn't fall below 1,200 a day for women or 1,500 a day for men.
- Diets under 800 calories a day count as very low-calorie diets. They aren't suitable for everyone and are only used in specific cases under medical supervision.
That's why SUMAAI defaults to a pace of 0.5% of your body weight per week (between 0.25% and 1%, depending on your goal and timeframe). It never suggests fewer than 1,200 calories for women or 1,500 for men, never goes below your basal rate, and never moves your target more than 150 calories from one week to the next.
Why the scale doesn't drop the same every week
You've probably heard that 3,500 calories equal one pound of fat (about 7,700 kcal per kilogram). It's a handy starting point for the first few weeks, but it has real limits:
- You burn less as you get lighter. A smaller body needs less energy, so the same calorie limit creates a shrinking deficit over time. An analysis led by Kevin Hall in The Lancet (2011) showed that the static 3,500-calorie rule ignores this adaptation, and a 2013 paper found it overestimates long-term weight loss.
- Water and gut contents shift day to day. Salt, carbs, your menstrual cycle or a hard workout can move the scale even when your fat mass hasn't changed.
Watch the trend over several weeks and your weekly average rather than any single weigh-in. One day over your limit doesn't undo anything if your week as a whole is still in a deficit.
How to stick to a deficit without feeling starved
- Lead with protein at every meal. It helps you feel full and protects muscle while you lose weight.
- Fill half your plate with vegetables and add fiber (beans, lentils, oats, whole fruit). You get volume for very few calories.
- Watch what you drink. Soda, juice, alcohol and sweetened coffee drinks add calories without filling you up.
- Log what you eat, even roughly. It's hard to know whether you're in a deficit if you don't know what you're eating.
- Move more through the day. Walking and taking the stairs count toward what you burn too.
- Sleep well and plan the week's meals so you're not improvising when you're hungry.
When to talk to a professional first
This guide is for information only and is not a substitute for advice from a health professional. Check with your doctor or a registered dietitian before starting a calorie deficit if you're pregnant or breastfeeding, have or have had an eating disorder, are under 18, have diabetes or another chronic condition, or take medication such as insulin or a GLP-1 treatment (Ozempic, Wegovy, Mounjaro, Zepbound).