What Is a Calorie Deficit? How It Actually Works
Health · 5 min read · Last updated: June 2026A calorie deficit means you eat fewer calories than your body burns, so it taps stored body fat to cover the gap — and that's the only way fat loss happens. Every diet that works, from keto to intermittent fasting, works because it puts you in a deficit, whether or not it says so. To create one, you need two numbers: how much energy you burn in a day (your maintenance, or TDEE) and how much you eat. Find your maintenance with our TDEE Calculator, then set a target with the Calorie Calculator.
How a calorie deficit works
Your body needs energy every minute — to pump blood, breathe, think, and move. It measures that energy in calories. When the calories you eat fall short of what you spend, your body makes up the difference by breaking down stored fat (and, if the deficit is steep, some muscle). This is just the first law of thermodynamics applied to the body: energy can't appear from nowhere. The U.S. CDC frames healthy weight loss the same way — a steady, moderate energy deficit over time.
One pound of body fat holds roughly 3,500 calories of stored energy. That figure is a rule of thumb rather than a perfect law, but it explains why a deficit of about 500 calories a day tends to produce about one pound of loss per week.
How big should a calorie deficit be?
For most people, a moderate deficit of 500 calories per day is the sweet spot: fast enough to see progress, slow enough to protect muscle and stay sustainable. The CDC recommends aiming for about 1 to 2 pounds per week as a realistic, lasting pace. Here's how deficit size maps to expected weekly loss:
| Daily deficit | Approx. weekly fat loss | Notes |
|---|---|---|
| 250 cal | ~0.5 lb | Gentle, very sustainable |
| 500 cal | ~1 lb | Common, balanced target |
| 750 cal | ~1.5 lb | Aggressive; watch hunger |
| 1,000 cal | ~2 lb | Upper limit; muscle-loss risk |
How do I find my maintenance calories?
Your maintenance level — total daily energy expenditure, or TDEE — is your resting metabolism plus the energy you burn moving. It's driven by your weight, height, age, sex, and activity level. Most calculators estimate it with the Mifflin–St Jeor equation, which research has found to be among the most accurate for the general population. Once you know your TDEE, your deficit target is simply that number minus 250 to 500. Get your estimate from the TDEE Calculator.
Why the scale doesn't always cooperate
A deficit reliably reduces fat over time, but daily weight bounces around because of water, sodium, glycogen, and digestion. A single salty meal can add a pound of water overnight that has nothing to do with fat. Judge progress over two to four weeks, not day to day. It also helps to keep protein high (to preserve muscle) and to strength-train — set your protein target with the Macro Calculator if you want a number to aim for.
Don't go too low
A bigger deficit isn't automatically better. Very low intakes cause fatigue, hunger, nutrient gaps, and faster muscle loss, and they're hard to stick to — which is why crash diets so often rebound. As a general floor, many health resources suggest not dropping below roughly 1,200 calories a day for women or 1,500 for men without medical supervision. Slow and steady wins.
Find your number
The whole strategy comes down to two figures. Start with the TDEE Calculator to learn what you burn, then use the Calorie Calculator to set a sensible deficit for your goal. If you also want context on where your weight stands, our guide What is a healthy BMI for your height? puts the numbers in perspective.
Frequently asked questions
How big should a calorie deficit be?
A moderate deficit of about 500 calories per day is a common, sustainable target that yields roughly one pound of fat loss per week. Larger deficits speed things up but are harder to maintain and risk muscle loss.
How many calories are in a pound of fat?
One pound of body fat stores roughly 3,500 calories. That's why a daily 500-calorie deficit adds up to about a pound of loss per week, though real-world results vary.
Can a calorie deficit be too aggressive?
Yes. Very low intakes can cause fatigue, nutrient shortfalls, and muscle loss, and they're hard to sustain. Most guidance suggests not going below roughly 1,200 calories a day for women or 1,500 for men without medical supervision.
This article is for general education only and is not medical or nutritional advice. Talk to a qualified professional before starting any diet, especially if you have a health condition.