The math, worked out by hand
How to Calculate a Calorie Deficit
Calculating a calorie deficit is not complicated once you understand three numbers: how many calories your body burns at rest, how much more it burns once daily activity is added, and how large a gap you want to open between what you burn and what you eat. This page walks through that math step by step with a full worked example, so you can see exactly where the calculator's numbers come from, or work out your own by hand if you prefer.
The Basic Formula
At its simplest: Calorie deficit = Total Daily Energy Expenditure (TDEE) minus calories eaten. If your TDEE is 2,200 calories and you eat 1,700, your deficit is 500 calories that day. The hard part is not the subtraction, it is getting an accurate TDEE in the first place, which takes two steps. The calorie deficit calculator runs both steps for you automatically, using your own age, sex, height and weight.
Step 1: Estimate Your BMR
Your Basal Metabolic Rate (BMR) is the number of calories your body burns just to stay alive: breathing, circulating blood, repairing cells, keeping your organs running, even if you stayed in bed all day. The most widely recommended formula for estimating it is the Mifflin-St Jeor equation:
- Men: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) + 5
- Women: BMR = (10 x weight in kg) + (6.25 x height in cm) - (5 x age in years) - 161
Worked example: a 30-year-old woman, 165 cm tall, weighing 70 kg.
BMR = (10 x 70) + (6.25 x 165) - (5 x 30) - 161 = 700 + 1,031.25 - 150 - 161 = 1,420 calories.
That is her estimated calorie burn if she did nothing but rest for 24 hours.
Step 2: Turn BMR Into TDEE With an Activity Multiplier
Almost nobody spends the whole day resting, so the next step multiplies BMR by an activity factor to estimate total daily burn, including movement and exercise:
| Activity level | Definition | Multiplier |
|---|---|---|
| Sedentary | Little to no exercise, desk job | BMR x 1.2 |
| Lightly active | Light exercise 1 to 3 days a week | BMR x 1.375 |
| Moderately active | Moderate exercise 3 to 5 days a week | BMR x 1.55 |
| Very active | Hard exercise 6 to 7 days a week | BMR x 1.725 |
| Extra active | Very hard training plus a physical job | BMR x 1.9 |
Continuing the example: our 70 kg woman exercises moderately, 3 to 4 days a week.
TDEE = 1,420 x 1.55 = 2,201 calories. That is roughly what she burns in a typical day, and the number she needs to eat below to lose weight.
Step 3: Pick a Deficit Size
Once TDEE is known, the deficit is a choice, not a calculation, and the size of that choice determines how fast weight comes off:
| Weekly goal | Daily deficit | Example at 2,200 TDEE |
|---|---|---|
| 0.5 lb/week (slow) | 250 to 300 calories | Eat about 1,900 to 1,950/day |
| 1 lb/week (moderate) | 500 calories | Eat about 1,700/day |
| 1.5 lb/week (faster) | 750 calories | Eat about 1,450/day |
| 2 lb/week (upper limit for most) | 1,000 calories | Eat about 1,200/day |
Most guidelines advise against a deficit larger than about 1,000 calories a day without medical supervision, since it becomes difficult to hit essential nutrient targets and increases the risk of losing muscle along with fat. Choosing 1 lb/week, our example woman's target becomes 2,201 minus 500, or 1,701 calories a day.
Step 4: Estimate Weeks to Goal
Once the daily deficit is set, estimating a timeline is straightforward, using the same 3,500-calorie-per-pound approximation the deficit itself is built on:
Weeks to goal = (current weight - goal weight) x 3,500 / daily deficit, then divided by 7 to convert days into weeks.
Finishing the example: our woman weighs 70 kg (about 154 lb) and wants to reach 65 kg (about 143 lb), an 11-pound loss, at a 500-calorie daily deficit.
Total calories needed: 11 x 3,500 = 38,500 calories. Days needed: 38,500 / 500 = 77 days. Weeks needed: 77 / 7 = about 11 weeks, roughly two and a half months.
The calculator above runs all four of these steps automatically the moment you enter your details, including this weeks-to-goal estimate if you fill in an optional goal weight.
Mistakes That Throw the Math Off
Overestimating activity level. A 20-minute walk most days is a good habit, but it does not make someone "very active." Most people with a desk job are genuinely sedentary or lightly active, and picking too high an activity level inflates TDEE, which inflates the calorie target, which quietly erases the deficit.
Underestimating what gets eaten. Cooking oil, a handful of nuts, a splash of dressing, a few bites off someone else's plate: none of it feels significant in the moment, but it adds up to real calories fast. Tracking honestly for even one week is usually the single biggest correction most people need to make.
Not recalculating as weight changes. TDEE drops as body weight drops, since a lighter body burns fewer calories both at rest and during the same activities. A deficit that was accurate at a higher starting weight can shrink toward nothing after losing 15 or 20 pounds if the numbers are never run again.
Skip the Arithmetic
The math above is exact but tedious to redo by hand every time you want to test a different weekly goal or check your numbers after a change in weight. The calculator on this site's homepage runs all four steps instantly and lets you compare goals side by side without any manual multiplication. Once you have your target, our weight loss tips guide covers the practical side of actually sticking to it.
Frequently asked questions
What is the exact formula for a calorie deficit?
How do I calculate my BMR by hand?
How many calories is a pound of fat?
How long will it take to lose 20 pounds?
Why is my calculated TDEE different from what an app told me?
Do I need to recalculate my deficit as I lose weight?
Does this page store the numbers from the worked example or any numbers I look at?
Ready to run your own numbers?
Plug your age, height, weight and activity level into the full calculator.
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