How is the calorie deficit worked out?

We estimate your basal metabolic rate with the Mifflin-St Jeor equation, multiply by an activity factor to get your total daily energy expenditure (TDEE), then subtract a deficit based on your weekly goal. About 7,700 kcal equals one kilogram of body fat.

How big a deficit is safe?

Most guidance suggests losing 0.25–1 kg per week. The calculator warns if your target falls below a commonly cited minimum of 1,200 kcal (women) or 1,500 kcal (men) — consider a slower pace or professional advice if so.

Is my data sent anywhere?

No. Every calculation runs entirely in your browser and nothing is uploaded or stored.

How many calories is 1 kg of fat?

Roughly 7,700 kcal. To lose 0.5 kg per week you need a 3,850 kcal weekly deficit, which is about 550 kcal per day. Losing 1 kg per week requires a 7,700 weekly deficit, or about 1,100 kcal per day — aggressive for most people.

Why did the calculator warn my target is too low?

It applies a safety floor of 1,500 kcal/day for men and 1,200 for women. If a fast weekly goal pushes your target below that, it flags it so you choose a slower pace. Eating too little risks muscle loss and nutrient gaps.

Will I keep losing exactly 0.5 kg a week forever?

No. As you get lighter your TDEE falls, so the same intake becomes a smaller deficit and loss slows. Recalculate every few kilograms and adjust your target to keep progress steady.

Do I need to exercise to be in a deficit?

No — a deficit is created by intake below TDEE, achievable through diet alone. Exercise raises your TDEE (and the activity factor here), letting you eat more at the same deficit, and helps preserve muscle while losing fat.

What is the Calorie Deficit Calculator?

Free calorie deficit calculator — estimate your maintenance calories (TDEE) and a daily target to lose weight at your chosen pace. Runs entirely in your browser, nothing is uploaded. It runs free in your browser on Gera Tools, with nothing uploaded.

Calorie Deficit Calculator

Name: Calorie Deficit Calculator
Creator: Gera Tools
License: https://creativecommons.org/licenses/by/4.0/

Calorie deficit calculator

A calorie deficit means eating fewer calories than your body burns — it is the one mechanism behind every form of fat loss. This calculator works out your maintenance calories (TDEE) and then a safe daily intake to hit your chosen rate of loss, so you can stop guessing and eat to a number. It is built for anyone starting a diet who wants a realistic, sustainable target rather than a crash plan.

How it works

The tool computes your BMR with the Mifflin-St Jeor equation (10 × kg + 6.25 × cm − 5 × age, +5 men / −161 women), multiplies by your activity factor to get TDEE, then subtracts a daily deficit. The deficit comes from your weekly goal using the rule that 1 kg of body fat ≈ 7,700 kcal:

daily deficit = (kg per week × 7,700) ÷ 7

A safety floor (1,500 kcal men, 1,200 women) flags targets that are too aggressive.

Example

A 30-year-old man, 178 cm, 80 kg, lightly active, aiming to lose 0.5 kg/week:

BMR = 10×80 + 6.25×178 − 5×30 + 5 = 1,768 kcal
TDEE = 1,768 × 1.375 = 2,431 kcal (maintenance)
Deficit = (0.5 × 7,700) ÷ 7 = 550 kcal/day
Target = 2,431 − 550 = 1,881 kcal/day

Weekly goal	Daily deficit	Approx. result
Lose 0.25 kg	275 kcal	gentle, easy to sustain
Lose 0.5 kg	550 kcal	recommended for most
Lose 1 kg	1,100 kcal	aggressive, often hits the floor

Everything runs in your browser — nothing is uploaded.

Activity levels explained

The activity multiplier is the single biggest source of variation between people with the same height and weight. The five levels used here correspond to:

Label	Multiplier	Typical description
Sedentary	1.2	Desk job, no deliberate exercise
Light	1.375	1–3 light workouts or walks per week
Moderate	1.55	3–5 moderate sessions per week
Active	1.725	Hard training 6–7 days per week
Very active	1.9	Physical job plus heavy training, or twice-daily sessions

Most people overestimate their activity level. If you sit most of the day and walk 20 minutes three times a week, “light” is correct. “Moderate” requires genuine cardiovascular effort three to five times a week — a 30-minute brisk walk does not qualify. When in doubt, choose the lower level and adjust if the projected rate of loss is faster than what you observe over two or three weeks.

Why your TDEE changes as you lose weight

The Mifflin-St Jeor equation uses your current weight. As you lose weight, your BMR falls — a lighter body burns fewer calories at rest. This means a fixed daily intake produces a decreasing deficit over time, and weight loss slows. For example, if you start at 90 kg and lose 5 kg, your TDEE may drop by 100–150 kcal, shrinking your deficit without any change in your eating or exercise.

The practical implication is to recalculate every 4–6 weeks as you lose weight, and either reduce intake or increase activity to maintain the same deficit.

How diet affects the accuracy of the 7,700 kcal/kg rule

The 7,700 kcal/kg rule applies cleanly to pure fat loss. In practice, a calorie deficit causes some loss of glycogen (and the water stored with it), some muscle loss if protein intake is too low or the deficit too large, and some fat loss. The glycogen and water component can cause rapid initial weight loss that exceeds the fat-only prediction. Longer-term, well-structured deficits with adequate protein (1.6–2.2 g per kg of body weight is a common recommendation) preserve more muscle and make the 7,700 rule more predictive.

Common mistakes

Underestimating calories in is the most common reason deficits don’t produce expected results. Studies consistently show people underestimate their intake, particularly for calorie-dense foods like oils, nuts, and sauces. Weighing food rather than estimating volumes is more accurate.
Overestimating calories out from exercise is also very common. Fitness trackers overestimate calorie burn, sometimes significantly. Many people unconsciously eat back more than they burned when they exercise.
Not accounting for the thermic effect of food. Protein has a higher thermic effect than fat or carbohydrate — about 20–30% of its calories are used in digestion. Higher-protein diets slightly raise effective TDEE.
Expecting linear progress. Water retention, hormonal cycles, gut contents, and glycogen fluctuations all move the scale. Progress is best assessed by comparing weekly averages rather than day-to-day numbers.