How the Calorie Deficit Calculator Works

Summary

A calorie deficit calculator estimates how many calories you should eat each day to reach a goal weight within a given timeframe. It works by computing your Total Daily Energy Expenditure (TDEE), then subtracting (for weight loss) or adding (for muscle gain) the daily energy change needed to achieve your target.

The calculator supports four goal modes: NHS guidelines, fat loss, muscle gain, and body recomposition — each with different macro splits and protein targets backed by sports science research.

How it works

Step 1 — Estimate BMR

Basal Metabolic Rate (BMR) is the number of calories your body burns at complete rest. This calculator uses the Mifflin-St Jeor equation (1990), which the American Dietetic Association identified as the most accurate BMR formula for healthy adults.

For females, the constant changes: BMR (female) = 10W + 6.25H − 5A − 161

Step 2 — Calculate TDEE

TDEE accounts for your activity level by multiplying BMR by a standard activity factor:

Step 3 — Compute daily energy change

The widely-used clinical estimate is that 1 kg of body fat stores approximately 7,700 kcal of energy (Wishnofsky, 1958). This is used to convert a weight change goal into a calorie target:

Step 4 — Set daily calorie target

• Weight loss: daily calories = TDEE − daily deficit
• Weight gain: daily calories = TDEE + daily surplus
• Recomposition: daily calories = TDEE (eat at maintenance)

Goal modes

The calculator offers four modes, each with a different protein target and macro split:

Why high protein during a deficit? The ISSN 2017 Position Stand recommends 1.6–2.4 g/kg/day during energy restriction to preserve lean mass. Longland et al. (2016) showed that 2.4 g/kg during a 40% deficit produced gains in lean mass alongside fat loss when combined with resistance training.

Worked example

Safety warnings

The calculator displays warnings when the plan may be unsustainable or unhealthy:

• Large deficit (over 1,000 kcal/day): Hard to sustain and risks muscle loss. Consider a longer timeframe.
• Fast weight loss (over 1 kg/week): The NHS recommends 0.5–1 kg/week. Faster rates increase muscle loss.
• Very low calories (under 1,200 kcal/day): Not recommended without medical supervision. Risk of nutrient deficiency.

Inputs explained

• Current weight / Goal weight — the start and end points. The calculator works for both loss and gain.
• Height, Age, Sex — used to compute BMR via the Mifflin-St Jeor equation.
• Activity level — selects the TDEE multiplier. Be honest; overestimating activity is the most common error.
• Timeframe (weeks) — how quickly you want to reach your goal. Longer timeframes produce smaller, more sustainable deficits.
• Goal mode — determines the protein target and macro split.

Outputs explained

• Daily calorie target — how many calories to eat per day to reach your goal.
• TDEE — your estimated maintenance calories (what you’d eat to maintain current weight).
• Deficit/Surplus — the daily energy gap between TDEE and target.
• Weekly weight change — projected rate in kg per week.
• Macros — protein, carbs, and fat in grams, based on the selected goal mode.
• Weight projection chart — a linear projection of weight over the timeframe.

Assumptions & limitations

• Linear weight loss is a simplification. Real weight loss is non-linear — early weeks include water/glycogen loss, and metabolic adaptation reduces TDEE over time. The calculator assumes a constant TDEE throughout.
• 7,700 kcal/kg is an average. The actual energy content per kg lost varies: early weight loss may be ~4,800 kcal/kg (due to water), while steady-state fat loss is closer to 7,700 kcal/kg.
• TDEE is an estimate. Mifflin-St Jeor predicts BMR within 10% for most people, but individual variation exists. Use TDEE as a starting point and adjust based on real-world results after 2–3 weeks.
• The surplus calculation uses the same 7,700 kcal/kg. For muscle gain, the actual energy cost per kg of lean tissue is lower (~1,500–2,500 kcal/kg), so the calculator produces a conservative (slightly high) surplus estimate.
• Macro protein targets are based on goal weight for deficit/gain modes and current weight for recomposition. This follows the convention that protein needs scale with the lean mass you’re trying to preserve or build.

Verification

All values verified against manual Mifflin-St Jeor + activity multiplier calculation.