Weight Loss Calculator

How the weight loss calculator works

The calculator answers one question: how much do you need to eat each day to get from your current weight to your goal weight by the date you picked?

1. Your TDEE. First it estimates how many calories you burn a day: your basal metabolic rate from the Mifflin-St Jeor equation (Mifflin et al., 1990), multiplied by an activity factor from 1.2 to 1.9. The TDEE calculator explains this step in detail.

2. The deficit. Each kilogram of body weight lost needs a total deficit of about 7,700 kcal (about 3,500 kcal per pound). The calculator spreads the total over the days until your target date:

Daily deficit  = (current weight − goal weight in kg) × 7,700 ÷ days to target
Daily calories = TDEE − daily deficit
Weekly loss %  = kg lost per week ÷ current weight × 100

3. Safety checks. The result is flagged if:

• daily calories fall below 1,200 kcal for women or 1,500 kcal for men, the usual minimums without medical supervision;
• you would lose more than 1% of your body weight a week. When that happens the calculator also shows how long the goal would take at 1% a week;
• your goal weight gives a BMI under 18.5, the start of the underweight range;
• your target is below your BMR. That isn’t dangerous on its own, but it is hard to sustain.

Worked example

A 35-year-old woman, 165 cm (5 ft 5 in) and 80 kg (176 lb), lightly active, wants to reach 70 kg (154 lb) in 20 weeks:

• BMR = 10 × 80 + 6.25 × 165 − 5 × 35 − 161 = 1,495 kcal
• TDEE = 1,495 × 1.375 = 2,056 kcal
• Total deficit = 10 kg × 7,700 = 77,000 kcal over 140 days = 550 kcal a day
• Daily calories = 2,056 − 550 = 1,506 kcal
• Pace = 0.5 kg (1.1 lb) a week, which is 0.63% of her body weight. No warnings.

If she wanted the same 10 kg in 8 weeks, the deficit would be 1,375 kcal a day and her target only 681 kcal. That is far below the 1,200 kcal floor and a loss of 1.56% of her body weight a week, so the calculator shows both warnings. At a pace of 1% a week she would need about 12.5 weeks, and even that means about 1,176 kcal a day. A pace of 0.75% a week (about 17 weeks at 1,396 kcal) or 0.5% a week (25 weeks at 1,616 kcal) is more realistic.

Calorie deficit by rate of loss

The daily deficit needed for common weekly targets, and what each target is as a share of body weight:

⚠ = faster than 1% of body weight a week. Deficits use 7,700 kcal per kg (about 3,500 kcal per lb).

A 2 lb a week target is fine at 250 lb but too fast at 150 lb. That is why the calculator judges pace as a percentage of your weight rather than a fixed number.

Why the plan slows down

The 7,700 kcal/kg rule comes from Wishnofsky (1958) and works well over a few weeks. Over longer periods it overestimates weight loss, because as you get lighter your BMR and the energy cost of moving both fall. Kevin Hall and colleagues at the US National Institutes of Health modelled this in The Lancet (2011) and found real loss is slower than the simple rule predicts, especially after the first few months.

What to do about it:

1. Recalculate every 4–6 weeks with your new weight. Your target will drop a little each time.
2. Judge progress on a weekly average, not single weigh-ins. Water can swing your weight by 1–2 kg (2–4 lb) from day to day.
3. Keep protein high and lift weights. That helps make sure what you lose is fat rather than muscle. The protein calculator gives a daily target.
4. Plan meals around your target with the macro calculator, which splits your calories into protein, carbs and fat.

When to get help

Speak to a doctor or registered dietitian before starting if you are pregnant or breastfeeding, under 18, have diabetes or another medical condition, take medication that affects your weight, or have a history of disordered eating. Stop and get advice if you feel dizzy, faint or constantly exhausted while dieting.