Anyone who has lost weight knows that progress is faster at the start than at the end. The reason is arithmetic, not willpower: a lighter body uses less energy, so the same food intake that created a 500-calorie gap at 82 kg creates a smaller gap at 75 kg. Simple calorie deficit calculators ignore this and return a straight-line timeline that is usually too short. This calculator recalculates your maintenance calories each week as your weight changes, then tells you how long it will take, what you will weigh at 4, 12 and 26 weeks, and whether your intake would stop working before you reach your goal.
It remains a simplified model. Real-world weight changes also depend on body composition, water balance, hormones and activity.
How to use the weight loss calculator
- Enter your sex, age, height and current weight.
- Enter your goal weight. It must be lower than your current weight.
- Choose an activity level that matches a typical week, and a starting daily deficit.
- Read the intake you should keep, the weeks to goal, and the weights at 4, 12 and 26 weeks.
- Check the plateau figure. If your goal is below it, the plan needs to change.
Weight loss formula
Intake = maintenance at starting weight − daily deficit
Weekly loss (kg) = (maintenance(w) − intake) × 7 ÷ 7,700
Repeat each week until weight reaches the goal
Maintenance uses the Mifflin-St Jeor equation (Mifflin et al., Am J Clin Nutr 1990;51:241–247) and the activity multipliers 1.2, 1.5, 1.75 and 2.2, the midpoints of the Physical Activity Level categories in the Institute of Medicine’s Dietary Reference Intakes (2005). The conversion of 7,700 kcal per kilogram of lost weight is the traditional approximation; Hall et al. (Lancet 2011;378:826–837) showed that a fixed conversion overstates long-term loss when intake is held constant, which is what this weekly update corrects for.
Worked example
A 35-year-old woman, 168 cm, 82 kg, sedentary, wants to reach 70 kg with a 500 kcal starting deficit.
- Starting maintenance = 1,841 kcal, so the intake she follows is 1,341 kcal.
- Week 1 loss = 500 × 7 ÷ 7,700 = 0.45 kg. Each kilogram lost lowers maintenance by 12 kcal (10 kcal × 1.2), so the daily gap narrows by about 5.5 kcal a week.
- Time to goal: 31 weeks, compared with 26 weeks by the fixed-deficit rule.
- Weight at 4, 12 and 26 weeks: 80.2, 76.9 and 71.7 kg.
The week-by-week curve
| Week | Weight | Lost so far | Maintenance at that weight |
|---|---|---|---|
| 0 | 82.0 kg (181 lb) | 0.0 kg | 1,841 kcal |
| 4 | 80.2 kg (177 lb) | 1.8 kg | 1,819 kcal |
| 8 | 78.5 kg (173 lb) | 3.5 kg | 1,799 kcal |
| 12 | 76.9 kg (169 lb) | 5.1 kg | 1,779 kcal |
| 20 | 73.8 kg (163 lb) | 8.2 kg | 1,742 kcal |
| 26 | 71.7 kg (158 lb) | 10.3 kg | 1,717 kcal |
| 40 | 70.0 kg (154 lb) | 12.0 kg | 1,697 kcal |
Why weight loss slows
- A smaller body burns less. Each kilogram lost lowers resting energy use by around 10 kcal and activity energy slightly more.
- Adaptation. The body can reduce energy expenditure by more than weight loss alone explains; this model does not include it, so real progress may be somewhat slower.
- Water shifts. The first week often shows a large drop that is mostly fluid and glycogen.
- Adherence drifts. Portions and activity tend to creep back over months.
How to plan a sustainable loss
Health agencies suggest aiming for around 1 to 2 pounds (0.5 to 1 kg) a week, and the US CDC notes that people who lose weight gradually are more likely to keep it off. A 5 to 10 percent reduction in weight is associated with clinically meaningful health gains (Jensen et al., Circulation 2014;129(suppl 2):S102–S138); see the Weight Loss Percentage Calculator. To estimate a calendar date use the Goal Weight Date Calculator, and to check your calorie target use the Calorie Calculator or the Calorie Deficit Calculator.
This page is for information only and is not medical or dietary advice. Speak to a doctor or registered dietitian before starting a weight-loss plan, especially if you have a medical condition, take medicines, are pregnant or have a history of disordered eating.
Sources
- Mifflin MD, St Jeor ST, et al. Am J Clin Nutr 1990;51:241–247.
- Hall KD, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet 2011;378:826–837.
- Institute of Medicine. Dietary Reference Intakes for Energy, 2005 (Physical Activity Level categories).
- Jensen MD, et al. 2013 AHA/ACC/TOS guideline for the management of overweight and obesity in adults. Circulation 2014;129(suppl 2):S102–S138.
- US Centers for Disease Control and Prevention. Steps for Losing Weight.
Frequently asked questions
How is this different from a simple calorie deficit calculator?
A fixed-deficit calculator assumes the gap between intake and expenditure stays the same until you reach your goal. In reality, as you get lighter your maintenance calories fall, so the same intake produces a smaller deficit. This calculator recalculates your maintenance each week, which gives a more realistic, longer timeline.
What is the plateau weight?
It is the weight at which your daily maintenance calories equal the intake you are following. At that weight, weight loss stops. If your goal is below it, the calculator says so, and you would need to reduce intake further or increase activity.
Is this the Hall model?
No. The NIH Body Weight Planner, based on the work of Kevin Hall and colleagues (Lancet 2011;378:826–837), models energy balance in more detail, including changes in body composition and adaptation. This calculator uses a simpler version that captures the main effect, falling energy needs, and should be treated as an approximation.
How fast is it safe to lose weight?
Common guidance, including from the US CDC, is about 1 to 2 pounds (0.5 to 1 kg) a week for most adults. Faster loss is hard to maintain and may need medical supervision.
Why does the real world differ?
Water weight, hormones, changes in activity and the accuracy of your food log all affect results. Use the timeline as a guide and adjust based on your actual progress over several weeks.