Body Weight Planner
Enter your current weight, goal weight, and timeline. Get a fixed daily calorie target and a week-by-week projection that recalculates estimated maintenance as your weight changes.
Built and reviewed by Jesse · Last reviewed
Enter your stats and goal to build a weight plan.
- Enter your stats. Sex, age, height, current weight, and activity level — these set your TDEE (maintenance calories).
- Enter your goal weight. The weight you want to reach. Lower than current = weight loss plan; higher = weight gain plan.
- Choose a timeline. How many weeks you want to reach your goal. The planner applies general daily-adjustment guardrails of a 1,000 kcal deficit and a 500 kcal surplus.
- Read the adjusted projection. You get a fixed daily intake, the starting and final-week energy gap, and milestones that recompute estimated maintenance as weight changes.
The plan is built from two steps. First, your TDEE (Total Daily Energy Expenditure) is estimated using the Mifflin-St Jeor equation and your activity level. Second, the planner searches for a fixed daily intake that reaches the requested goal near the requested week while staying inside its general planning guardrails.
It then runs that intake week by week. At each step it recomputes Mifflin-St Jeor maintenance from the projected weight, so the same intake produces a smaller deficit during loss or a smaller surplus during gain. The 7,700 kcal-per-kilogram conversion remains a simplifying assumption, and the result explicitly compares this maintenance-adjusted curve with the older straight line.
The most common mistake in weight loss is setting a deficit that is too aggressive to maintain. Very large deficits — beyond about 1,000 kcal/day — may produce faster initial results but can increase muscle loss, fatigue, and hunger. A slower adjustment is often easier to sustain.
A smaller change takes longer but leaves more room for ordinary meals and training. The planner does not label any pace safe for a particular person. Its deficit, surplus, and lower-intake limits are general planning guardrails; health history, medication, pregnancy, and disordered-eating risk require individualized advice.
Daily weight fluctuates by 1–2 kg or more even with perfect adherence to a plan. The main driver is water: sodium, carbohydrate intake, hydration, and hormonal cycles all shift the amount of water your body retains day to day. This noise sits on top of the underlying fat-loss trend and can completely mask a week of good progress.
The most reliable approach is to weigh yourself at the same time each day — ideally in the morning, after using the bathroom, before eating — and track a rolling 7-day average rather than individual readings. The trend over weeks is what matters, not any single number. For a realistic sense of what that trend looks like over months, see the guide on how long it takes to lose weight.
Maintenance-adjusted energy balance model
weekly weight change (kg) = (intake - current TDEE) x 7 / 7700
next week's TDEE = Mifflin-St Jeor at projected weight x activity factor- 7700
- = kcal stored per kilogram of body fat (approx. 3500 kcal/lb)
- TDEE
- = Total Daily Energy Expenditure (Mifflin-St Jeor BMR x activity factor)
- intake
- = the fixed daily calorie target the planner solves for
- current TDEE
- = maintenance recomputed from the projected weight at the start of each week
The model captures maintenance drift caused by body-weight change only. It does not model adaptive thermogenesis, activity changes, adherence, water, or body composition. General guardrails limit the starting deficit to 1,000 kcal/day, the surplus to 500 kcal/day, and loss targets to the site's sex-specific planning floor; these are not individualized medical safety thresholds.
How does this calculator work?
It estimates your starting Total Daily Energy Expenditure (TDEE) with the Mifflin-St Jeor equation, finds a fixed calorie intake for your goal and timeline, and then recomputes estimated maintenance at each projected weekly weight. That makes the deficit or surplus shrink over time instead of pretending it remains constant.
How long does it realistically take to lose weight?
A gradual rate is easier for many people to sustain. Faster rates can increase the risk of muscle loss and may be harder to maintain. The planner uses a general 1,000 kcal/day deficit guardrail and flags when the requested timeline would require a larger adjustment; this is not an individualized medical safety limit.
What is the 7,700 kcal per kilogram rule?
It is a planning approximation for converting an energy imbalance into weight change: about 7,700 kilocalories per kilogram, or 3,500 kcal per pound. A real kilogram lost or gained is not a fixed package of body fat; water, glycogen, and lean tissue also change. The planner therefore uses the constant to compare trajectories, not to promise an exact biological result.
Why is there a cap on the daily deficit or surplus?
Very large deficits can increase fatigue and the risk of losing lean mass, while large surpluses may add more fat relative to muscle. The planner therefore uses general adjustment guardrails of a 1,000 kcal/day deficit and a 500 kcal/day surplus. They are planning defaults, not individualized medical limits.
Will I actually lose exactly this much per week?
Probably not exactly, and that is normal. Weight fluctuates with water retention, sodium, carbohydrate stores, hormonal cycles, and digestion. The milestones are an estimated trend under fixed intake and activity. If you weigh regularly, compare rolling averages taken under similar conditions rather than treating one reading as the result.
How is this different from the Calorie Deficit Calculator?
The Calorie Deficit Calculator lets you choose a deficit size and tells you how much you would lose. This planner works the other way: you enter a goal weight and a deadline, and it tells you what deficit is needed. Use this one when you have a specific target and timeline; use the Calorie Deficit Calculator when you want to explore different deficit sizes and their effects.
Can I use this to plan weight gain?
Yes. Enter a higher goal weight and the planner switches to a surplus, capped by a general 500 kcal/day planning guardrail. The projected change is body weight, not muscle: the calculator cannot determine how much gain would be lean tissue versus fat, because training, experience, genetics, and adherence are not inputs.
Do I need to adjust my target as I lose weight?
Yes. This planner adjusts its projection for the maintenance change caused by body weight, but it cannot observe your actual activity, adherence, body composition, or adaptive changes in energy expenditure. Re-run it with your measured weight and a newly calibrated maintenance estimate every 8 to 12 weeks, or sooner if your trend differs materially from the projection.
What if I do not know what goal weight to pick?
This planner assumes you already have a target in mind — it works out the calories and timeline to reach it. If you are still deciding what that target should be, start with the Ideal Weight Calculator, which shows a healthy weight range for your height and sex, then bring a number from that range back here to build the plan. Picking a goal from a healthy range rather than a single figure tends to produce a more realistic timeline.
How is this different from the NIDDK Body Weight Planner?
The NIDDK Body Weight Planner uses Kevin Hall's validated dynamic model and represents more physiological adaptations. FitCalcs uses a smaller, transparent model: it recomputes Mifflin-St Jeor maintenance as projected body weight changes but does not model adaptive thermogenesis, changing movement, or body composition. Use this tool for an inspectable planning estimate, not as a substitute for the NIDDK model or clinical advice.
References
- Mifflin MD, St Jeor ST, Hill LA, et al. A new predictive equation for resting energy expenditure in healthy individuals. Am J Clin Nutr. 1990;51(2):241-247.
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826-837.
FitCalcs calculators provide general estimates for healthy adults and are not medical advice.
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Calculate your Total Daily Energy Expenditure (TDEE) and find out how many calories you burn per day.