Calorie Deficit & Weight Loss Planner
Daily calories to reach your goal weight, from the NIH body-weight model.
Projected weight
Week-by-week projection
How it was calculated
For general information only, not medical advice. Consult a qualified health professional for advice about your situation.
About the Calorie Deficit & Weight Loss Planner
Tell the planner where you are, where you want to be and either the date you want to get there or how fast you want to go. It answers with the calories to eat each day — worked out two ways.
The classic 7,700 kcal per kg rule (3,500 kcal per pound) assumes weight falls in a straight line. It does not: as you lose weight your body needs less energy, so the same intake brings smaller losses each week. The planner therefore also runs the dynamic adult body-weight model of Hall et al. (Lancet, 2011) — the model behind the NIH Body Weight Planner — which tracks fat, lean tissue, glycogen, water and the slowing of metabolism day by day. You get the daily calorie target from both, a projected timeline, the calories to keep the weight off afterwards and plain warnings if a plan is too fast or too low in calories.
How to use it
- Enter your sex, age (18+), height, current weight and goal weight. Add your starting weight if you have already lost some.
- Choose your usual activity level. If unsure, the NIH default of 1.6 describes mostly sitting at work plus walking or cycling at least once a week.
- Under Plan by, choose Target date and pick the date, or Weekly rate and enter how much to lose (or gain) each week.
- Read the daily calories from the NIH model, the 7,700-rule figure, the projected weight curve and the calories to maintain your new weight. Download the week-by-week projection if you want to track against it.
Examples
NIH model: eat about 1,789 kcal/day (607 below her maintenance of 2,396) · 7,700 rule: 1,968 kcal/day — which the model says would leave her near 73 kg on the date · To stay at 70 kg afterwards: about 2,187 kcal/day
Rule: 550 kcal/day deficit (1,846 kcal/day), goal in 20 weeks · Model: the same intake takes about 30 weeks; to arrive in 20 weeks eat about 1,665 kcal/day
Why the 7,700 kcal rule overestimates weight loss
The rule assumes each kilogram of weight lost contains about 7,700 kcal (3,500 kcal per pound), so it divides your total deficit by 7,700. That holds for the first weeks only. As you lose weight you have less tissue to maintain and you burn less moving a lighter body, and your metabolism adapts — so a fixed deficit shrinks over time and weight loss slows until it levels off. Hall et al. showed that the body-weight response to a change in intake is slow, with half the eventual change taking about a year, and offered a better rule of thumb for an average overweight adult: every permanent change of 100 kJ (about 24 kcal) a day leads to an eventual weight change of about 1 kg, half of it in about a year and 95% in about 3 years. Thomas et al. compared the 3,500-kcal rule with seven closely supervised weight-loss studies and found it grossly overestimates actual loss.
How the NIH model works
The model starts from your energy balance today: maintenance calories = Mifflin-St Jeor resting rate × your activity level, and a body-fat estimate from your BMI, age and sex (Jackson et al.) unless you enter a measured value. Each day it updates five things with a 4th-order Runge-Kutta step:
- Fat and lean mass — an energy gap is split between them depending on how much fat you have (people with more fat lose a larger share as fat)
- Glycogen and its water — the fast early drop on any diet
- Extracellular water — which follows changes in salt and carbohydrate intake
- Adaptive thermogenesis — metabolism slowing by about 14% of the change in intake, over about two weeks
To find the calories for a target date the planner searches for the constant daily intake that lands exactly on your goal weight that day. Our implementation reproduces the NIH Body Weight Planner’s own published code to within a kilocalorie on test cases; it assumes your activity stays the same and carbohydrate stays about half of your calories.
How fast, and how low?
- The CDC notes that people who lose weight gradually and steadily — about 1–2 lb (0.5–0.9 kg) a week — are more likely to keep it off. The planner warns when a plan averages more than about 1% of your body weight a week.
- The 2013 AHA/ACC/TOS guideline recommends prescribing 1,200–1,500 kcal/day for women and 1,500–1,800 for men, or a deficit of 500 or 750 kcal/day; the planner warns below 1,200 or 1,500. It also says that a sustained loss of just 3–5% brings clinically meaningful benefits.
- Under 1,000 kcal/day: the NIH Body Weight Planner asks for at least 1,000 kcal/day because food-group targets and nutrient recommendations will not be met below that. Under 800 kcal/day is a very-low-calorie diet, which the guideline restricts to medical settings with close supervision.
- Goals below a BMI of 18.5 are not planned at all.
Sources
- Hall KD et al. Quantification of the effect of energy imbalance on bodyweight, Lancet 2011;378:826-37 (full text)
- NIH/NIDDK Body Weight Planner — the published JavaScript was used to generate our reference test values
- Thomas DM et al. Can a weight loss of one pound a week be achieved with a 3500-kcal deficit? Int J Obes 2013;37:1611-3
- Jensen MD et al. 2013 AHA/ACC/TOS guideline for the management of overweight and obesity in adults, Circulation 2014;129:S102-38
- CDC, Losing weight
- Mifflin MD et al. A new predictive equation for resting energy expenditure, Am J Clin Nutr 1990;51:241-7
Limitations
- Projections assume you eat the same number of calories every day and keep the same activity level. Real life varies — weigh yourself regularly and adjust.
- Both your starting energy needs and your body fat are estimated, so individual results can differ by several kilograms over a year or more (Hall et al. show about ±4 kg after several years from typical uncertainty in starting energy use).
- For adults aged 18 and over. Not for pregnancy or breastfeeding, children, or medical conditions and medicines that affect weight — ask your doctor.
- Exercise added during the plan is not modelled separately; enter your new typical activity level instead.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
How many calories should I eat to lose 1 kg a week?
By the 7,700 kcal rule, 1 kg a week needs a deficit of about 1,100 kcal a day, and even that only holds for the first weeks. For many people — especially smaller or less active women — that means eating below the usual minimum of 1,200 kcal (women) or 1,500 kcal (men). The CDC suggests about 1–2 lb (0.5–0.9 kg) a week. Enter your details with Weekly rate to see what the model predicts.
Why does the NIH model say I need to eat less than the 7,700 rule?
Because your energy needs fall as you lose weight. To reach a goal by a fixed date the model has to make up for the slowdown, so it asks for a bigger deficit at the start. The longer the plan, the bigger the difference.
Why do I lose weight quickly in the first week?
Part of the early loss is glycogen — carbohydrate stored in muscle and liver — and the water stored with it (about 2.7 g of water per gram). The model includes both, which is why its curve drops faster at first and then settles into slower fat and lean-tissue loss.
How many calories will I need after reaching my goal?
Fewer than before — a lighter body uses less energy. The planner shows the intake that keeps your new weight stable; if you go back to your old intake, the model predicts you will regain weight.
Can I use this to plan weight gain?
Yes. Enter a goal above your current weight and the planner shows the daily surplus from both methods. It tells you if the goal is in the overweight BMI range (25–29.9) and warns if it is in the obesity range (30 or more).
What if the planner says my goal cannot be reached?
Even eating nothing would not get you there by that date. Choose a later date or a closer goal — the AHA/ACC/TOS guideline notes that losing just 3–5% of your weight and keeping it off already improves health.