NIDDK Body Weight Planner: how to use it, and how to check it against your scale

The NIDDK Body Weight Planner is a free calculator from the US National Institutes of Health that tells you how many calories to eat to reach a goal weight by a date, and how many to eat afterwards to stay there. It takes about two minutes, needs no account, and is the one public tool that models your body's energy needs falling as you lose weight instead of assuming they stay fixed.

Use it when you want a calorie target tied to a date. Then treat the result the way the researchers who built it do: as a planning estimate to be checked against your scale, not a promise.

What the planner is, in one paragraph

The tool runs a mathematical model of human metabolism developed by Kevin Hall's group at the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). The model tracks fat mass, lean mass and how your energy expenditure changes as both shift, and it was tuned against data from controlled feeding studies. NIDDK's description of the planner · Hall et al., The Lancet, 2011

That is the difference from a plain calorie calculator. A plain calculator estimates what you burn today and subtracts a deficit. The planner also estimates what you will burn at 5 kg lighter, and at 10 kg lighter, and works out the intake that gets you to the goal on your chosen date despite that drop. If you have ever wondered why the old "3,500 calories per pound" arithmetic runs ahead of real scales, that article explains the maths; this one is about using the tool.

What you enter

Open the planner at niddk.nih.gov/bwp. There is a switch for metric or US units near the top. The fields, in order:

1. Current weight, sex, age and height. Use a morning weight from the last few days, not your lightest ever.
2. Physical activity level. This is a single number between about 1.4 (mostly sitting, little deliberate exercise) and 2.5 (a physically demanding job plus training). The default is 1.6: a desk job with a walk or a bike ride at least once a week. The "Estimate your level" button asks about your work, your daily movement and your exercise, then fills the number in for you. Use it. Most people who type a number directly pick one that is too high, and every calorie the planner then gives you is inflated by the same optimism.
3. Goal weight. Enter a weight, not a wish. If it lands below a BMI of about 18.5 for your height, pick a higher one; the planner will run the numbers regardless, but a target in that range is not a healthy plan.
4. Goal date, or number of days. Six months is a sensible first choice for 5–10 kg. A closer date produces a lower calorie target, and the planner will happily produce one that is too low to live on. It does not apply the safe floors a coach would.
5. Change in physical activity. Optional. If you genuinely will train more, say by how much, and the planner lowers the size of the calorie cut it needs from food. If you are not sure, leave it at zero and change it later, when the training habit is real.

There is also an Expert Settings switch. Unless you have measured values to enter, leave it closed; the defaults are the researchers' best estimates for a person of your size.

The three numbers it gives you

Press Calculate and you get three calorie figures. Each one has a different job.

• Calories to maintain your current weight. The model's estimate of what you burn today. This is the number to compare with other calculators, and with the intake that has kept your weight steady recently. If you have logged food honestly for two weeks at a stable weight, your own average is better than any model's guess.
• Calories to reach your goal by the date. The target. Notice that it is lower than "maintain minus a round number", because the planner has already allowed for your expenditure falling along the way.
• Calories to maintain your goal weight. The number people skip and should not. It is what a lighter you will need to stay lighter, and it is always lower than today's maintenance. Plan for it now: the reason so much lost weight returns is that people go back to eating today's maintenance in a smaller body.

The planner also draws the expected weight curve. Screenshot it. That curve is what you will check your scale against.

A worked example

Take the person from our calorie calculator: a 35-year-old woman, 170 cm, 80 kg, moderately active, aiming for 72 kg in six months.

The flat arithmetic says: 8 kg × 7,700 kcal per kg of fat ≈ 61,600 kcal to lose, so a 550 kcal daily deficit gets there in about 112 days. That is the "3,500 calories per pound" logic, and it is where the planner departs.

The planner's model recognises that each kilogram she loses takes a little of her daily expenditure with it. Using the same dynamic logic as our calorie deficit calculator, roughly 24 kcal a day per kilogram of eventual change, her maintenance at 72 kg is about 190 kcal a day lower than it is at 80 kg. So a fixed intake that starts as a 550 kcal deficit is closer to a 360 kcal deficit by the end, and the weight loss slows accordingly. Her outcome under the planner is one of two things: either the same intake reaches 72 kg later than 112 days, or reaching it on the date needs a lower intake than the flat rule suggested.

Two practical consequences:

• If she trusts the flat rule and the scale "stalls" at week ten, nothing is wrong. The maths was.
• The planner's "maintain goal weight" number for her will sit roughly 200 kcal below today's maintenance. Eating today's number at 72 kg is a slow regain, not a plateau.

Where people go wrong with it

Overstating activity. The single biggest error. A 1.8 instead of a 1.6 adds a few hundred calories to every figure, and the plan fails on paper before it starts. Use the estimator, and if in doubt round down.

Choosing the date by impatience. A three-month date for 10 kg will return a target far below what is sustainable. If the reach-goal figure is under about 1,200 kcal for a woman or 1,500 for a man, move the date, not your appetite. How low is too low.

Ignoring the third number. See above. Maintenance at your goal is the number the whole exercise is for.

Expecting a straight line. The scale moves with water, salt, carbohydrate and your cycle by a kilogram or more in a day. The planner's curve is a trend; compare it with a trend.

Treating the output as measurement. It is a model of an average person of your size and activity. Your own expenditure can sit 10–15 percent either side of it, which is why the check below matters more than the calculation.

How to check it against your scale

The planner gives you a prediction. Two weeks of records tell you whether it fits you.

1. Eat at the reach-goal number as closely as you honestly can, and log everything, including oils, drinks and the weekend.
2. Weigh yourself each morning after the bathroom and before breakfast, and use the weekly average, not the daily reading.
3. After 14 days, compare your weekly average with the planner's curve for day 14. If you are within about half a kilogram of it, keep going. If you are losing clearly faster, the model underestimated your expenditure and you can add 100–150 kcal. If you are losing clearly slower and your log is complete, take 100–150 kcal off, or give it another week before deciding; water alone can hide two weeks of fat loss.

That loop, predict, log, compare, adjust, is the whole method. The planner does the first step well. The other three are habits, and they are where most plans actually fail. What usually goes wrong.

Planner, deficit calculator or coach?

Use the planner for the plan. Use whatever you will actually open every day for the record.

FAQ

Is the NIDDK Body Weight Planner free and official?

Yes. It is published by the National Institute of Diabetes and Digestive and Kidney Diseases, part of the US National Institutes of Health, at niddk.nih.gov/bwp. It is free, needs no account, and stores nothing you type.

Is the Body Weight Planner accurate?

It is more realistic than a fixed-deficit calculator because it models expenditure falling as you lose weight, and its model was validated in controlled studies. For any one person the result is still an estimate with an error band of a few hundred calories, which is why the two-week scale check matters.

Why does the planner give me fewer calories than my app?

Most apps subtract a fixed deficit from today's maintenance. The planner also accounts for the maintenance you will have lost by the goal date, so to arrive on time it asks for a lower intake. The same reason applies to the maintain-goal number.

Can I use it if I am under 18, pregnant or breastfeeding?

No. NIDDK states the tool is for adults 18 and over and not for pregnant or breastfeeding women. Anyone with a medical condition that affects weight should plan with their doctor.

Does it work in kilograms and centimetres?

Yes. There is a units switch at the top of the tool.

Sources

• NIDDK, Body Weight Planner and the tool itself at niddk.nih.gov/bwp
• Hall KD et al., Quantification of the effect of energy imbalance on bodyweight, The Lancet, 2011
• NIH News in Health, Body Weight Planner, 2015
• Validity of two weight prediction models for community-living patients, Scientific Reports, 2023

The easy way to do this

HeyCoach turns the planner's number into a daily record without another app. Say "set my calories to 1,700" and the target is live. Send meals as a photo or a line of text, reply "weight 79.4" each morning, and the evening check-in shows how the week compares with the plan. When the scale disagrees with the prediction, the log is already there to explain why, and the target is one message away from changing.