Free Weight-Loss Calculators and Tools

Key takeaways

• BMI is a screening measure for adults 20 and older; the CDC says it should be considered with other factors, not on its own.^[1]
• Calorie estimates use the Mifflin-St Jeor equation, built from measured resting energy use in 498 healthy adults.^[2]
• The timeline estimator follows the NIH Body Weight Planner approach and shows a range, never a promised date.^[3]
• NICE advises keeping your waist to less than half your height, a waist-to-height ratio under 0.5.^[6]

In short: InstaTuck’s free weight-loss calculators estimate your BMI, daily calories, protein and macros, body fat, a realistic timeline and more, using published formulas that each tool explains in plain words. Everything runs in your browser: what you type is not sent to us or stored anywhere, and the weight tracker keeps its data on your own device. The results are estimates to start a conversation, not a diagnosis.

Start here: do these next 1-2-3

Body measurements

Calories and nutrition

Which calorie tool? The calorie calculator gives a quick daily target, the TDEE calculator shows how your maintenance number is built, and the weight-loss timeline calculator turns a plan into a date range.

Progress and planning

Costs, decisions and conversions

To put two or three treatments side by side, use the comparison builder on our compare page.

Which tool answers which question

• “Where am I starting from?” The BMI calculator plus the waist-to-height calculator.
• “How much should I eat?” The calorie calculator, then the protein calculator or the macro calculator.
• “How long might it take?” The weight-loss timeline calculator, which shows a range rather than a date.
• “Is it working?” The weight tracker and the weight change percentage calculator.
• “What does my workout add?” The calories burned calculator.
• “What could treatment cost, and what fits me?” The GLP-1 cost comparison and Find My Options.

Using the numbers together

No single number tells the whole story. The CDC notes that BMI does not distinguish between fat, muscle and bone, and does not show where in the body fat is carried. Waist size adds that missing piece: according to the NHLBI, for adults with a BMI of 25 or more, a waist of more than 35 inches for women or more than 40 inches for men raises the risk of heart disease and type 2 diabetes. Looking at BMI, waist-to-height and, if you like, the body fat estimate side by side gives a fuller picture than any one of them.

Calorie results are a starting point, not a rule. Use the estimate for a few weeks, compare it with the trend in your weight tracker, and adjust; real bodies rarely match a formula exactly, and weight moves up and down from day to day for reasons that have nothing to do with fat. If the numbers raise questions about your health, bring them to your clinician, who can put them together with your history, an exam and lab tests.

Measuring for accurate results

Good inputs make better estimates. Every tool accepts pounds and inches or kilograms and centimeters, and our pounds to kilograms and inches to centimeters converters use the exact factors. For your waist, the NHLBI advises standing, placing the tape around your middle just above your hipbones, and measuring just after you breathe out. Weigh yourself at the same time of day, on the same scale, in similar clothing, and compare weekly averages rather than single days. For the body fat estimate, take each measurement twice and use the average.

If a result surprises you, check the units first: a height entered in centimeters as inches, or a weight in kilograms entered as pounds, is the most common reason a number looks wrong.

If you take a GLP-1 medication

Calorie calculators assume a normal appetite, and GLP-1 medications such as semaglutide and tirzepatide reduce it, so many people eat well below any target without trying. In that situation the protein calculator is often the more useful tool: a 2025 joint advisory from four obesity and nutrition societies puts enough protein and resistance training among the top priorities for protecting muscle and bone during treatment. The GLP-1 cost comparison helps with the practical side, and our guide to what to eat on a GLP-1 turns the numbers into meals. Your prescriber and a registered dietitian can set targets that fit you.

Your privacy: the tools run in your browser

Every calculation happens on your device. Your height, weight, age and measurements are not sent to InstaTuck, not saved on our servers and not added to the page address. The weight tracker saves entries in your browser’s local storage so you can come back to them; you can export a CSV or clear everything at any time. Find My Options never stores your answers. There are no advertising pixels on any tool page. The details are in our privacy policy and consumer health data notice.

How the tools work: methods and sources

Each tool page shows its formula in plain words, its limits and its references. The main methods:

• BMI uses weight divided by height squared and the CDC adult categories (under 18.5 underweight, 18.5 to under 25 healthy weight, 25 to under 30 overweight, 30 or more obesity) for adults 20 and older. The CDC describes BMI as a screening measure to be considered with other factors, not a diagnosis.
• Calories and TDEE start from the Mifflin-St Jeor equation, developed from measured resting energy use in 498 healthy adults aged 19 to 78, multiplied by an activity factor.
• The timeline estimator is informed by the NIH Body Weight Planner, which accounts for the way energy needs fall as weight falls, so it shows a range rather than a promised date.
• Protein and macros start from the protein RDA of 0.8 grams per kilogram a day and the acceptable range of 10 to 35% of calories from protein, then show the higher ranges studied during weight loss.
• Calories burned uses MET values (multiples of resting energy use) from the Compendium of Physical Activities, times your weight and the time spent.
• Waist-to-height divides waist by height. NICE, the UK’s health guidance body, advises keeping your waist to less than half your height, a ratio under 0.5.

Limits and safety

Calculators work with averages, and your body is not an average. They are built for adults; anyone under 18 is pointed to growth-based advice from a pediatrician instead of targets. They never suggest very low calorie targets without a note to talk with a clinician, and they do not apply to pregnancy or breastfeeding. If you have a health condition, take regular medication or have a history of an eating disorder, use the numbers only together with a professional. Our guides help you use the results well: start with calorie deficit, nutrition for weight loss or how to lose weight.

Questions to ask a professional

• My BMI is in a certain range: what other measures should we look at for my health?
• Is the calorie target from this calculator safe for me, given my health and medications?
• How much protein should I aim for, given my kidney function and activity level?
• Which numbers should I track between visits, and how often?

Frequently asked questions

Are InstaTuck calculators free?

Yes. All tools are free and need no account. They run in your browser, and nothing you type is sent to us.

How accurate are the calculators?

They use published formulas that work well on average but can be off for an individual. Treat results as estimates and check them against your real progress or with a clinician.

Which calculator should I use to plan weight loss?

Start with the calorie calculator for a daily target, then the protein calculator. Use the weight-loss timeline calculator for a realistic date range and the weight tracker to follow your progress.

Where is my weight tracker data saved?

Only in your browser's local storage on your device. We cannot see it. You can export a CSV or clear it at any time.

Can teenagers use the calculators?

The tools are built for adults. Teens are still growing, so the tools show no targets for anyone under 18 and point to advice from a pediatrician.

References

1. Adult BMI Categories. Centers for Disease Control and Prevention. (accessed October 7, 2026) Government page
2. Mifflin MD, St Jeor ST, Hill LA, Scott BJ, Daugherty SA, Koh YO. A new predictive equation for resting energy expenditure in healthy individuals. American Journal of Clinical Nutrition, 1990. doi:10.1093/ajcn/51.2.241 · PMID 2305711 (accessed October 7, 2026) Other
3. Body Weight Planner. National Institute of Diabetes and Digestive and Kidney Diseases (NIH). (accessed October 6, 2026) Government page
4. Wolfe RR, Cifelli AM, Kostas G, Kim IY. Optimizing Protein Intake in Adults: Interpretation and Application of the Recommended Dietary Allowance Compared with the Acceptable Macronutrient Distribution Range. Advances in Nutrition, 2017. doi:10.3945/an.116.013821 · PMID 28298271 (accessed October 6, 2026) Review
5. Herrmann SD, Willis EA, Ainsworth BE, et al.. 2024 Adult Compendium of Physical Activities: A third update of the energy costs of human activities. Journal of Sport and Health Science, 2024. doi:10.1016/j.jshs.2023.10.010 · PMID 38242596 (accessed October 7, 2026) Review
6. Overweight and obesity management (NG246): identifying and assessing overweight, obesity and central adiposity. National Institute for Health and Care Excellence (NICE), 2025. (accessed October 7, 2026) Guideline
7. About Body Mass Index (BMI). Centers for Disease Control and Prevention. (accessed October 7, 2026) Government page
8. Assessing Your Weight and Health Risk. National Heart, Lung, and Blood Institute (NIH). (accessed October 6, 2026) Government page
9. Mozaffarian D, Agarwal M, Aggarwal M, et al.. Nutritional priorities to support GLP-1 therapy for obesity: A joint Advisory from the American College of Lifestyle Medicine, the American Society for Nutrition, the Obesity Medicine Association, and The Obesity Society. Obesity, 2025. doi:10.1002/oby.24336 · PMID 40445127 (accessed October 6, 2026) Society statement

Facts checked on October 6, 2026

Educational information, not medical advice. Talk with a qualified healthcare professional about your own situation. In an emergency call 911.