In short
- We estimate the calories your body uses at rest with the Mifflin-St Jeor equation.
- We multiply that by an activity factor for your whole day to estimate maintenance: the calories that would keep your weight about the same.
- For a cut or bulk, we take a percentage off or add a percentage on, with limits and safety rules.
- Your real weight trend over 3–4 weeks then tells you how close the estimate is for you.
1. Resting energy
Resting energy is what your body uses at complete rest: breathing, circulation, keeping warm. It's usually the largest part of daily energy use, but it isn't your total.
Mifflin-St Jeor equation:
- Males: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) + 5
- Females: (10 × weight in kg) + (6.25 × height in cm) − (5 × age in years) − 161
US units are converted first (1 kg = 2.20462 lb, 1 inch = 2.54 cm). The calculator accepts adults aged 18–80, heights of 120–230 cm (3′11″–7′6″) and weights of 35–300 kg (77–660 lb).
How accurate is it? A systematic review found Mifflin-St Jeor more likely than other common equations to come within 10% of measured resting energy, but it noted real errors for individuals and less evidence for older adults and some ethnic groups [2]. In a study of 362 healthy adults, it was within 10% of measured resting energy for 56% of participants, and accuracy varied by sex, BMI, age and ethnicity [3]. That's why we show a range alongside your number, and why your own weight trend matters more than any formula.
2. Activity: from resting energy to maintenance
Maintenance = resting energy × activity factor. The factor (called physical activity level, or PAL) is your total daily energy divided by your resting energy. It covers your job, walking, general movement and exercise, not just workouts.
| Level | Factor | Typical day |
|---|---|---|
| Sedentary | 1.40 | Desk job or mostly sitting, little walking, little or no planned exercise |
| Lightly active | 1.55 | Mostly sitting job with some daily walking or light activity, or light exercise 1–3 times a week |
| Moderately active | 1.70 | On your feet for much of the day, or a desk job plus moderate exercise 3–5 times a week |
| Very active | 1.90 | Physically demanding job, or a fairly active day plus hard exercise 6–7 days a week |
| Extra active | 2.10 | Very demanding physical work plus hard training, or long endurance training most days |
Why these factors. They come from measurements of real daily energy use, not from copying other calculators. The FAO/WHO/UNU expert report groups lifestyles as sedentary or light (1.40–1.69), active or moderately active (1.70–1.99) and vigorous (2.00–2.40) [4]. The NIH Body Weight Planner uses a similar range, from 1.4 for sedentary people to 2.5 for very active [5]. Our factors sit in the lower-middle of those bands, because people often overrate their activity. Many calculators use older textbook factors of 1.2–1.9; their "sedentary" 1.2 is below what these studies measured for sedentary adults.
Limits of activity factors. Five levels can't describe everyone. Two people with the same label can differ by hundreds of calories a day. Daily steps are only used as a cross-check and for your first step; they're never added on top, because walking is already part of your activity level.
3. Cutting, maintaining and bulking
| Goal and pace | Change from maintenance | Limit |
|---|---|---|
| Cut, conservative | −10% | at most 400 kcal a day |
| Cut, moderate | −20% | at most 750 kcal a day |
| Cut, aggressive | −25% | at most 1,000 kcal a day; only offered with a BMI of 25 or more |
| Maintain | 0 | no deficit or surplus |
| Bulk, conservative | +5% | at most 250 kcal a day |
| Bulk, moderate | +10% | at most 400 kcal a day |
| Bulk, aggressive | +15% | at most 600 kcal a day, with a warning |
Why percentages. A fixed number like 750 calories is a much bigger share of a small person's needs than a large person's. Percentages keep the change in proportion. For cutting, a review of evidence for keeping muscle suggests losing about 0.5–1% of body weight a week; faster loss takes a larger share from lean mass [6]. For bulking, a review suggests roughly 10–20% above maintenance for beginners and 5–10% for experienced lifters, aiming for about 0.25–0.5% of body weight gained a week [7]. Extra calories beyond that mainly add fat, not more muscle.
The results show these weekly guidelines as a way to check progress, not as predictions. We don't use the "3,500 calories = 1 pound" rule to forecast weight change; the body adapts as weight changes, so that rule overstates long-term loss [8].
4. Safety rules
- No cut is suggested if weight is already low for height (BMI under 18.5); maintenance is shown with advice to see a professional.
- The calculator won't suggest a target below 1,500 calories a day for males or 1,200 for females. This is a cautious limit for the tool, not proof that any intake is safe. If a cut would go lower, the target is set at the limit with advice to see a doctor or registered dietitian. If maintenance is already within 100 calories of the limit, no cut is suggested.
- The aggressive cut isn't offered with a BMI under 25. With less weight to lose, a larger deficit takes more from muscle and is harder to sustain.
- A caution is shown for bulking with a BMI of 30 or more, and for an aggressive bulk with a BMI of 25 or more.
- The calculator is for adults. It isn't suitable if you're pregnant or breastfeeding, have a medical condition or take medication that affects your weight or energy needs, or have a history of disordered eating. In those cases, please get individual advice from a professional.
BMI is only used for these rules and is never shown. It's a rough measure that doesn't distinguish muscle from fat.
5. Nutrition targets (Improve Life)
- Protein: 1.8 g per kg of reference weight when cutting, 1.6 when maintaining or bulking; Higher protein adds 0.4. Gains in muscle from resistance training plateau at around 1.6 g/kg a day [9], and higher intakes help keep muscle during a deficit [6, 10]. Reference weight is your weight, or if your BMI is above 25, the weight at a BMI of 25 for your height, because these g/kg amounts were studied in leaner people. Protein stays between 10% and 30% of calories (35% for Higher protein).
- Fat: 30% of calories (25% when bulking, to leave more carbohydrate for training), 22% for Higher carbohydrate, 33% for Higher fat; all inside the 20–35% guideline range [11].
- Carbohydrates: the remaining calories. Grams are rounded so that protein × 4 + carbs × 4 + fat × 9 equals the target exactly.
- Fiber: 14 g per 1,000 calories [12]. Fiber is counted inside carbohydrates, as on US food labels, not added on top.
- Changing the preference never changes the calorie target.
6. Finding your real maintenance
The best estimate of your maintenance comes from your own data:
- Eat close to one calorie target for 3–4 weeks, keeping a rough record of intake.
- Weigh yourself the same way (morning, after the bathroom, before eating) on at least 3–4 days a week, and use weekly averages.
- If the weekly averages hold steady, your intake is close to maintenance. If they're moving, they tell you roughly how far off you are.
- Adjust by about 100–200 calories at a time, then give it another 2–3 weeks.
Don't change your target because of one weigh-in, and don't keep cutting calories every time progress slows.
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.
- Frankenfield D, Roth-Yousey L, Compher C. Comparison of predictive equations for resting metabolic rate in healthy nonobese and obese adults: a systematic review. J Am Diet Assoc. 2005;105(5):775–789.
- Hasson RE, Howe CA, Jones BL, Freedson PS. Accuracy of four resting metabolic rate prediction equations: effects of sex, body mass index, age, and race/ethnicity. J Sci Med Sport. 2011;14(4):344–351.
- FAO/WHO/UNU. Human energy requirements: report of a joint FAO/WHO/UNU expert consultation. FAO Food and Nutrition Technical Report Series 1. Rome; 2004.
- National Institute of Diabetes and Digestive and Kidney Diseases. Body Weight Planner. niddk.nih.gov/bwp.
- Helms ER, Aragon AA, Fitschen PJ. Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. J Int Soc Sports Nutr. 2014;11:20.
- Iraki J, Fitschen P, Espinar S, Helms E. Nutrition recommendations for bodybuilders in the off-season: a narrative review. Sports. 2019;7(7):154.
- Hall KD, Sacks G, Chandramohan D, et al. Quantification of the effect of energy imbalance on bodyweight. Lancet. 2011;378(9793):826–837.
- Morton RW, Murphy KT, McKellar SR, et al. A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. Br J Sports Med. 2018;52(6):376–384.
- Hector AJ, Phillips SM. Protein recommendations for weight loss in elite athletes: a focus on body composition and performance. Int J Sport Nutr Exerc Metab. 2018;28(2):170–177.
- Institute of Medicine. Dietary Reference Intakes for Energy, Carbohydrate, Fiber, Fat, Fatty Acids, Cholesterol, Protein, and Amino Acids. National Academies Press; 2005.
- U.S. Department of Agriculture and U.S. Department of Health and Human Services. Dietary Guidelines for Americans, 2020–2025.
These rules were written from published evidence by LifesRank. They haven't yet been reviewed by a registered dietitian or doctor; we plan to have them reviewed before launch.