What are the numbers behind a diet?
Modern diet planning reduces to a handful of recurring questions: how many calories should I eat, how should I split them between protein/fat/carbs, how much protein do I actually need to build or maintain muscle, how much water should I drink, and how fast will I lose or gain weight if I run a calorie deficit or surplus. This page gives each its own calculator. The numbers come from three different sources of evidence: macronutrient ratios (the cut/maintain/bulk templates) come from sports-nutrition guidelines (ISSN, ACSM); protein requirements come from nitrogen-balance studies that find the minimum intake needed to maintain muscle protein synthesis at different activity levels; water, calorie balance, and timeline come from energy-balance and fluid-balance physiology. None of these is a perfect predictor of what an individual will actually achieve — body composition is influenced by hormones (insulin, leptin, thyroid), microbiome, sleep, stress, and individual genetics — but they are the best simple rules the field has, and they work on average.
Three vocab distinctions to keep straight: BMR (basal metabolic rate) is calories burned at complete rest. TDEE (total daily energy expenditure) is BMR × activity factor and represents what you actually burn. Calorie target is TDEE ± goal adjustment (cut, maintain, bulk). Macronutrients are protein (4 kcal/g), fat (9 kcal/g), and carbohydrates (4 kcal/g) — the calorie-bearing nutrients. Micronutrients are vitamins and minerals, not counted in calories but essential for the metabolism that uses the calories.
Macros — splitting calories between protein, fat, carbs
Once you have a daily calorie target, the next question is how to split it across the three macronutrients. The page gives three built-in templates: Cut (40% protein / 25% fat / 35% carbs) — high protein to protect muscle while running a deficit, moderate carbs to fuel training. Maintain (30/25/45) — a balanced template for active adults not actively dieting. Bulk (30/20/50) — higher carbs to fuel muscle growth and recovery. With the sample (2000 kcal, cut): protein = 2000×0.40/4 = 200 g (800 kcal), fat = 2000×0.25/9 ≈ 56 g (500 kcal), carbs = 2000×0.35/4 = 175 g (700 kcal). These ratios are starting points; the actual optimal split depends on training type (endurance athletes need more carbs, strength athletes more protein), food preferences, and metabolic health (people with insulin resistance often do better on lower carbs).
Protein — how much do you actually need?
Protein is the only macronutrient with a defined RDA: 0.8 g/kg/day is the minimum that prevents deficiency in sedentary adults (a 70 kg adult = 56 g/day, about two chicken breasts). The ISSN updated this in 2017 for active populations: 1.2–1.6 g/kg for general training, 1.6–2.2 g/kg for muscle building or fat loss in athletes. With the sample (70 kg, moderate training 1.6 g/kg), protein = 112 g/day (448 kcal), or about 37 g/meal for 3 meals and 28 g/meal for 4 meals. Beyond ~2.2 g/kg, additional protein shows diminishing returns for muscle growth in most studies — excess is oxidized for energy or stored as fat. Whey, casein, egg, soy, and pea are complete proteins (all essential amino acids); rice and beans together are complete; a single plant source usually isn't. Per-meal doses around 0.4 g/kg maximally stimulate muscle protein synthesis, but the daily total matters more than distribution.
Water — daily fluid intake
The European Food Safety Authority (EFSA) recommends 35 mL/kg/day for sedentary adults in temperate climates (about 2.5 L for a 70 kg adult); the U.S. Institute of Medicine gives ~3.7 L for men and ~2.7 L for women. Add +350 mL per 30 minutes of exercise (~11.67 mL/min, the page's value) and apply a climate factor: ×1.0 normal, ×1.2 dry/arid, ×1.3 hot/tropical. With the sample (70 kg, 60 min/day exercise, normal climate): base = 2450 mL, exercise addition = ~700 mL, total = 3200 mL (13 cups / 14 glasses). Note: "8 glasses a day" (~1.9 L) is a 1945 U.S. Food & Nutrition Board rule of thumb that under-estimates modern intake needs. Most of daily water comes from food (~20%) and metabolic water (~10%); you do not need to drink all of it as plain water.
Calorie balance — surplus, deficit, or equilibrium
Energy balance is intake − expenditure: positive = surplus (weight gain), negative = deficit (weight loss), zero = maintain. The page converts the daily balance to a weekly number and projects the weight change using the canonical 7700 kcal/kg fat conversion (i.e. losing 7700 kcal more than you burn = losing 1 kg of fat). With the sample (TDEE 2500, intake 2000): daily deficit = −500 kcal, weekly = −3500 kcal, projected weight change = −0.45 kg/wk (≈ −1.95 kg/month, ≈ −1.0 lb/wk). Note the 7700 number is a population average; actual body-fat energy density ranges from 6500–8800 kcal/kg depending on hydration and lean-tissue fraction. Also, water and glycogen shifts cause ±1–2 kg of week-to-week noise unrelated to fat change.
Weight timeline — when will the goal be reached?
The timeline panel projects how long a given daily deficit takes to reach a target weight change: days = target_kg × 7700 / daily_deficit. With the sample (500 kcal/day deficit, 5 kg target): 5×7700/500 = 77 days (≈ 11.0 weeks). The page also reports milestones at 1, 2, 3, 4, 5 kg (and up to 20 kg) so you can see the day-count to each sub-goal. Important caveats: real weight loss is rarely linear — the first 1–2 weeks drop faster than average because of water and glycogen depletion, then plateau as the body adapts; metabolic adaptation (lower BMR and lower NEAT) means the same deficit produces less fat loss over time. A 500 kcal/day deficit that loses 0.5 kg/wk in week 1 may produce 0.3 kg/wk by month 3 without further reduction. A safe, sustainable target is 0.5–1.0% of body weight per week — faster loss trades muscle for speed.
Daily targets — putting it all together
The Targets panel combines BMR (Mifflin-St Jeor — same as the body-metrics page), an activity multiplier, and a goal adjustment (±500 kcal for cut/bulk): target = round(BMR × activity + goalAdj). With the sample (male, 175 cm, 70 kg, 30 yr, moderate 1.55, weight loss −500): BMR = 1649, TDEE = 1649×1.55 ≈ 2556, target = 2056 kcal/day. The macro split follows the goal: cut → 40/25/35, bulk → 30/20/50, maintain → 30/25/45. For 2056 kcal cut: 206 g protein / 57 g fat / 180 g carbs. The panel also reports water (35 mL/kg rounded to nearest 100 mL) and fiber (14 g per 1000 kcal — the U.S. Adequate Intake for adults).
Common misconceptions
- Macros must sum to 100% of calories. Protein and carbs are 4 kcal/g, fat is 9 kcal/g; alcohol is 7 kcal/g. If you drink, it displaces other macros; the page does not account for alcohol calories.
- 0.8 g/kg protein is the minimum, not the optimum. The RDA prevents deficiency; most active adults benefit from 1.2–1.6 g/kg. Plant-based eaters need ~20–30% more because plant proteins have lower digestibility and incomplete amino-acid profiles.
- 7700 kcal/kg is a population average. Real body-fat energy density varies from 6500 to 8800 kcal/kg; the 7700 number is mid-range. Adipose tissue is roughly 87% lipid; the rest is water, protein, and connective tissue.
- "Weight loss" and "fat loss" are different. The 7700 kcal/kg assumes pure fat. Real scale-weight loss includes water, gut mass, and glycogen — so the first week of any diet shows a bigger drop than fat loss alone explains.
Related tools: Body Metrics for the BMI and BMR/TDEE calculations that underlie Targets, Fitness & Exercise for the MET-based calorie-burn side of the energy-balance equation, and Clinical Medicine for the medical context where rapid weight loss is a red flag (hyperthyroidism, diabetes, malignancy).
