Every method named in this guide is detailed further on the editorial standards page.
Fat loss, muscle gain, or just holding steady: every nutrition plan runs through the same gate, energy balance. This guide walks the numbers in the order they actually matter, calories burned, calories eaten, and how the two combine, with a link to the calculator for each step.
Energy balance: the rule underneath every diet
Body weight tracks the gap between energy in (food and drink) and energy out (everything the body does). Run a deficit and the body draws on stored energy, mostly fat, to cover the gap. Run a surplus and the extra gets stored. This is why keto, vegan, and intermittent fasting can all produce weight loss for different people: each is just a different route to the same deficit.
A commonly used rule of thumb treats roughly 3,500 calories as equivalent to a pound of body fat, so a 500-calorie daily deficit points toward losing about a pound a week. The body isn't a perfect calculator: water, glycogen, hormones and adaptation all add noise week to week, but the trend follows the arithmetic over a couple of months.
BMR and TDEE: what you actually burn
Your basal metabolic rate (BMR) is what your body spends at complete rest, breathing, circulation, cell repair, and it's usually the largest single piece of your total burn. The BMR calculator finds it using the Mifflin-St Jeor equation, published in 1990 and the version most dietitians default to today.
Your total daily energy expenditure (TDEE) multiplies that BMR by an activity factor covering movement and exercise. TDEE is the number to actually plan around: eat at it to hold steady, below it to lose, above it to gain. The TDEE calculator does this in one step and shows targets for a mild cut, an aggressive cut, and a lean surplus.
Choosing a target: deficit or surplus
For fat loss, a moderate deficit of 300 to 500 calories below TDEE tends to be sustainable and protects both muscle and energy levels. Bigger deficits move faster but are harder to stick with and raise the odds of losing muscle, so they suit shorter pushes rather than months-long plans. One guardrail worth keeping: avoid eating below BMR for extended stretches. For muscle gain, a small surplus of 200 to 300 calories supports growth without piling on much fat; larger surpluses mostly add fat instead of muscle.
Protein and the rest of the macros
Calories decide whether you gain or lose; macronutrients, protein, carbohydrate and fat, decide what that change is made of and how you feel along the way. Protein comes first: it protects muscle during a deficit, is the most filling per calorie, and costs the body the most energy to digest. The protein intake calculator gives a range based on weight and goal, generally 0.7 to 1.0 grams per pound for active people. Fill the rest with carbohydrates for training energy and fats for hormone health, weighted by preference.
BMI and a reasonable goal weight
Before chasing a specific number, it helps to know roughly where you stand. The BMI calculator gives a quick screen of weight relative to height using the same ratio the CDC publishes, and the ideal weight calculator shows a sensible range using several established formulas side by side. Treat both as a starting point rather than a verdict: neither can see muscle mass, which is why very lean, muscular people often read as "overweight" by BMI alone.
Hydration, briefly
Water affects performance, appetite and recovery, and mild dehydration is often mistaken for hunger. The National Academies' Adequate Intake figures put total water needs around 3.7 liters a day for men and 2.7 for women, including water from food; the popular "half your body weight in ounces" line is a rule of thumb, not that federal number. The water intake calculator turns either into a concrete daily target, and pale yellow urine remains the simplest real-world check.
Putting the pieces together
A workable plan looks like this: find TDEE, subtract a moderate deficit or add a small surplus, set a protein target, hydrate, and judge the trend over two to four weeks instead of reacting to daily swings. Nudge calories by 100 to 200 if the scale stalls for two to three weeks straight. Consistency beats precision here; the best plan is the one you'll actually repeat.
Calculators used in this guide
- Calorie Deficit - Calories to lose, maintain or gain weight.
- Macro Calculator - Protein, carb and fat targets for your goal.
- Pace Calculator - Running pace, speed and finish times.
- Sleep Calculator - Best bedtimes and wake times by sleep cycle.
- Body Fat Calculator - Body-fat % from the U.S. Navy method.
- BMI Calculator - Body mass index and your healthy weight range.
- BMR Calculator - Calories your body burns at rest, plus daily needs.
- Water Intake Calculator - How much water to drink per day for your body.
- TDEE Calculator - Total daily calories burned + targets for cut, maintain, gain.
- Ideal Weight Calculator - Healthy weight by height using 4 methods.
- Protein Intake Calculator - Daily protein target for your weight and goal.
A few common questions
How many calories should I eat to lose weight?
Start at TDEE and subtract 300 to 500 calories for steady loss of about a pound a week; avoid going below BMR for long stretches.
What's the difference between BMR and TDEE?
BMR is calories burned at rest; TDEE is BMR multiplied by an activity factor, your real daily burn.
How much protein do I need?
Roughly 0.7 to 1.0 g per pound of body weight when active or building muscle; less if sedentary.
Why isn't the scale moving even in a deficit?
Water, glycogen and hormones mask fat loss week to week; judge the two-to-four-week trend and adjust by 100 to 200 calories if truly stalled.
Is BMI accurate?
It's a useful screen but can't tell muscle from fat, so pair it with body-fat context.