How to work out how many calories you actually need
Start with your resting burn, then add activity. The BMR Calculator uses the Mifflin-St Jeor equation to estimate what your body burns at complete rest; the Maintenance Calorie Calculator multiplies that by an activity factor to give TDEE, the number you eat at to stay the same weight.
From there the direction is simple arithmetic. The Calorie Deficit Calculator sets a daily target for a chosen weekly loss, and the Weight Gain Calculator does the reverse for a surplus. Once the total is set, the Macro Calculator splits it into protein, carbs and fat.
What BMI and body composition numbers really mean
BMI is a screening ratio of weight to height, nothing more. The BMI Calculator is genuinely useful across populations but says nothing about an individual's muscle mass, which is why heavily trained athletes routinely score as overweight. It was never designed to diagnose anyone.
Composition measures add the missing context. The Body Fat Calculator uses the US Navy tape method, the Lean Body Mass Calculator estimates non-fat tissue, and the Waist to Hip Ratio Calculator captures fat distribution, which tracks metabolic health risk better than total weight alone. Take tape measurements at the same time of day for comparable results.
When to use heart rate and hydration calculators
Heart rate tools are most useful when you are structuring training rather than just exercising. The Max Heart Rate Calculator gives the age-based ceiling, and the Target Heart Rate Calculator uses the Karvonen method with your resting rate to set zones — more personal than a flat percentage of max.
Hydration is simpler and mostly about a sensible baseline. The Water Intake Calculator scales daily fluid to body weight, which you then adjust upward for heat and hard training. For cycle and pregnancy planning, the Ovulation Calculator and the Pregnancy Due Date Calculator apply the standard clinical dating rules.
Frequently asked questions
- Is BMI accurate for athletes?
- No — BMI cannot distinguish muscle from fat, so muscular people are often classed as overweight or obese despite low body fat. Use a body fat or lean body mass estimate alongside it.
- How many calories should I eat to lose 1 pound a week?
- Roughly a 500 calorie daily deficit, since about 3,500 calories is commonly used as the energy in a pound of fat. Calculate your maintenance level first, then subtract.
- What is a healthy body fat percentage?
- Typical healthy ranges are around 10-20% for men and 18-28% for women, rising modestly with age. Athletes often sit lower and very low levels carry their own risks, particularly for hormonal health.
- How is a pregnancy due date calculated?
- The standard method is Naegele's rule: 280 days, or 40 weeks, from the first day of the last menstrual period. It assumes a regular 28-day cycle, so an early ultrasound is more reliable if your cycle is irregular.
- What is the difference between BMR and TDEE?
- BMR is what your body burns at complete rest just to stay alive; TDEE is BMR plus everything you do — movement, exercise and digestion. TDEE is the number that matters for diet planning, and it is usually 1.2 to 1.9 times BMR depending on activity.