Fitness and Diet Calculators
These five calculators give the numbers most diet and training plans start from: body mass index, daily calorie needs, a protein target, a water target and a weight-loss plan with a date. They suit people starting a diet or training block, coaches who want to show clients where a figure comes from, and anyone checking a number from a fitness app.
5 free tools · no sign-up · no ads
All Fitness Calculators (5)
BMI Calculator
Calculate Body Mass Index and health recommendationsCalorie Calculator
Calculate daily calorie needs and recommendationsProtein Calculator
Calculate daily protein requirementsHydration Calculator
Calculate daily water intake needsCalorie Deficit Planner
Plan calorie deficit for weight loss goals
Which tool do I need?
Check whether my weight is in a healthy range for my height
Enter height and weight in metric or feet, inches and pounds and see where the result falls on the WHO adult scale.
Find my maintenance calories
Get BMR and TDEE from age, sex, height, weight and one of five activity levels, plus loss and gain targets.
Get a date for reaching a goal weight
Choose a 250, 500 or 750 kcal deficit and see the daily target, expected weekly loss and estimated goal date.
Set a protein target while cutting or building muscle
Get grams per day from weight, activity and goal, split across three meals.
Work out how much water to drink on training days
Estimate daily water from age, weight, activity and climate, shown in ml, litres and 250 ml glasses.
About Fitness Calculators
They chain naturally. The BMI Calculator places weight on the WHO adult scale. The Calorie Calculator estimates basal metabolic rate with the Mifflin-St Jeor equation and multiplies it by an activity factor to get TDEE. The Calorie Deficit Planner uses the same equation, subtracts 250, 500 or 750 kcal and converts the deficit to weight at about 7,700 kcal per kilogram to give weeks and a goal date. During a deficit, the Protein Calculator sets a grams-per-kilogram target from activity and goal, and the Hydration Calculator adds a daily water figure from weight, activity and climate.
Know the limits. BMI cannot tell muscle from fat, and its adult bands do not apply under 20. The activity factor adds more uncertainty than the equation itself, so the best check is your own weight trend over two or three weeks. The deficit planner refuses aggressive first targets and never sets intake below 1,200 kcal for women or 1,500 for men. These are estimates for healthy adults, not medical advice.
Guides
- What BMI Measures: the Formula, the Cut-offs and Where It FailsHow kg/m² and 703 × lb/in² work, where the 18.5, 25 and 30 cut-offs came from, why children use percentiles, and what BMI cannot see about body composition.
- BMR and TDEE Explained: How Calorie Calculators Estimate Your NeedsThe Mifflin-St Jeor equation worked by hand, what the 1.2 to 1.9 activity factors mean, how accurate the estimate is, and how to correct it with your own data.
- Calorie Deficits and the 3,500 Calorie Rule: Why Loss Slows DownWhere the 3,500 kcal per pound rule came from, why Hall et al. showed it overpredicts long-term loss, safe deficit sizes, intake floors and how to re-plan.
- How Much Protein Do You Need? From the 0.8 g/kg RDA to TrainingWhat the 0.8 g/kg RDA covers, why the ISSN suggests 1.4 to 2.0 g/kg for people who train, how age and dieting change the number, and how to hit it with real food.
Frequently asked questions
Which fitness calculator should I use first?
The Calorie Calculator, because maintenance calories anchor everything else. A loss plan, a protein target and a water target all make more sense once you know roughly how much energy you use. Use BMI for context and the deficit planner once you have a goal weight.
Why do different TDEE calculators give different answers?
They use different equations and activity multipliers. Mifflin-St Jeor, Harris-Benedict and Katch-McArdle can differ by a hundred calories or more for the same person, and activity labels such as moderately active are interpreted differently. Pick one, stick with it and adjust from real results.
How often should I recalculate my calorie and protein targets?
Every 4 to 5 kg of weight change, or whenever your activity changes a lot. Energy needs fall as weight falls, so a target that worked at the start of a diet will produce slower loss later. Protein targets based on body weight move too.
Do these fitness calculators accept pounds and inches?
Yes. All five have a metric or imperial switch. BMI takes feet, inches and pounds; the Calorie, Protein and Deficit calculators take height as total inches, so 5 ft 9 in is entered as 69. Results are converted back for display where it matters.
Are fitness calculators accurate for very muscular people?
Less so. BMI often reads muscular athletes as overweight, and Mifflin-St Jeor was built mostly on non-athletes, so it can be off for them in either direction. Treat the figures as a starting estimate and adjust from your own intake and weight records.