All-in-One Health & Fitness Calculators

How Calorie Needs & Intake Are Estimated

Your daily calorie needs depend on your basal metabolism (BMR) plus the energy you burn through movement and digestion.

Step 1 – Calculate Your BMR

We first estimate your BMR (Basal Metabolic Rate) using the Mifflin–St Jeor equation:

• Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
• Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

Step 2 – Apply Activity Factor

We multiply BMR by an activity factor to get maintenance calories:

• Sedentary: × 1.2
• Lightly active: × 1.375
• Moderately active: × 1.55
• Very active: × 1.725
• Super active: × 1.9

Step 3 – Adjust for Your Goal

Then we adjust calories up or down depending on your target:

• To lose fat: a modest calorie deficit (for example −250 to −500 kcal/day).
• To maintain: stay close to your maintenance calories.
• To gain: a small surplus (for example +250 to +500 kcal/day).

Reminder: Extreme deficits or surpluses can be risky. It’s safer to change gradually and get professional advice for big goals.

What Is BMR and Why It Matters

BMR (Basal Metabolic Rate) is the number of calories your body burns at complete rest — just to keep you alive and functioning (breathing, circulation, cell repair, etc.).

The BMR Formula

We use the Mifflin–St Jeor equation:

• Men: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age + 5
• Women: BMR = 10 × weight(kg) + 6.25 × height(cm) − 5 × age − 161

From BMR to Daily Calories

Real life includes movement. To estimate total daily needs you:

• Multiply BMR by an activity factor (sedentary, light, moderate, active, etc.).
• Then adjust slightly to lose fat, maintain, or gain muscle.

Using BMR in Practice

• Gives you a baseline to build a meal plan around.
• Helps explain why people of different sizes need different calories.
• Best used along with progress tracking over several weeks.

Note: BMR is an estimate. Health conditions, medications, and body composition can all affect real-world calorie needs.

What Is Body Fat Percentage?

Body fat percentage tells you how much of your total body weight is made up of fat mass. Unlike BMI, it focuses on body composition rather than just weight and height.

How This Body Fat Formula Works

This calculator uses the U.S. Navy tape measure method:

• Inputs: gender, height, neck, waist, and hip (for females).
• Idea: compares your waist and hip size to your height and neck to estimate fat vs. lean mass.
• Measurements are taken in centimeters and converted to inches for the formula.

Typical Body Fat Ranges

• Men
• 2–5% – Essential fat
• 6–13% – Athletes
• 14–17% – Fitness
• 18–24% – Average
• 25% and above – Obese

• Women
• 10–13% – Essential fat
• 14–20% – Athletes
• 21–24% – Fitness
• 25–31% – Average
• 32% and above – Obese

Limitations

• Less accurate than DEXA, Bod Pod, or ultrasound scans.
• Needs consistent measuring technique (same tape, same spot, same time of day).
• Results can shift with posture, hydration, and breathing.

Use your body fat % to track trends over time, not to obsess over a single number.

How “Ideal” or Healthy Weight Is Estimated

There is no one perfect weight for any height, but many tools use the “normal” BMI range to give a rough healthy weight range.

The BMI-Based Weight Range

This calculator works backwards from the standard normal BMI range (18.5–24.9):

• Lower weight limit: 18.5 × height(m)²
• Upper weight limit: 24.9 × height(m)²

We take your height, convert it to meters, then calculate both ends of the range.

How to Use This Number

• Think of it as a reference range, not a strict target.
• Some people feel healthiest slightly outside this range (e.g., muscular athletes).
• Combine it with body fat %, waist size, energy, strength, and medical guidance.

Note: “Ideal weight” is individual. Mental health, lifestyle, and medical conditions all matter too.

Understanding Target Heart Rate Zones

Your heart rate during exercise is a simple way to gauge how hard you’re working and which energy systems you’re training.

Estimating Maximum Heart Rate

A common quick formula is:

• HRmax ≈ 220 − age

It’s not perfect, but it gives a reasonable starting point.

Training Zones (as % of HRmax)

• 50–60% – Very light: warm-up, recovery, gentle movement.
• 60–70% – Light: comfortable, often called the “fat-burning zone”.
• 70–80% – Moderate: improves aerobic fitness and endurance.
• 80–90% – Hard: high-intensity cardio and speed work.
• 90–100% – Very hard: short bursts, not for beginners.

Using Resting Heart Rate (Optional)

If you provide resting heart rate, we use the Karvonen formula to personalize zones based on heart rate reserve (HRmax − HRrest).

Always listen to your body. If you feel dizzy, overly short of breath, or unwell, stop and seek medical advice.

How Much Water Do You Really Need Per Day?

Water supports digestion, nutrient transport, temperature control, and joint health. The typical “8 glasses a day” rule is very generic — this tool tailors hydration to your body and lifestyle.

How the Water Intake Formula Works

The calculation follows practical hydration guidelines:

• Base: weight (in pounds) × 0.67 = ounces of water per day.
• Exercise: +12 oz for every 30 minutes of activity.
• Climate: hot environments add ~15% extra need.

Why Proper Hydration Matters

• Improves focus, mood, and energy.
• Supports digestion and regular bowel movements.
• Helps regulate temperature during exercise or heat.
• Supports kidney function and natural detoxification.

These are general guidelines. People with kidney, heart, or other conditions should follow medical advice.

How Ovulation & Fertile Window Are Estimated

Ovulation is the release of an egg from the ovary. You are most fertile in the days just before and around ovulation.

Cycle-Based Estimate

This calculator assumes a regular menstrual cycle and estimates:

• Ovulation usually happens about 14 days before the next period.
• If your cycle is 28 days, ovulation is around day 14.
• If your cycle is 30 days, ovulation is around day 16, and so on.

Fertile Window

Sperm can live for several days inside the female reproductive tract, and the egg survives for about 12–24 hours after ovulation. So the most fertile days are:

• About 5 days before ovulation, up to
• About 1 day after ovulation.

Limitations

• Cycles can vary month to month.
• Stress, illness and travel can shift ovulation.
• People with irregular cycles may find predictions less accurate.

This tool is for education and planning only. It does not guarantee or prevent pregnancy.

How Pregnancy Due Date Is Estimated

A pregnancy due date is an estimate of when your baby might be born. Most calculators use the first day of your last menstrual period (LMP) and assume a typical 28-day cycle.

Naegele’s Rule – Classic Method

The most common method is called Naegele’s rule:

• Start with the first day of your last period (LMP).
• Add 280 days (40 weeks), or roughly
• Add 9 months and 7 days.

Cycle Length Adjustment

If your cycle is not 28 days, the estimate is shifted:

• Longer cycle → ovulation likely later → due date moves slightly later.
• Shorter cycle → ovulation likely earlier → due date moves slightly earlier.

Why It’s Only an Estimate

• Only a small percentage of babies arrive exactly on the due date.
• Most births happen between 37 and 42 weeks.
• Early ultrasounds can fine-tune the clinical due date.

Always rely on your doctor or midwife for medical decisions and official due dates.

Understanding the Menstrual Cycle & Period Predictions

The menstrual cycle is the time from the first day of one period to the first day of the next. Many people have cycles of around 28 days, but anything from about 21–35 days can be normal.

What This Calculator Does

Using the first day of your last period, your average cycle length, and your usual period length, it estimates:

• The next few period start dates.
• The approximate duration of each period window.

Why Predictions Can Change

• Cycles can be affected by stress, illness, travel, contraception, and more.
• Teenagers and people approaching menopause often have more irregular cycles.
• Some medical conditions (e.g., PCOS, thyroid disorders) affect cycle timing.

If your periods are very irregular, very heavy, or very painful, it’s important to talk to a healthcare professional.