I Tracked My Body Fat for 90 Days and Built a Calculator That Actually Makes Sense

For three months, I weighed myself every morning and took body measurements every Sunday. I used a caliper, a tape measure, and a scale that probably lies to me about hydration levels.

The goal wasn't to get ripped. It was to understand whether any of these measurements actually mean something day to day.

The Problem With Most Health Calculators

Most body fat calculators fall into one of two camps:

1. Too simple — plug in height and weight, get a BMI number that tells you nothing about your actual composition.
2. Too complicated — requires measurements you need a degree to take correctly, plus an email signup and a paid subscription.

Neither is useful for someone who just wants to know "am I making progress?"

Building Something Practical

I put together a calculator that uses the Navy Method — it takes neck, waist, and hip measurements and estimates body fat percentage. The math has been around since the 80s and correlates reasonably well with DEXA scans for most people:

function navyBodyFat(gender, neck, waist, hip, height) {
  if (gender === 'male') {
    return 86.010 * Math.log10(waist - neck) -
           70.041 * Math.log10(height) + 36.76
  }
  return 163.205 * Math.log10(waist + hip - neck) -
         97.684 * Math.log10(height) - 78.387
}

The inputs are simple enough that anyone can take them with a tape measure. The output gives you a ballpark number that's consistent enough to track trends over time.

What 90 Days of Data Taught Me

Three things stood out:

1. Daily weight is useless; weekly trend is everything. My weight would swing 2-3 pounds daily due to water, food, and sleep. The weekly moving average was the only signal worth watching.
2. Body fat percentage changes slowly. Like, frustratingly slowly. In 90 days of consistent training, I moved maybe 2%. But that's real — if a calculator tells you you dropped 5% body fat in a month, it's broken.
3. Consistency beats precision. Taking measurements at the same time, under the same conditions, with the same method matters more than which formula you use.

Try It

The calculator I built is at bodycalctool.com. It includes BMI, body fat estimation, and a few other health metrics. No frills, no accounts — just input fields and numbers.