HesapHanem

Bioimpedance Body Fat Estimate Calculator

Enter your BIA resistance, reactance, height, weight and sex to estimate body fat.

How it is calculated

Fat-free mass (FFM) comes from a regression equation: FFM = −4.104 + 0.518 × (H² ÷ R) + 0.231 × W + 0.130 × Xc + 4.229 × S. H is height in cm, R resistance in ohms, W weight in kg, Xc reactance in ohms, and S is 1 for men and 0 for women. Fat mass is weight − FFM and body fat % is fat mass ÷ weight × 100. The tool gives no result for height outside 50 to 272 cm, weight outside 2 to 650 kg, resistance of zero or less, negative reactance, or an FFM that comes out zero, negative or not below body weight. The equation was developed on a specific adult sample, so it can be off for athletes, children or very high body fat, and the ohm readings must come from your device in the same state each time.

Worked example

A man, 175 cm, 80 kg, resistance 480 ohm, reactance 55 ohm. H² ÷ R = 30,625 ÷ 480 = 63.80, times 0.518 = 33.05. Weight term: 0.231 × 80 = 18.48. Reactance term: 0.130 × 55 = 7.15. Sex term: 4.229. FFM = −4.104 + 33.05 + 18.48 + 7.15 + 4.229 = 58.80 kg. Fat mass = 80 − 58.80 = 21.20 kg, so body fat = 21.20 ÷ 80 × 100 = 26.49%.

Try this example →

Common mistakes

  • Entering the impedance (the combined value) in the resistance field; resistance and reactance are two separate readings.
  • Swapping resistance and reactance, which gives a wildly wrong fat-free mass because resistance sits in the denominator and is usually several times larger.
  • Measuring after a workout, a meal or when dehydrated; resistance changes with body water and shifts the result by several points.
  • Choosing the wrong sex: the equation adds 4.229 kg of fat-free mass for men, so with the example values the same readings give 26.49% as a man but 31.78% as a woman.
  • Entering height in metres or feet and inches. The field expects centimetres, and values below 50 give no result.

Sources

About this calculator

Bioelectrical Impedance Analysis (BIA) estimates body composition by sending a tiny, imperceptible electrical current through the body and measuring how much it is impeded. That impedance breaks down into two components: resistance, the opposition to current flow through the body's water and electrolytes (lean tissue, which holds more water, conducts better and shows lower resistance than fat), and reactance, the delay caused by cell membranes acting like tiny capacitors, which reflects cell membrane health and integrity.

Using your height, weight, sex, and these resistance and reactance readings, regression equations (built from BIA validation research against reference methods like DEXA and hydrostatic weighing) estimate your fat-free mass (FFM); subtracting FFM from total body weight then gives your estimated body fat mass and percentage.

This is the same underlying technology used in home smart scales, handheld body-fat analyzers, and clinical devices, and it is popular precisely because it's fast and non-invasive, though results can shift with hydration status, recent exercise, or meals — clinicians also use raw resistance/reactance readings (via phase angle) as a marker of nutritional and fluid status in conditions like dialysis.

Was this helpful?

You can pick up to 3 reactions.

Comments

  • Be the first to comment.

Popular calculators

Loan Calculator

Monthly payment, total interest and full amortization schedule.

Percentage Calculator

Percent change, percent of, and ratio calculations.

Discount Calculator

Calculate final price, savings, and discount amount.

BMI Calculator

Body mass index from height and weight (metric or imperial).

Age Calculator

Compute your exact age in years, months and days from a birth date.

Compound Interest Calculator

Calculate the final amount and interest earned with compounding.

All Calculators