Why the result is a range and not a decimal
A body fat percentage with a decimal point looks precise and is not. The US Navy equation was validated against hydrostatic weighing with a standard error of about 3.5 percentage points for an individual. A three-site skinfold with a practised tester is about the same. A BMI-based equation is closer to 4.5. Even DEXA, the usual reference, disagrees with the four-compartment model by a point or two depending on hydration and the scanner's software.
So a tape measurement that returns 18.3 percent really means somewhere between about 15 and 22. This calculator says so. When you enter more than one method, it combines them with inverse-variance weighting, the same approach used to pool studies in a meta-analysis: each method counts in proportion to how precise it is, and the combined error shrinks. Two independent methods that agree narrow the range to roughly 2.5 points. Two that disagree by more than six points are reported as a disagreement rather than quietly averaged, because that gap usually means one measurement was taken wrong.
The three methods, and which one to trust
The tape method comes from Hodgdon and Beckett's 1984 work for the US Navy. Men need height, neck and waist; women add hips. It works because the difference between waist and neck circumference tracks abdominal fat well in most adults. It is the most repeatable home method because a tape does not need skill, only consistent placement: neck just under the larynx, waist at the navel after a normal exhale, tape snug but not pressing into the skin.
The skinfold method uses the Jackson and Pollock three-site equations (chest, abdomen and thigh for men; triceps, suprailiac and thigh for women) to estimate body density, then converts it to fat with the Siri equation. It is accurate in trained hands and poor in untrained ones. If your skinfolds put you five points away from the tape, the caliper technique is the usual suspect, not the tape.
The BMI method is Deurenberg's 1991 equation, which predicts fat from BMI, age and sex. It is included because sometimes a tape is not to hand, but it cannot tell muscle from fat, so the calculator uses it only when no direct measurement exists and labels it reference only. A lifter at 90 kg and 178 cm has a BMI of 28.4, which this equation reads as roughly 25 percent fat whether the person is soft or shredded.
The South Asian adjustment
The BMI equation was built on European populations. Deurenberg-Yap's Singapore work in 2000, and several Indian studies since, found that South Asians carry between three and six percentage points more body fat at the same BMI, with more of it stored around the organs. That is why the Indian obesity guidelines set the overweight cut-off at a BMI of 23 rather than 25.
When you select South Asian, the tool adds four points to the BMI-based estimate. The tape and skinfold methods are not adjusted, because they measure where the fat sits rather than inferring it from weight. If the BMI estimate reads noticeably higher than your tape result, that difference is the equation doing what it should.
What the calculator refuses to accept
Most body fat calculators will happily compute a number for a 150 cm person who weighs 240 kg with a 42 cm waist. This one checks whether the inputs can belong to one human being: BMI between 12 and 70, waist between 30 and 80 percent of height, neck between 15 and 32 percent of height, waist at least two centimetres larger than the neck, hips at least 85 percent of the waist for women, and skinfold totals that make sense against BMI. When something fails, the result is withheld and the message says which measurement to re-check. Nothing is silently clamped into range.
Reading the bands
For men, essential fat sits around 3 to 5 percent, competition condition 5 to 9, lean and athletic 10 to 14, fit 15 to 19, average 20 to 24, and above that the health risk climbs. For women the same bands sit roughly eight points higher: essential 10 to 13, athletic 14 to 20, fit 21 to 24, average 25 to 31. Because the result is a range, the tool tells you which bands the range touches instead of pinning you to one label. Abs generally become visible around 12 percent for men and 20 for women, though where you store fat matters more than the number.
From percentage to kilos
The number that matters for planning is not the percentage but the fat mass behind it. At 85 kg and 22 percent you carry about 18.7 kg of fat and 66.3 kg of everything else. To reach 15 percent with lean mass held, the target weight is 66.3 divided by 0.85, which is 78 kg, so the fat to lose is 7 kg. The calculator does this arithmetic from the range, so the target weight also comes as a range, and it hands the number to the fat loss timeline calculator, which turns kilos into weeks.
Lean mass is rarely fully held during a diet. The fat loss timeline models how much lean tissue a given rate of loss costs, and a body fat measurement taken every four to six weeks under the same conditions is the only way to know what actually happened.
How to measure so the number is worth something
Measure in the morning, before food, after the bathroom, and before training. Take each circumference twice and use the average. Keep the tape parallel to the floor. For skinfolds, pinch on the right side of the body, pull the fold away from the muscle, place the caliper one centimetre below the fingers, and read after two seconds. Repeat the whole set on the same weekday every few weeks. A single measurement is a snapshot with a three-point error; the trend over months is what tells you whether the plan is working.