Weight is an enduring obsession among cyclists although it would be fair to say that until recently riders have tended to
invest far too much time worrying about bicycle and component weight when matters of aerodynamic drag and power output
should have been higher priorities. Notwithstanding these issues it remains a fact that a lighter combination of rider
and bike will tend to accelerate faster, lose less power to rolling resistance, and climb faster than a heavier
combination and so system weight minimisation forms a part of any serious race preparation programme.

The limits of minimisation in terms of bicycles and components are defined by the structural limitations of the
materials used to build them or, more and more relevant, the minimum machine weight of 6.8 kilograms enforced by
the UCI for professional racing. But what is the effective minimum when it comes to the rider? Generally this is
taken to be the weight at which a rider minimises body fat percentage down to the level of just “essential fats”
– 5-6% of body weight in men and 9-13% in women. Any lower and the rider compromises the body’s ability to
function normally - leading to power loss – or tends to fall ill – again leading to power loss as a result of detraining.

Regardless of how close to these limits a rider may be able or want to be management and therefore monitoring of body
fat levels is of serious importance for competitive or aspiring competitive bike riders. With this in mind the calculator
here provides some tools that can be used for body fat percentage estimation using common statistical models based on
medical research.

Measurement of Skinfolds to determine body density – The Durnin-Womersley method

Evaluation of body fat percentages is typically a two stage process beginning with some method used to determine
body density. The gold standard method of “underwater weighing” AKA "hydrostatic weighing" is accurate (around +/-4%) but
inacessible and impractical for routine use. “Field” measurement of body density typically relies on measurement of skinfold
“pinches” at a small number of specific “sites” on the body and a statistical model (such as that of Durnin & Womersley)
to convert these skinfold measurements – usually an aggregate thickness in millimetres - into an estimated density in kilos/m^3.

Body fat calipers can be obtained very cheaply from retailers as common as Amazon and Ebay. To compute body density
from skinfolds using this calculator and the Durnin-Womersley equations simply specify the subject's sex, age grouping and
apropriate skinfold thicknesses (bicep, tricep, sub-scapular and super-iliac). Sex and age are important because they
determine empirical assumptions made about the distribution between subcutaneous (skinfold) fat and visceral (body cavity)
fat in the subject.

From Body Density to Body Fat Percentage – The Siri Method

Body fat has a density of ~0.9 kg/litre while lean tissue and bone has a density of ~1.1kg/litre. It follows that given
an estimate of body density and some simple maths we can estimate a subjects body fat percentage. There are a couple of
alternative methods available to make this calculation but this calculator implements the "Siri" method. Along with
the resulting body fat percentage we also show the corresponding American Council on Exercise adiposity classification
ranging from "Essential Fat (Only)" though "Obese".

Body Mass Index (BMI) – A Rough Estimator of Adiposity (Leanness)

BMI (weight in kilos x height in metres x height in metres) is a well-known, fast, but less accurate estimator of leanness.
The obvious limitation is that no “one size fits all” estimator is appropriate to everybody since
genetic and lifestyle factors cause great variability in the relationship between height, weight, and body compoosition.
The Body Mass Index is also associated with an adiposity classification from "Very Severely Underweight" to "Obese Class III"
and our calculator reports this too.

From BMI to Body Fat Percentage – The Deurenberg et al Equations

If you don't have body fat calipers but want to estimate the body fat percentage of yourself or another athlete then the
equations of
Deurenberger et al
can be used to aproximate a fat percentage from the Body Mass Index computed easily above.

Theoretical Watts per Kilo at Essential Fat Percentage

To demonstrate the kind of wattage per kilo improvement that may be possible for a rider achieving "essential fat only"
leanness we provide some simply calculations based on current sustainable power at some duration of interest, the
currently evaluated and potential fat percentages. Illustrating the "cost of fat" at current power and weight we also
compute the additionla watts that would be required to achieve the same wattage per kilo at current "fatness".

Professional Cyclists

Notwithstanding the limitations of the combined BMI and Deurenberg et al estimator of body fat percentage we thought it would be
fun to let you run such an estimator on any rider in the CPL rider database. Casual analysis of these numbers suggests they
may be on the high side for elite athletes competing at the highest level.