The Hacker's Diet
The Hacker's Diet is a diet plan created by the founder of Autodesk, John Walker, outlined in an electronic book of the same name, that attempts to aid the process of weight loss by more accurately modeling how calories consumed and calories expended actually impact weight. John Walker notes that much of our fat free mass introduces signal noise when trying to determine how much weight we're actually losing or gaining. With the help of a graphing tool (Excel is used in the book), he addresses these problems. Factoring in exercise, and through counting calories, we can calculate our total energy expenditure (basal metabolic rate, thermic effect of food, and day-to-day exercies) and cut back our calorie intake, or increase our exercise to lose weight.

# Precautions
John Walker has nothing to do with medicine or nutrition in his professional life. As he writes, it "is a diet book by somebody who spent most of his life fat."
Consuming too few calories may result in muscle loss in addition to fat loss. Though not proposed in the electronic book, many of the tools shown on this page allow their users to track body fat percentage as well. Tracking body fat percentage will allow you to determine your body's fatty mass.

# Dieting as an engineering problem
Walker describes the diet as approaching weight loss "as an engineering problem", claiming that his approach enabled him to reduce his weight from 215 pounds to 145 pounds in a year, and keep it stable afterwards.
Walker cuts the problem down to the barest elements, modeling the human body as a "rubber bag" where one can ignore many small variables such as food type, frequency, metabolic rates and even exercise as not greatly important to the central problem. Calories consumed compared to calories used is the key, according to Walker; if one eats more calories than one burns, one gains weight; if one eats fewer calories than one burns, one loses weight.  All that is necessary to consistently lose weight, at the desired rate, is to monitor the intake, monitor the weight loss rate and make the desired proportional adjustments to reach the desired goal, a very simple control systems problem.

# The body as a system
Walker also goes to some lengths to introduce the reader to simple feedback and control systems, providing spreadsheets to demonstrate feedback, oscillation and data smoothing to illustrate his arguments.  Data smoothing is a key element of the monitoring system, preventing the dieter from becoming discouraged by short term failure to lose and to be able to concentrate on the long term trend.
While the diet is a fairly straightforward calorie-counting approach, what makes it successful for many is the unique focus on feedback through monitoring of weight using engineering principles.  Techniques are presented for Excel aided or paper and pencil data smoothing to allow the dieter to adjust the diet for themselves using the long term trend and to not be discouraged by short term fluctuations based on water retention or other factors.
Another important factor in the diet's approach is using the trend line as a control system to allow the dieter early warning of relapse after the target weight is reached.  As Walker states "The vast majority of people who lose weight end up, in relatively short order, gaining back every pound they lost." A quick check of the trend line provides an easy way to make small adjustments in intake, allowing much greater control of weight for life.

# Tools
Walker uses Excel spreadsheets to log weight and produce charts, but provides a list of other software packages that may be used.
Online services which provide tracking and charting services of the kind described in the book include The Hacker's Diet Tracker and PhysicsDiet.com. For the Palm, the program The Hacker's Diet provides mobile tracking and charting services.
As of August, 2007, Walker provides a free online tracker and grapher at his Fourmilab website.

# Exercise
Walker says "You don't exercise to lose weight (although it certainly helps). You exercise because you'll live longer and you'll feel better."
Even though Walker says that weight loss is not the goal of exercise, he devotes an entire chapter of the book to it. Walker suggests combining the diet with exercises derived from the Royal Canadian Air Force 5BX exercise program.