If your design is questioned, do you have the calculations to support it readily at hand, in a complete and presentable format? Do you ever shuffle through stacks of notes to double check a pipe or control valve size?

When an engineer working on a project I was managing left the company and we looked for information in his files, the only design calculations recognizable as such were those produced by various vendors' equipment sizing software programs. We needed design data to respond to a question from the client regarding system sizing and it was embarrassing, to say the least, that we could not immediately provide an answer.

In the "good old days" when I was an engineering student, professors still taught that a complete and clearly written problem solution was as important as getting the correct answer. Back then, professional engineering examinations consisted entirely of written problem solutions, and this rule was emphasized as critical to obtaining a passing score.

I still find that writing up a formal calculation to document a "back-of-the-envelope" design computation often provides an opportunity to realize that I've overlooked an important factor, or made an unthinking assumption along the way. This benefit is obtained from a calculation structure that forces a clear statement of the problem, and the assumptions and methodology by which a solution is to be reached. While specific format is a matter of personal or organizational preference, a good calculation should address the following key elements.


State the intended objective of the calculation. It may be obvious to you, but not necessarily to a reviewer. This can be as simple as a single sentence like "Determine the required airflow rate for electrical room cooling," or a more involved statement of the need for and use of the results of the calculation.


Provide all information that a qualified reviewer not familiar with the details of the project would need to understand your approach. This should include given conditions and parameters, design criteria, and solution methods. In electrical problems, this section frequently includes a simple one-line diagram or schematic representation of the relevant part of the distribution system.


List all references used for design criteria, material and equipment data, and solution methodology. Common documents that should be readily available to a qualified reviewer such as the National Electrical Code, or ASHRAE standards, may merely be cited, as long as the citation is complete enough for the reviewer to locate the pertinent information. The relevant portions of documents that may not be readily available should be photocopied and attached to the calculation sheet.


List the assumptions and any data from standard sources on which your calculation is based. When doing a handwritten calculation, I usually leave some extra room here; in the middle of the process I invariably realize that I need to make an additional assumption and can then go back and add it to the list.


Work through the computation in an orderly manner. Include units, not just quantities, in equations. When complex expressions are involved, present them first with variable names and reference citations before presenting them in numerical form. Don't skip steps unless they are painfully obvious.


Highlight key intermediate results and final results so they are easy to pick out on the page. One useful method is to underline intermediate results and draw a box around the final result.


State how the result of the computation is to be applied to your design, such as "10,000 cfm at 85 degrees F is an adequate ventilation rate." or "Select a circuit breaker interrupting rating of 35,000-A RMS SYM as the lowest standard rating that exceeds the calculated short circuit current by at least 15%."


Finally, ask another qualified engineer to review your calculation for errors and indicate agreement or disagreement with your conclusion. A well-organized presentation will make this an efficient task. Completed and checked calculations should be indexed and filed.ES