Commercial bidding is a process that should be done by estimators. I started in commercial and left for residential, but my father moved from commercial plumbing to estimating and eventually to expediting.
You need the skill and ability to do take-off on plans - you have to be able to read blueprints well and figure every single fitting and every foot of pipe, as well as the cost of every specified fixture.
You have to know what the amount of labor is for each of those things. You have to be able to figure in problems of weather and material shortages and you have to understand the lien process and legal aspect of bidding. (For example, the commercial companies I worked for placed a lien on the property the day material was delivered to the job.)
You have to be able to understand the spec sheets. If the job calls for brazed joints on L copper, then that's what you bid. You don't try to "get by" with cheaper or alternate materials.
If you need equipment rental or sub-sub contractors for something like core drilling, you need to know how to add that. You need to know the percentage to allow for your own markup on subs you hire. And you need to know the profit margin you need.
There are courses and books and all manner of ways to learn this. And of course I'm speaking of larger commercial jobs, not some single office building or a pair of bathrooms in a welding shop.
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