No bid for plumbing work should ever be submitted without first checking up the estimate by some empirical rule to see if the amount figured is about what it should be. When a contractor has estimated and installed a number of operations similar in character - as, for instance, the work in apartment houses - he knows from his experience about what a like installation should amount to, and ought to tell offhand whether his estimate is right or wrong; but even under such conditions it is safer to check up the work by some simple method, such as given for checking labor by the day-per-fixture rule.

A simple method is to determine approximately what the labor and material per fixture will amount to, then multiply the number of fixtures in the building by that price. The product should agree approximately with the estimate as calculated; if it does not, the estimator should go very carefully over his figures and items to see where the discrepancy lies. After estimating and installing a number of operations a contractor, by averaging the price per fixture he received for the various classes of buildings will arrive at the approximate costs per fixture, which he can then use as checks on similar buildings, until changes in the prices of materials force other factors to be determined. It is a very simple matter to determine the approximate cost per fixture. For instance, if an apartment house of forty bath rooms and kitchens were to be estimated upon, and the class of goods used were to be such that the kitchen sinks cost $3 each, the laundry trays $20 per set, the water closet $25, bath tub $36, lavatory $20, and if two days were allowed per fixture for the labor, roughing-in and finishing, labor for plumber and helper costing $5 per day, while the soil pipe, lead pipe, wrought pipe, cocks, valves and other materials averaged about $5 per fixture - then the six fixtures in bath room and kitchen, there being two laundry trays and each compartment representing a fixture, would average $33.16 each. Knowing the cost per fixture and the number of fixtures in the building, which would be 6 x 40 = 240, the cost of the operation could be approximately determined by multiplying 240 by $33.16 = $7,958.40. If the cost of the operation, as determined by the itemized estimate, were to run several hundred dollars either above or below this approximate cost, the estimator should satisfy himself that no mistake has been made in his qualities or calculations.