I'm in the construction trades and one of my most useful tools is my ProjectCalc. This calculator makes it easy for me to figure material for my jobs. You can get this at all the popular home improvement stores, like Lowe's and Home Depot. Not only does it work as a standard calculator, but I can also calculate the amount of material I need in all these materials: Bricks & Blocks, Drywall, Gravel, Lumber, Paint, Roof Shingles, Studs, Tile, & Wallpaper. It helps me find the diagonal, so I can make sure an area is perfectly square. You can figure how many square feet a gallon of paint will cover. You can figure in feet, inches, fractions, decimal feet, decimal inches, fractions, yards, meters, as well as square and cubic formats.
I use it mostly to figure how much dirt or gravel I need for a job. It only takes me a few seconds to punch in the numbers and get an accurate estimate.
A watch battery that lasts a long time powers this calculator. The screen is large and easy to read and it has a hinged cover that keeps the dust and dirt out and also helps protect it from damage on the job site.