Learning a little mathematics will keep you from wasting gas and welding rods and ruining a lot of projects, and it is absolutely necessary if you want to become a master welder. You will only need to understand some math basics to become a very good welder, these include fractions, measuring, understanding simple formulas and knowing your way around the mathematics of geometrical shapes.
Fractions show up in welding all the time. A good welder can tell you how many times 3/4 goes into four and a half without having to look for a calculator--most calculators do not handle fractions anyway. Welding seams and penetration depths are almost always described in terms of fractions. A little practice with any good mathematics book will make you an expert at adding, subtracting, multiplying and dividing fractions. More and more, decimals (a special kind of fractions) are being used and the ability to convert between decimals and ordinary fractions is needed. Problems like how to write 3/16 of an inch in decimals come up frequently. Start learning with a calculator then try to do more and more in your head--it will save a lot of time.
Expert welders say "measure twice and weld once", but even two measurements are insufficient if you are doing it incorrectly. Making accurate measurements will save you from making a lot of mistakes. In some jobs the measurements will be in metric, so it is important to be familiar with millimeters and centimeters as well as with inches. It would be unfortunate to miss out on a lucrative job with a foreign employer because you can't understand the metric system. Remember that the rest of the world uses metric and that the workplace is becoming more and more international.
There are some formulas used in welding, so you should feel comfortable with formulas and know how to use them. There will be formulas involving shapes and formulas involving gasses. Understanding formulas not only means knowing how to use the formulas in calculations, it means looking at a formula and knowing what it means. For example, one of the gas laws says that the pressure times the volume equals the temperature times a constant. A master welder will look at this formula and instantly tell you what happens to the temperature if the pressure goes down and the volume stays the same.
A knowledge about the mathematics of basic geometrical shapes is essential for good welders. For one thing, this will allow you to accurately estimate how much supplies you will need for a job. If you are welding around 1,000 ten inch disks you will need to know how to calculate the 1,000 circumferences before you know how much gas and how many rods you will need. There is a similar problem with welding metal boxes--if you do not know the geometry you will have no idea how much supplies are needed for the job.