The correct spoke length is absolutely essential for a properly built set of wheels. You would think this would go without saying, but I am never really surprised when I see expensive factory built wheels that don't have the correct spoke length. Either too short or too long, they are missing the mark of high quality workmanship. You would think that a factory which may be building thousands of the same model would get that right, but not always. This failure can lead to premature spoke breakage.
So, how does a wheel builder arrive at the most ideal spoke length? Years ago before the era of handy online spoke calculators it was a much more challenging job. I used to have a little black book that recorded every wheel that I built with the hub model, rim model, lacing pattern and what the corresponding best spoke length for those parts was. We also had at our disposal the Sutherland Handbook for mechanics which had suggested spoke lengths. It generally was quite helpful but it didn't have every answer among its pages.
Fast forward a few decades and companies like DT Swiss started to develop their own online spoke calculators. In many ways these were a God send for mechanics and wheel builders everywhere. Today there are several reliable online spoke calculators.
How do they work? Initially someone had to develop some pretty sophisticated software to number crunch an infinite array of measurements to bring these to life. For those actually using them, it's really just a matter of plugging in the correct numbers on hub dimensions and rim ERD and the program does all the hard math for you and spits out it's suggested spoke lengths.
Are those suggested spoke lengths absolutely reliable? In most cases they are, but now and again I get a suggested length that isn't exactly ideal. Not a world away but a slightly different length would have been better. I always keep my print outs and make notes for future builds.
To ensure predictable accuracy its wise to take your own ERD measurements before beginning. By the way, ERD stands for the "effective rim diameter" . These numbers are often supplied by rim manufactures but they are sometimes not accurate. This mistake can lead to the wrong spoke length being recommended. An investment in a pair of WheelSmith rim rods will give you a proper ERD measurement.
A word or two on taking the hub measurements. Invest in a decent quality digital vernier calliper. It will pay for itself eventually. Hub measurements are often supplied by the manufacturer but double check that they haven't made an error in what they have published. It might save you having to get new spokes cut.
There are a number of online spoke calculators I'd like to mention. I have had reliable results from DT Swiss, prowheelbuilder.com and wheelpro.co.uk .
I often will use more than one online calculator to check to see how close a recommendation they make. Sometimes they are almost identical and other times there may be a difference of a half millimetre or so. If there is a notable difference in recommendations I'll invoke the opinion of a third calculator. As you investigate various online calculators you'll become familiar with their strengths, weaknesses and idiosyncrasies.
My new favourite calculator is an app that's on my Iphone. It was developed by Douglas Pepelko and it can be procured through the Itunes App store. It's called Quick Spoke and it's simply amazing! I would highly recommend this very handy spoke calculating app. It has a long list of hubs with all the measurements already there for you. It also has a comprehensive list of rims with their ERD's. There are also other features like the information relating to bracing angle of the spokes as well as the percentage of tension on each side of the wheel. It also enables you to save your calculations for future reference. For the wheel builder on the go, this is the best app out there. At $1.99 you can hardly go wrong.
There are several other online spoke calculating websites that are out there. I have had the best results with the ones I have mentioned. That's not to say you won't find a different one that's ideal for you. Whichever one you decide upon keep in mind that you are a thousand miles ahead of the days when nothing like this even existed. Keep a record of your various wheel builds so that you can reference them one day in the future. Most important, try to have some fun building those wheels!