If I'm doing it for a client I go for the exact measurements, there's nothing far worse than a necklace than turns out to be quite a choker. However, as hobbyists our enthusiasm gets the best of us and we just want to create and do away with the math.
Luckily, there's a great array of jewelry and beading design tools that focus on Measurements and Sizes:
Necklace Planning Board
The Necklace Planning Board is the best guide for designing necklaces. It helps you plan your design, the placement of your beads as a whole. You can foresee what your necklace will look like without the string or chain. As you are creating, you would soon realize what textures and colors would go together seamlessly.
Photo and Product sold online by Beadfx.com
Some jewelry shops refer to them as Bead Design Boards. Some have a bead counter which provides a visual placement for round beads. Measurements also go by inches. Some boards have 3 rows (Beadalon brand) enough for you to envision multi-strand necklaces. Like in making movies, the Necklace Planning Board is the cutting room floor for necklaces. Best of all, they serve as a bead tray to keep all your beads from rolling away.
Bracelet Sizer
The Bracelet Sizer is used to get the right size of the wrist before you can start on your bracelet. This is a good peg especially if you are receiving orders online and you can't personally measure your client's forearm.
This Bracelet Sizer can be found at Beadfx.com
In any bracelet, the thread or chain will curve around the wrist, the bracelet sizer helps you allot the right amount of space or allowance between the skin and the beads. Please consider that not all beads are round- some are stone flat, irregularly shaped or bulky. If you are creating a bracelet of mixed beads and stones, you need to compensate for the length of the thread.
Bracelet Gauge
They look like metal bracelets and like handcuffs, they provide an opportunity for you to do a fitting on your client. The client can now gauge how tight or loose she may want her bangle, cuff or bracelet to be.
Check out this Bracelet Gauge at Beadfx.com
Ring Sizer
This is the most popular among the jewelry measurement tools- this is how they get the right fit for engagement rings. Imagine, buying something that pricey then it doesn't fit. As for jewelry making, Ring Sizers are there to gauge finger diameter.
I usually use my ring sizer as a dummy finger for freeform peyote stitch projects (aka glass bead rings). In Manila, I bought mine at Wellmansons in Quiapo for P150, I got the aluminum kind though I would have preferred the wooden one (P400-P500).
Here are some online shops that sell Jewelry Measurement tools:
