How Many Stones Do I Need?? Calculating Halos - the Nitty Gritty

I probably get a version of this question once a week: how many 1.5mm stones do I need to go around a 6.8x7.6 pear shape? Or some such. So this blog is overdue. And the next time you ask me this question, I can just send you to a nice and useful link, lol. Explaining math is not my strong suit but I am going to give it my best shot. Note that in actuality this is not difficult but it can be tricky to get across if you don’t regularly teach this stuff. 

The simplest halo calculation can be done using just round stones. I’ll show you how that works and once you have that process down, it can be adapted for other shapes. Note that all these calculations will be approximate but you can get it right within one or two stones and that is enough. You need extra halo gems anyway because setters don’t want to worry about losing a tiny stone or breaking it.  

Ok here goes. 

Let’s use a 5mm round stone and 1mm halo stones as an example.  

Step 1: add the size of the halo stone to the diameter of the center: 5+1 = 6. The 6mm represents the halo going through the center of the halo gems around the middle stone. Call that the outer circumference. What we are calculating is the diameter of that circumference. 

Step 2: Multiply that new diameter (6) by three to get the circumference. [Explanation for more advanced math people: technically you should use Pi of 3.14... But for our purposes 3 is close enough]. So 6x3 = 21.  

Step 3: Take the circumference (21) and divide it by the halo stones. In this case, it’s 21:1=21. [But if the halo stones were 1.3mm, that’s the figure you would use as your divisor.] So you need 21 stones. If there is a gap between the gems for extra millgrain, pave beads or you are laying out more than one size stone in the halo, you may need to use CAD. But this you can do on your own. 

Now let’s move on to other gemstone shapes. Many gem shapes adapt easily because you can average out the diameter. I.e. a 6x4mm oval will have a circumference equal to a 5mm round. A 6x4 pear shape has a little bit of a smaller circumference but it has a tip, so its best to approximate with the nearest round shape as well, which is again 5mm (the average between the two measurements). A trillion shape gem may use less but again it has 3 corners that you need to go around, a square has four. For a square, use the sides of the square and add them together. If it’s 5mm plus a 1mm halo, then you have four 6mm sides. So that’s 24mm in total. 

And what about gaps between the gems for setting? The answer to that question depends on what type of gems you are setting. Diamonds can touch so you don’t need to subtract out gaps. Colored gems need a wee gap between them so they don’t scratch each other. So then you might want to subtract a mm or two. But unless your halo has 3mm stones (that’s very big for a halo), don’t bother with that. You need extra gems anyway and the number you arrive at with this math is closer to the minimum number of gems you need than the maximum. 

For ease in calculation, I am here re-attaching an image from an earlier blog where I discussed halos but didn’t explain the math. You can print this or save it and use it as a reference guide. 

Here’s one more example: 

How many 1.5mm stones are needed for a 10x8mm pear shape?  

That means we want to use a 9mm round as an approximation for the math. We now add  1.5mm. So we have 10.5mm for the diameter. We now multiply by 3 for the circumference, so that’s 31.5 (and if you actually used Pi it would be more like 32). Now we divide the halo stone size into it. That means we need 21 stones. And that is probably enough because it’s a pear so it’s narrower than an oval. But I would probably acquire 22.  

Now here are some additional factors you want to consider when calculating. 

Hope this helps!