Two sample beads sit close together on outside size: 19.5 mm and 18.8 mm. Close enough that you might expect them to be much the same bead.
The hole openings, measured with my caliper, are approximately 3.0 mm versus 14.9 mm. The 14.9 mm hole opening is almost five times the 3.0 mm opening! Close outside measurements, but a huge difference at the opening. Without measuring the opening - you'd be none wiser.
That's precisely the reason our bead measurement chart lists two numbers for one bead, an outside measurement and a separate opening measurement. For a simple round or near-round bead, the outside dimension is usually the main size reference. For shaped beads, the measured span needs to be stated clearly. The opening is its own separate reading. Hence two measurements are displayed.
Here's what we propose to guide you: Use a real bead to get some real figures, using my samples here as a reference - then use those figures on the Shop by Size page.
Why outside size and hole opening are two separate readings
A bead is essentially a shape with an opening through its center. The outside measurement is one number - the hole opening is a separate reading, and they're different measurements. That's why it's listed twice in our table. The top example shows an outside of 19.5mm, an opening of 3.0mm. Close on the outside, a gaping difference at the opening, here a big outside size had little hole. Compare that with its nearest neighbor which was an outside of 18.8, yet a 14.9 mm opening, a wide hole, so close outside and a massive hole opening. In this pair, the outside measurements alone would not have told us the hole openings.
They give you different pieces of useful info. The outside measurement gives you a feel for the proportions of the bead and how much physical space it occupies in a design. The hole opening is a separate reading. On the sample here (table below, listed to right), the outside has already been measured and the hole opening has also been noted. Recording the outside size and the hole opening separately keeps the two numbers from getting mixed up.
What outside dimension do you measure?
If the bead is round or nearly round this isn't too complicated - the widest relevant span along the line of the perforation, the way the bead sits on a string. On something such as a flat oval bead where you're looking to give the largest possible impression - perhaps for a focal bead in a piece; or on an irregularly shaped bead where one axis is considerably longer than another; you might want to take a second measurement. If a shaped bead has a much wider perpendicular span, record that second outside dimension separately. If the task is maximum overall size on a raised or decorated piece, measure across the most protruding span.
How we measure beads with a digital caliper
This piece of equipment is a digital caliper. Nothing exotic, but perfect for getting a rough guide. It can also display inches if needed. In the samples, the number on the display changes only on intervals of .1 mm, so 0.1, 0.2, etc. So the numbers you see on the chart land on a tenth. That's the displayed step, not a claim that the bead itself is accurate to a tenth.
When measuring the outside dimensions bring the flat jaws into firm, stable contact with the widest relevant part of the bead. According to the 799 electronic caliper manual on Starrett's digital calipers, you shouldn't use excessive measuring pressure.
The hole opening requires careful alignment. For the photographed samples here, you simply line the caliper up to the opening and read the opening measurement it shows. If the bead is not aligned with the opening, re-align it and take the reading again.
The outside measurement is straightforward. The hole opening takes more care, since you need the bead aligned to the opening against the caliper jaws. Precision-tool references like the Mitutoyo quick guide to precision measuring instruments lay out the external and internal measurement functions clearly. In our own bead checks, if two readings differ, alignment is the first thing we re-check.
Five real beads - measured both ways
The chart below records the outside and the corresponding opening for five of my sample beads. The outside measurement is on the left, and the hole opening on the right.
| Sample | Outside | Hole opening |
|---|---|---|
| Bead 1 | 29.7 mm | 2.7 mm |
| Bead 2 | 19.5 mm | 3.0 mm |
| Bead 3 | 18.8 mm | 14.9 mm |
| Bead 4 | 17.1 mm | 3.3 mm |
| Bead 5 | 27.5 mm | 2.3 mm |
Take another look at these five: Bead 1 is the largest outside measurement in this sample set at 29.7 mm yet carries only 2.7mm of hole! And the next in the group (#5), 27.5 mm across, has a hole of just 2.3 mm. Bead #2 is 19.5 mm outside - hardly any difference to bead 3, with an 18.8mm outside size. Yet the first bead only has a 3.0 mm hole opening while the second bead has a massive 14.9 mm! In these samples, outside size alone did not tell us the hole opening. It all comes down to which figure you require.
If a reading does not make sense, check alignment first
Sometimes you come up with a figure that doesn't match a previous reading, or doesn't match the span you meant to measure. When two readings differ, alignment is the first thing we re-check.
Alignment is the first thing to check. Re-seat the bead against the jaws and take the measurement again.
Check the outside span again - did you maybe measure a different span by mistake? Or measure a bead that's irregular or raised? If the readings still differ after re-aligning, those are the next things to check.
Use the two numbers to check the right product details
Use your outside and hole opening measurements and see which bead category in our range your numbers correlate with. The outside size relates to how a bead sits, and how much physical space it takes up. The hole opening is a separate measurement. If you need to check a material, wire, or finding, compare that item's actual diameter with the measured opening.
So having these two numbers gives you a solid reference to work from. Use the Shop by Size page as a size reference, then confirm the listed dimensions and opening on the product you're considering.
Written and reviewed by ZOE, Business Owner at ZoeDIY.
