Measuring a shank size depends on what you’re measuring, but the process almost always involves the same core tool: a caliper. Whether you’re sizing a drill bit, router bit, ring band, or machining accessory, you need the diameter (and sometimes the width or length) of the cylindrical portion that fits into something else. Here’s how to get an accurate measurement for the most common types.
What a Shank Actually Is
A shank is the smooth, non-cutting portion of a tool or the structural band of a ring. On a drill bit, it’s the part that fits into the chuck. On a router bit, it’s the cylinder that slots into the collet. On a ring, it’s the band that wraps around your finger. In every case, you’re measuring the same basic geometry: a cylinder’s diameter, and sometimes its width or thickness.
The Right Tool for the Job
A digital caliper is the standard instrument for measuring any shank. They’re inexpensive, read out in both inches and millimeters, and give you precision to the hundredth of an inch. A micrometer works too, especially for very small shanks, but calipers handle the widest range of sizes you’ll encounter at home or in a shop.
Avoid using a ruler or tape measure. Shanks are typically measured to tolerances of a few thousandths of an inch, and eyeballing against a ruler will leave you guessing between sizes.
How to Measure a Drill Bit Shank
Place the shank (the smooth end, not the fluted cutting end) between the outer jaws of your caliper. Position the jaws perpendicular to the shank, not parallel, so you’re capturing the widest point of the cylinder. Slide the object as close to the body of the caliper as possible to minimize play in the sliding mechanism. Close the jaws until they sit flush against the shank, apply moderate pressure, and wiggle the bit slightly to confirm there’s no gap.
Avoid clamping at the very tips of the jaws. The tips are angled to fit into tight spaces and can tilt the bit off-axis, giving you a false reading without any obvious sign of error.
Common straight shanks on household drill bits come in fractional sizes: 1/8 inch (0.125″), 1/4 inch (0.250″), 3/8 inch, and 1/2 inch. If your caliper reads 0.248″, that’s a 1/4-inch shank with normal manufacturing tolerance. Match your reading to the nearest standard fraction.
SDS and Hex Shanks
Not all drill shanks are smooth cylinders. SDS and SDS-MAX shanks have grooves that lock into rotary hammer drill chucks to prevent slipping under impact. Hex shanks have a six-sided profile for use in impact drivers. For these, measure across the widest flat-to-flat distance on a hex shank, or measure the smooth cylindrical portion of an SDS shank. Manufacturers specify which shank types and sizes their tools accept, so matching the diameter gets you to the right compatibility.
Measuring Tapered Shanks
Some industrial drill bits and lathe accessories use Morse tapers, which are cone-shaped rather than straight. This means the diameter changes along the length, and you need to measure at a specific point to identify the correct taper number.
The most common approach is to measure the large end diameter. Take your caliper reading and round up to the nearest standard size. For example, a reading of 0.698″ rounds up to 0.700″, which identifies the shank as a Morse Taper #2. You can also measure the small end diameter or the overall taper length and cross-reference those against a Morse taper chart, but the large end is the easiest and most reliable reference point.
Router Bit Shanks
Router bits come in two standard shank diameters: 1/4 inch and 1/2 inch. If the markings on your bit have worn off, a quick caliper measurement tells you which one you have. This matters because using the wrong shank size in your router’s collet means the bit either won’t fit or won’t be held securely. A 1/4-inch shank is the more common size for lighter, general-purpose work. A 1/2-inch shank offers more rigidity and less vibration for heavier cuts with larger bits.
How to Measure a Ring Shank
For jewelry, “shank size” usually refers to two dimensions: width and thickness. Width is the measurement from one edge of the band to the other as you look at the ring from the side (how tall it appears on your finger). Thickness is how far the band extends from the inside surface to the outside surface.
Use the same digital caliper technique. For width, place the jaws on the top and bottom edges of the band. For thickness, measure from the inner surface to the outer surface at the thickest point. Industry standard ring widths come in even millimeter increments: 2mm, 4mm, 6mm, 8mm, 10mm, and 12mm. Odd sizes like 5mm and 7mm exist but are less common and may require a custom order.
For men’s wedding bands, widths generally break down into three categories: narrow (under 6mm), regular (6 to 8mm), and wide (over 8mm). Women’s bands tend to fall in the 2mm to 4mm range.
Comfort Fit Changes the Interior
If you’re measuring a ring to determine finger size compatibility, keep in mind that comfort fit bands have a slightly rounded interior. This rounding makes the ring sit differently than a flat fit band, which has a completely flat inner surface. A comfort fit ring generally runs about a half size larger than a flat fit ring of the same stated size, so if you’re comparing two rings or ordering a replacement, factor that difference in.
Tips for Accurate Readings
- Zero your caliper first. Close the jaws completely with nothing between them and confirm the display reads 0.000. If it doesn’t, use the zero button to reset.
- Measure perpendicular, not parallel. On any cylindrical object, perpendicular jaw placement captures the true diameter. Parallel placement can slide off-center and give you a smaller reading.
- Take multiple readings. Measure at two or three points along the shank and average them. Wear, manufacturing variation, or slight bending can make the diameter inconsistent.
- Keep the shank close to the caliper body. The farther out toward the jaw tips you measure, the more mechanical play you introduce. Slide the object toward the body for the tightest, most stable contact.
- Round to the nearest standard size. Shanks are manufactured to standard dimensions. If your reading is 0.499″ instead of 0.500″, you have a 1/2-inch shank with normal tolerance, not some nonstandard size.