How to Size a Ring with a Steel Ring Mandrel

By Jewellery Tools UK — Hand Tools for Jewellers & Silversmiths | Published: 2026-10-26

Category: How-to Guides

Learn how to use a steel ring mandrel to check US ring sizes, true oval shanks, and form new bands with confidence.

Quick Answer

To size a ring accurately, slide it onto a steel ring mandrel until it rests snugly at the correct US size, then read the engraved half-size markings. For forming or truing, use the mandrel as a solid anvil and tap gently with a rawhide or nylon mallet.

Why a Steel Mandrel Beats Plastic or Wood

A steel ring mandrel is the reference tool for ring work because its solid steel construction gives it the mass and rigidity to take real hammer blows without deflecting. Plastic or wooden mandrels may be fine for gentle sizing checks, but they will not hold up to forming or truing work.

The working surface of a good steel mandrel is hardened, smooth, and evenly tapered, which allows rings to slide on and off easily and provides a consistent surface for hammering. This is why our guide to choosing a ring mandrel emphasizes these qualities first.

Know Your Ring Sizes

US ring sizes are based on the inner circumference of the ring, and the mandrel's engraved scale from 1 to 16 with half-size markings lets you read the size directly. If you are sizing a ring that is not perfectly round, you may need to check it at several points along the taper to get an accurate reading.

Remember that the mandrel measures the inside diameter, not the outside. A thick band will feel tighter than a thin one at the same size, so always consider the ring's width and profile when interpreting the reading.

Step-by-Step Ring Sizing and Forming

Prepare the ring and mandrel: Clean the ring to remove any dirt or grease, and make sure the mandrel is dry and free of debris. Hold the mandrel by its grip handle, with the tapered end pointing upward. A clean ring slides more smoothly and gives a more accurate reading.
Slide the ring onto the mandrel: Gently push the ring onto the mandrel from the smaller end, letting it settle at the point where it fits snugly without forcing. Do not force a ring past a tight spot, as this can scratch or distort it. The ring should sit firmly but not be stuck; if it is too tight, stop and check the size again.
Read the size: Look at the engraved scale at the point where the ring rests. Read the number and any half-size mark directly below the ring's inner edge. For an oval ring, rotate it to find the smallest reading, which is the true size. Always read at the inner edge, not the outer, for accurate sizing.
True an oval shank: If the ring is out of round, place it on the mandrel at the correct size and tap gently with a rawhide or nylon mallet around the high spots to encourage it back to round. Work slowly and check frequently to avoid over-correcting. Use light taps and check after each pass to prevent distortion.
Form new work on the mandrel: For forming a new band, place the metal strip on the mandrel at the desired size and hammer it around the taper, moving the metal gradually. The solid steel mandrel provides a stable anvil for this work. Always use a non-marring hammer to avoid damaging the mandrel's surface or your work.

The Tool We Recommend

Ring Sizing & Forming Mandrel — Solid Steel, Round, US Sizes 1–16, 13 Inch, Grip Handle: This solid steel ring mandrel is engraved with US sizes 1–16 including half-size markings, and its hardened, smooth, evenly tapered surface is built to take hammer blows without deflecting. You get accurate size readings and a durable forming surface for truing or shaping rings, all in one tool.
Ring Sizing & Forming Mandrel — Solid Steel, Round, US Sizes 1–16, 13 Inch, Grip Handle