Home / Guides / How to measure your scope tube diameter

How to measure your scope tube diameter

Rings clamp the main tube, so the tube diameter has to be measured to the millimetre before any ring will hold securely.

Find the main tube

The main tube, or body tube, is the straight central section of the scope between the wide objective bell at the front and the eyepiece and turret housing. Rings only ever clamp this straight section, so that is what you measure, not the bell or the saddle.

The four common sizes are 1 inch (25.4mm), 30mm, 34mm and 35mm. There is no meaningful overlap between them, so getting the reading right matters.

Take the measurement

Close a caliper gently across the straight tube at a point clear of the turret saddle. A reading of 25.4mm is a 1 inch tube; 30mm, 34mm and 35mm read as their names. Measure in two positions to be sure you are on the round tube and not catching a taper near the bell.

If you cannot get to bare tube, the scope specification or the manufacturer listing states the tube diameter, and it is usually printed on the box or in the manual.

Why the match must be exact

Rings are cut for one tube diameter. A 30mm ring clamped onto a 1 inch tube leaves a gap and cannot grip properly; the scope will slip under recoil or crush unevenly if overtightened. A 1 inch ring will not close around a 30mm tube at all.

The correct route from 1 inch to 30mm rings, or the reverse, is a purpose-made insert or shim designed for that step, not simply overtightening. Match the ring to the measured tube first, then decide on height.

A note on inserts

Some ring sets ship with reducing inserts so one ring body can hold two tube sizes, most often 30mm rings with 1 inch inserts. These are a proper engineered solution and hold well. Avoid improvising with tape or foam, which lets the tube shift and can mark the finish.

Related

Sources: Vortex · Warne