Scope ring height calculator

Will your scope's objective bell clear the barrel? Enter five measurements and see the gap, with a live front-view diagram.

Units

Measure the bell, not the lens (a 56 mm scope's bell is larger than 56 mm).

Flip caps add to the bell radius. Use 0 with no cap.

From the ring maker's spec.

Include any taper from a canted (MOA) rail at the bell's position.

Measure where the bell sits over the barrel.

Front view, to scale: barrel (bottom), rail top (dashed), scope bell with cap (top). Red marks the gap.

How we calculate clearance

Everything is measured vertically from the bore axis (the barrel's centerline) at the point where the bell sits:

Clearance = ring height + rail height above bore − (bell diameter ÷ 2 + cap thickness) − barrel diameter ÷ 2.

Why the barrel radius is subtracted: if rail height is measured from the bore axis, the barrel's top surface sits half a barrel diameter above that axis, so it eats into the gap. If you measure rail height straight down to the top of the barrel instead, enter 0 for the barrel diameter. For a canted rail, rail height at the bell ≈ height at the front ring − distance to the bell × tan(cant); 20 MOA over 3 in is about 0.017 in (0.44 mm).

What the result means

These thresholds are our rule of thumb, not a maker's spec. Lower is generally better for cheek weld, so choose the lowest rings that clear. More: choosing scope ring height.