What do the numbers on a rifle scope mean?
Quick answer: the two headline numbers are magnification × objective lens diameter in millimeters. On a 3-9x40, the 3-9x is a variable zoom from 3x to 9x, and the 40 is the width of the front lens in mm, which sets how much light the scope gathers. A single magnification like 4x means a fixed scope. Higher zoom reaches farther but narrows the view and magnifies shake and mirage; a bigger objective is brighter but heavier and sits taller over the bore. Everything else (tube size, MOA/MRAD clicks, focal plane, parallax) describes how you adjust and aim the scope.
The first number: magnification
The first part is magnification, how many times closer the target appears. When it is a range like 3-9x, the scope is variable: it zooms from 3x up to 9x, so you can dial 3x for a close, wide view and 9x to reach out. A single number like 4x means a fixed-power scope with no zoom. More magnification is not automatically better: as you zoom in, the field of view narrows, and any mirage, wobble or heartbeat is magnified too. Match the range to your distance: 3-9x or 4-12x for general hunting, a 1-6x or 1-8x LPVO for an AR-15, and 5-25x or 6-24x for long-range precision. See our best rifle scopes by use case.
The second number: objective lens diameter
The number after the x is the diameter of the front (objective) lens in millimeters. A bigger objective gathers more light, for a brighter image at dawn and dusk and at high magnification, but it also adds weight, costs more, and sits higher over the bore so it needs taller rings. 40 mm to 44 mm is the all-round standard that suits most hunting; 50 mm and 56 mm pull in more light for long-range and low-light work at a real weight penalty, which is why a long-range scope like the Vortex Strike Eagle 5-25x56 carries a 56 mm objective.
Tube diameter: 1 inch vs 30 mm
The main body tube is usually 1 inch (25.4 mm) or 30 mm, and it matters for two reasons. First, it sets your ring size, the rings must match the tube. Second, a larger 30 mm (or 34 mm) tube generally has room for more internal adjustment travel, so you can dial more elevation for long-range shooting. A 1-inch tube keeps a hunting scope lighter and cheaper; a 30 mm tube is common on precision scopes for the extra travel. Tube diameter does not change image brightness, that is the objective lens.
Turret clicks: MOA and MRAD
The turrets adjust your point of impact in clicks measured in MOA (minute of angle) or MRAD (milliradian, or mils). One click is typically 1/4 MOA, about a quarter inch at 100 yards, or 0.1 MRAD, about 0.36 inch at 100 yards. The unit tells you how far each click moves your shot: to move impact 1 inch at 100 yards, you dial about 4 clicks of 1/4 MOA. Neither unit is more accurate; just make sure your reticle and turrets use the same one. For the full breakdown see MOA vs MRAD, and our free MOA/MRAD adjustment calculator turns a miss distance into the exact clicks to dial.
Focal plane, parallax and eye relief
Three more specs shape how the scope behaves. The focal plane is where the reticle sits: a first focal plane (FFP) reticle grows and shrinks with the zoom so its holdover marks stay true at any magnification, while a second focal plane (SFP) reticle stays one size and is only true at one power (usually the top). Parallax is the distance the scope is focused to; it is fixed on most hunting scopes (often 100 yards, or 50 for rimfire) and adjustable via a side-focus knob on higher-power scopes, where it matters most above about 10x. Eye relief is how far back your eye can sit and still see the full image; more eye relief is safer on hard-recoiling magnums so the ocular does not hit your brow.
The numbers at a glance
| Spec | What it is | Why it matters |
|---|---|---|
| First number (3-9x) | Magnification range | How many times closer, from low to high zoom. 3-9x hunting, 1-6x/1-8x for an AR, 5-25x long range. |
| Second number (40) | Objective lens diameter (mm) | Light gathering. Bigger = brighter but heavier and taller over the bore. 40-44 mm all-round. |
| Tube diameter | 1 inch or 30 mm | Sets ring size and internal travel. 30 mm usually allows more elevation for long-range dialing. |
| Turret click value | MOA or MRAD | 1/4 MOA (about 1/4 in at 100 yd) or 0.1 MRAD (about 0.36 in). How far each click moves impact. |
| Focal plane | FFP or SFP | FFP holds the reticle true at any zoom; SFP is only true at one magnification (usually the top). |
| Parallax | Fixed or adjustable (yards) | When the aiming distance is set. Adjustable side focus matters most above about 10x. |
| Eye relief | inches | How far back your eye sits and still sees a full image. More is safer under recoil (magnums). |
So which numbers should you choose?
- 3-9x40 or 4-12x40 for all-round hunting inside 300 yards: the classic default. See best scope for deer hunting.
- 1-6x24 or 1-8x24 for an AR-15 or close-to-mid: a low power variable optic that starts at 1x.
- 6-24x50 or 5-25x56 for long-range precision past 400 yards, usually FFP on a 30 mm tube. See best long range rifle scope.
- 2-7x32 or 3-9x40 rimfire for a .22, with a 50-yard parallax: rimfire vs centerfire scope.
FAQ
What does 3-9x40 mean on a rifle scope?
3-9x40 means the scope zooms from 3x to 9x magnification through a 40 mm objective (front) lens. The 3-9x is a variable magnification range: 3x for a close, wide view and 9x to reach out, with everything in between. The 40 mm is the diameter of the front lens in millimeters, which sets how much light the scope gathers. 3-9x40 is the classic all-round hunting size, which is why scopes like the Vortex Crossfire II 3-9x40 are so common.
What is the difference between 3-9x40 and 6-24x50?
The first pair is the magnification range and the second number is the objective lens in mm. A 3-9x40 zooms 3x to 9x with a 40 mm lens, ideal for general hunting inside 300 yards. A 6-24x50 zooms 6x to 24x with a bigger 50 mm lens, built for long-range precision where you want more magnification to see and dial small targets far away, like the Vortex Diamondback Tactical 6-24x50 FFP. More magnification narrows the field of view and shows more mirage and shake, so you only want it if you actually shoot at distance.
Is a bigger objective lens always better on a rifle scope?
No. A bigger objective (the second number, like 50 or 56 mm) gathers more light for a brighter image at dawn and dusk and at high magnification, but it adds weight, raises the scope higher over the bore (needing taller rings), and costs more. For general hunting a 40 mm to 44 mm objective is plenty; step up to 50 mm or 56 mm mainly for long-range or low-light shooting where the extra brightness earns its weight.
What do the other numbers mean, like 1 inch, 30 mm, MOA or MRAD?
The tube diameter (1 inch or 30 mm) is the main body size that sets your ring size and how much internal adjustment travel the scope has; a 30 mm tube usually allows more elevation for long-range dialing. MOA and MRAD are the units of the turret clicks: one click is typically 1/4 MOA (about 1/4 inch at 100 yards) or 0.1 MRAD (about 0.36 inch at 100 yards). They tell you how far each click moves your point of impact. See MOA vs MRAD and our free adjustment calculator for the math.
What magnification do I need on a rifle scope?
Match it to distance. For general hunting inside 300 yards, 3-9x or 4-12x is ideal and keeps a wide, forgiving view. For an AR-15 or close-to-mid work, a 1-6x or 1-8x LPVO starts at 1x for both-eyes-open speed. For long-range precision past 400 yards, 5-25x or 6-24x lets you see and dial small targets. More magnification is not automatically better: it narrows your field of view and amplifies mirage and shake.
Related: MOA vs MRAD · MOA/MRAD calculator · FFP vs SFP · what parallax is · how to mount a rifle scope · how to sight in a rifle scope · best rifle scopes 2026.