Co-witness means your iron sights and your red dot line up on the same point, so you can see and use the irons through the optic's window. Absolute co-witness (1.41 in, 35.8 mm above the rail) puts the dot right on top of the front sight post; lower 1/3 co-witness (about 1.535–1.63 in, 39–41.4 mm) raises the dot so the irons sit in the bottom third of the window. Taller 1.93 in (49.0 mm) mounts drop co-witness altogether for a more upright head position.
Which you choose changes what you see every time you shoulder the rifle, so it is worth understanding before you buy a mount or a dot that comes on one.
Key takeaways
- Absolute co-witness: dot and iron sights share the same line at 1.41 in (35.8 mm) above the rail.
- Lower 1/3 co-witness: the dot sits 3–6 mm higher, so the irons appear in the lower third of the window and clutter less of the view.
- At 1.93 in (49.0 mm) and 2.26 in (57.4 mm) the irons no longer show in the window; you gain a heads-up posture instead.
- Co-witness only matters if you actually fit iron sights; folding back-up sights can be left down until needed.
- A magnifier must be set to the same height as the dot, or the dot will not be centred when you flip it in.
What co-witness actually means
On an AR-pattern rifle, the iron sight line runs a fixed distance above the rail. Mount a red dot so its window is centred on that line and you can look through the glass and see the front sight post with the dot sitting on its tip. If the optic's battery dies or the emitter fails, you simply use the irons through the same window without moving your head. That redundancy is the whole point.
The height is measured from the top of the Picatinny rail to the centre of the optic's window. For exact figures, including millimetres and height over bore, see the red dot mount height chart.
The three main heights
Absolute co-witness: 1.41 in (35.8 mm)
The dot sits exactly on the iron sight line. With the irons raised you see the front post in the middle of the window with the dot on top of it. Maximum redundancy, but the post and rear aperture take up the middle of your sight picture, which some shooters find cluttered. Sights with a built-in mount at this height include the Holosun HS510C, which Holosun specifies at a 1.41 in dot height, and EOTech's XPS models.
Lower 1/3 co-witness: 1.535–1.63 in (39–41.4 mm)
The dot is raised slightly above the iron line, so when you look through the window the irons appear in the lower third, below the dot. You still have them, but they are out of the way. This is the most popular AR height. Be aware that makers use different figures: Aimpoint Micro-pattern mounts use 39 mm (1.535 in), while Holosun's rifle dots, such as the AEMS CORE X2 and the ARO EVO, ship on 1.63 in (41.4 mm) mounts. EOTech achieves lower 1/3 on the EXPS3 with a base raised by 7 mm.
Taller mounts: 1.7 in, 1.93 in and 2.26 in
Primary Arms notes that some lower 1/3 mounts reach 1.7 in (43.2 mm); above that, the irons drop out of the window. At 1.93 in (49.0 mm) and 2.26 in (57.4 mm) the aim is ergonomics, not redundancy. You shoot with your head more upright, which many find faster, and there is room for night vision or bulky ear defenders. Irons, if fitted, are no longer visible through the optic.
| Height | Inches / mm | Irons in the window? | Best for |
|---|---|---|---|
| Absolute co-witness | 1.41 in / 35.8 mm | Yes, centred on the dot | Maximum redundancy |
| Lower 1/3 co-witness | 1.535–1.63 in / 39–41.4 mm | Yes, in the lower third | All-round use with back-up irons |
| 1.93 in | 1.93 in / 49.0 mm | No | Upright, heads-up shooting |
| 2.26 in | 2.26 in / 57.4 mm | No | Night vision and very upright posture |
Which height should you choose?
- Back-up irons matter most to you: absolute co-witness, so the irons and dot share the same point.
- You want irons available but a clean view: lower 1/3, the all-round default on AR-pattern rifles.
- You want the most comfortable, upright posture: 1.93 in, and accept that the irons become a separate system.
- You do not fit iron sights at all: choose purely on comfort; co-witness is irrelevant.
Taller mounts also increase the gap between bore and sight, which changes both the close-range offset and where your zero crosses the line of sight again. Our zero distance chart shows the effect in centimetres.
Expert tip
If you choose lower 1/3 and plan to add a magnifier later, buy the magnifier with the dot's exact height in mind. A 1.535 in dot behind a magnifier set to 1.63 in is 2.4 mm out, which is enough to push the dot off-centre in the magnified view.
Pistol optics work differently
On a pistol, height is set by the slide cut and any adapter plate, not by a riser mount. Pistol shooters co-witness with taller "suppressor-height" iron sights that are visible through the lower part of the window. The rifle heights on this page do not apply; for pistols, the footprint is what matters, covered in our RMR vs RMSc footprint guide.
UK note
In England, Wales and Scotland, semi-automatic centrefire rifles are prohibited under section 5(1)(ab) of the Firearms Act 1968, which exempts .22 rimfire. Northern Ireland has the same rule in Article 45 of the Firearms (Northern Ireland) Order 2004. So co-witness heights here mostly apply to straight-pull centrefire ARs, .22 rimfire AR-pattern rifles and airsoft replicas. The heights are the same on any Picatinny rail, but if your rifle has no iron sights, pick the height that fits your face, not your irons.
Getting it right
Many rifle dots come with a mount at a fixed height, so check which height is in the box before you buy. Browse rifle dot sights for dots that include co-witness mounts, or look at red dot and magnifier combos if you want the height matching done for you. Then mount and zero it properly with our mounting and zeroing guide. We do not currently stock standalone AR riser mounts, so a separate 1.93 in or 2.26 in riser has to be bought elsewhere.
Frequently asked questions
What does lower 1/3 co-witness mean?
The optic is mounted slightly above the iron sight line, so the irons appear in the lower third of the window instead of the middle. You keep back-up sights without them cluttering the view.
Absolute or lower 1/3: which is better?
Neither is better in every case. Absolute gives the most direct back-up; lower 1/3 gives a cleaner view with the irons still usable. Lower 1/3 is the more popular choice on AR-pattern rifles.
Do I need co-witness if I have no iron sights?
No. Without irons, choose a height purely for comfort and cheek weld. Many shooters then prefer lower 1/3 or 1.93 in.
Can I co-witness with a holographic sight?
Yes. EOTech's XPS models sit at absolute co-witness height and the EXPS models use a raised base for lower 1/3, according to EOTech's specifications.
Shop this guide
- Rifle Dot Sights — many supplied on absolute or lower 1/3 mounts
- Holographic Sights — EOTech XPS and EXPS at co-witness heights
- Red Dot Sight Magnifiers — with spacers to match your dot's height
