The data come from Shimano’s compatibility charts and spec sheets and from SRAM’s caliper mounting document and brake manual, checked in October 2026. The diameter is etched on the rotor itself; you can tell the caliper mount by measuring the distance between the two bolts on the frame or fork. Rotors are sized in millimeters everywhere, the US included.
Rotor diameters and what they’re used for
| Diameter | Shimano | SRAM | Typical use according to SRAM |
|---|---|---|---|
| 140 mm | Yes (Shimano recommends it for the rear only) | Yes | Light road and gravel |
| 160 mm | Yes | Yes | Road, gravel, XC |
| 180 mm | Yes | Yes | XC, trail, road for heavier riders |
| 200 mm | Yes (2.2 mm thick rotor) | Yes | Trail and downhill |
| 203 mm | Yes | Yes (6-bolt) | Frames and forks with a Post 203 mount |
| 220 mm | Yes | Yes (designed for a 4-piston caliper) | Downhill and trail for heavy riders |
The recommended size depends on total weight. In SRAM’s chart, a rider and bike under 139 lb (63 kg) run 160 mm for trail and 180 mm for downhill; above 320 lb (145 kg), 220 mm for both. On the road, SRAM says to check with the frame manufacturer before fitting 140 or 180 mm, because many frames don’t allow them.
Caliper mounts: how to recognize them
| Mount | Bolt spacing | Where it’s used | Rotor without an adapter |
|---|---|---|---|
| Post Mount | 74 mm | Mountain bikes and e-bikes | The mount’s own size: 140, 160, 180, 200 or 203 mm |
| IS (International Standard) | 51 mm | Mountain bikes, less common today | None: always takes an adapter to Post Mount |
| Flat Mount, fork | 70 mm | Road and gravel | 140 mm; 160 mm with an F20 bracket or Shimano’s reversible one |
| Flat Mount, frame | 34 mm | Road and gravel | 140 mm; 160 mm with an adapter |
| Fork with through bolts | 34 mm | Road and gravel | Same as Flat Mount, with an FD20 bracket to go up |
On Post Mount the bolts run parallel to the wheel and thread into the frame; on IS they run perpendicular and pass through the tab. SRAM requires 9 to 13 mm of thread engagement on Post Mount and 5 to 7 mm on Flat Mount: if the bolt is too short or too long, replace it.
Post Mount adapters for bigger rotors
| Frame or fork mount | 160 mm | 180 mm | 200 mm | 203 mm | 220 mm |
|---|---|---|---|---|---|
| Post 140 | +20 | +40 | — | — | — |
| Post 160 | Direct | +20 | +40 | — | — |
| Post 180 | — | Direct | +20 | — | +40 |
| Post 200 | — | — | Direct | +3 | +20 |
| Post 203 | — | — | — | Direct | +17 |
Shimano calls them MA-P0020 (+20 mm: 140 to 160, 160 to 180, 180 to 200 or 200 to 220) and MA-P0040 (+40 mm: 160 to 200 or 180 to 220). SRAM handles +20 with spacers (20 S) or a bracket (20 P), and +40 with the 40 P bracket. The adapter’s number is how much the rotor diameter grows.
Center Lock or 6-bolt, and minimum thickness
| Rotor | New thickness | Minimum thickness | Hub mounting |
|---|---|---|---|
| Shimano standard | 1.75 mm | 1.5 mm | Center Lock (lockring at 40 N·m, 354 in-lb) or 6-bolt (2-4 N·m, 18-35 in-lb) |
| Shimano thick (RT-CL750, RT-6B750) | 2.2 mm | — | 200 and 220 mm; only with compatible calipers |
| SRAM CenterLine, Paceline X | 1.85 mm | 1.55 mm | Center Lock (40 N·m, 354 in-lb) or 6-bolt (6.2 N·m, 55 in-lb) |
| SRAM HS2 and 2 mm CenterLine | 2.0 mm | 1.7 mm | Center Lock or 6-bolt |
Shimano doesn’t list the minimum for its 2.2 mm rotors in its spec sheets: check the value etched on the rotor. Shimano’s newest 4-piston calipers (BR-M9220, M8220, M6220, MT620) use a different spacer to switch from 2.2 mm to 1.75 mm rotors. On hubs with 15 or 20 mm thru-axles, the internally splined Center Lock lockring won’t fit: use the externally splined one.
Torque specs for calipers and rotors from other brands are in the bike torque specs chart, and the axle each hub uses is in the bike axle sizes chart. How to change pads, stop rotor rub and bed in a new rotor is in our disc brake guide, and when and how to bleed the system, in how to bleed hydraulic brakes. The names of every brake part in six languages are in the brakes glossary.
Frequently asked questions
What adapter do I need to go from a 160 to a 180 mm rotor?
If your fork or frame is 160 mm Post Mount, a +20 mm adapter (Shimano MA-P0020, SRAM 20 P) or SRAM's 20 mm spacers. To go from 160 to 200 mm you need a +40 mm adapter.
When should I replace a disc brake rotor?
When it wears below the minimum thickness etched on the rotor: 1.5 mm on standard-thickness Shimano rotors, 1.55 mm on 1.85 mm SRAM rotors and 1.7 mm on 2 mm SRAM rotors. Measure it with calipers on the braking track, not at the edge.
Can I put a Center Lock rotor on a 6-bolt hub?
No, but the reverse works: a 6-bolt rotor on a Center Lock hub with an adapter such as the Shimano SM-RTAD05. The simplest option is to buy a rotor that matches your hub.
What size rotor do I need?
It depends on the combined weight of rider and bike. According to SRAM's chart, between 139 and 200 lb (63-91 kg) a 140-160 mm rotor is enough on the road and 160-180 mm for XC; downhill starts at 180 mm, and above 320 lb (145 kg) you go up to 220 mm.
Sources
- Shimano: caliper and rotor compatibility (C-461) and adapters (C-193)
- Shimano: caliper mount adapter and Center Lock rotor specifications (October 2026)
- SRAM: Caliper Mounting Specifications (mounts, adapters and recommended rotor size)
- SRAM: Disc Brake Rotor Overview
- SRAM: MTB disc brake user manual (minimum thickness and torque)
- BikeGremlin: when to replace bicycle brake rotors (minimum thickness by brand)