Chainring BCD Calculator
Bolt circle diameter from the distance between two adjacent holes, or the other way round. Works for chainrings, spiders, direct mount adapters and 6-bolt rotors, as long as the holes are equally spaced.
Last checked September 27, 2026. Runs in your browser, nothing is sent.
The formula
Adjacent bolt holes on a circle are the ends of a chord. For n equally spaced holes, the angle between neighbors is 360°/n, and half of it sets the chord.
BCD = d ÷ sin(180° ÷ n)
where d is the center to center distance between two adjacent holes. Reversed, d = BCD × sin(180° ÷ n). For 5 bolts sin(36°) is 0.5878, for 4 bolts sin(45°) is 0.7071, for 6 bolts sin(30°) is exactly 0.5.
Measuring the chord
- Center to centerInside edge to inside edge plus one hole diameter, or outside edge to outside edge minus one hole diameter. Same answer either way.
- Even patternsOn 4 and 6 bolt patterns, skip the formula: opposite holes are exactly one BCD apart.
- Odd resultsIf nothing standard is within about 1.5 mm, check for unequal spacing before you trust the number. Shimano 110 mm 4-bolt and some 96 mm patterns are asymmetric.
Common patterns and their hole spacing
Equally spaced patterns only. If you measure one of these chords on a ring, this is what you have.
| Pattern | Bolts | BCD | Adjacent centers | Opposite centers |
|---|---|---|---|---|
| Disc rotor, 6-bolt ISO | 6 | 44 mm | 22.00 mm | 44.0 mm |
| MTB triple inner, older 5-bolt | 5 | 58 mm | 34.09 mm | No opposite pair |
| MTB 4-bolt inner | 4 | 64 mm | 45.25 mm | 64.0 mm |
| Road triple inner | 5 | 74 mm | 43.50 mm | No opposite pair |
| SRAM 4-bolt MTB | 4 | 94 mm | 66.47 mm | 94.0 mm |
| Shimano 4-bolt MTB, symmetric versionAn asymmetric 96 mm version also exists. Check the crank before you machine. | 4 | 96 mm | 67.88 mm | 96.0 mm |
| MTB 4-bolt outer | 4 | 104 mm | 73.54 mm | 104.0 mm |
| Road compact 5-bolt | 5 | 110 mm | 64.66 mm | No opposite pair |
| Road standard 5-bolt | 5 | 130 mm | 76.41 mm | No opposite pair |
| Campagnolo 5-bolt | 5 | 135 mm | 79.35 mm | No opposite pair |
| Track 5-bolt | 5 | 144 mm | 84.64 mm | No opposite pair |
Patterns from the bolt circle table. Sources: Sheldon Brown, chainring bolt circle crib sheet, Wikipedia, Crankset.
Putting it on the drawing
The calculator tells you what a part is. The drawing has to tell a shop how to make it, and bolt circles are where drawings go wrong most often.
- One patternDimension the BCD, the hole count and the hole size as a single pattern with a positional tolerance from the center bore. Not five hole coordinates.
- Position, not sizeThe hole diameter can be loose. The position is what makes the ring sit flat and the bolts thread in without side load. ±0.05 mm true position is a normal working figure on a spider.
- Name the standard"104 mm BCD, 4-bolt" on the drawing settles it. A measured 73.5 mm chord with no pattern name invites a round of questions.
- Go deeperMachining chainrings and where tolerance belongs on a bike part drawing.
Questions
How do I measure the distance between two bolt holes accurately?
Measure center to center between two adjacent holes. With calipers, measure from the inside edge of one hole to the inside edge of the next and add one hole diameter, or from outside edge to outside edge and subtract one hole diameter. Both give the center distance. Take it twice on different pairs of holes and average.
Why is there a shortcut for 4-bolt and 6-bolt patterns?
With an even number of bolts, two holes sit directly opposite each other and the distance between their centers is the bolt circle diameter itself. On a 104 mm 4-bolt ring the opposite holes are 104 mm apart. A 5-bolt ring has no opposite pair, so you need the chord formula.
Does this work for Shimano 4-bolt road cranks?
No. Shimano 110 mm 4-bolt road cranks use unequal bolt spacing, and an asymmetric 96 mm MTB pattern also exists. The formula assumes equal spacing. Take those patterns from the crank maker's drawing, not from a measurement and a formula.
Does this work for disc brake rotors?
Yes. The 6-bolt ISO rotor mount is a 44 mm bolt circle with six M5 holes, so adjacent hole centers are 22.00 mm apart and opposite holes are 44 mm apart.
How should a bolt circle be dimensioned on a drawing?
As one pattern: the bolt circle diameter, the number of holes and a positional tolerance referenced to the center bore. Never as separate hole coordinates. Chained dimensions stack their tolerances and the ring ends up not sitting flat on the spider.
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