How to measure bicycle crank length: centres, markings and separate dimensions
Identify the BB and pedal axes, keep a stamped length separate from a ruler observation, and avoid confusing arm length with spindle length, Q-factor or pedal-circle diameter.
Published 9 October 2026 · OpenBikeFit
Crank field guide · not to scale
Locate axes, not arm ends.
Fictional radius 170 mm → diameter 340 mm.
BB axis to pedal axis defines L; the visible outline is different.
Fixed-BB planar construction, not an ideal crank length or physical measurement.
On this page
- 01Locate two axes rather than two edges
- 02Read the marking and preserve its identity
- 03A device setting is a separate record
- 04Position the ruler in the declared plane
- 05Radius, diameter and circumference answer different questions
- 06A shorter arm changes both extremes
- 07Do not substitute spindle length or Q-factor
- 08Carry a baseline rather than a purchase verdict
Understand the model first
Inspect the existing crank comparison
The free tool retains its access, hardware and measured re-check boundaries. Reading creates no bike record or installed claim.
Open toolA crank marked 172.5 mm does not have to look 172.5 mm long from one visible end to the other. Its working length joins two axes: the bottom-bracket axis about which the arm turns and the pedal axis carried by that arm. The shape extends around those axes. Before entering a calculator or comparing replacement parts, identify the actual quantity and the evidence for it. A photograph of the silhouette is not the same record as a centre-to-centre observation.
This guide teaches a non-destructive baseline. Keep the bicycle stable, avoid the drivetrain and do not loosen a pedal, crank bolt or bearing assembly to expose a hidden centre. If the reference cannot be identified without mechanical work, retain an unknown observation and use exact model documentation or a mechanic. The outcome here is a named dimension and its source, not a decision to replace a part.
Locate two axes rather than two edges
The bottom bracket, abbreviated BB, contains or supports the rotating crank spindle. In a side-view construction, its axis appears as a centre point. The pedal spindle axis supplies the second point. The shortest centre-to-centre separation in the plane of the crank rotation is the radius used by the simple pedal-circle model. An exposed fastener or cap may help locate an axis, but a decorative outline or offset feature is not automatically that centre.
Write which visible features locate each axis and how the tool reaches them. A pedal body can conceal the spindle centre, while a crank cover can obscure the BB axis. Do not replace an inaccessible centre with the nearest edge and still call the result crank length. Edge offsets can be used only when their exact geometry is known and declared; adding a guessed correction simply hides the uncertainty.
Read the marking and preserve its identity
Garmin’s Vector manual notes that crank length is often printed on the crank arm. That is useful identification evidence, not a guarantee that every number visible on an arm is its length. Retain manufacturer, model, side and the actual marking. A model code, batch number or spindle inscription answers a different question. If the marking is unclear, consult the exact specification rather than borrowing a number from a similar-looking product.
Keep specification evidence and observed measurement in separate entries. For example: declared length 172.5 mm from the identified model; physical centre-to-centre reading unresolved because the pedal centre is hidden. This is a better record than labelling the specification as a repeated measurement. Two readings of the same stamped number do not independently test axis location or ruler placement. Check each installed arm’s identity if the assembly is uncertain.
A device setting is a separate record
Some pedal-based devices also ask for crank length. Garmin’s Vector manual gives one device-specific entry procedure and refers other devices to their own instructions. A value displayed in a device menu does not physically measure the arm or prove the setting was entered correctly. Read the applicable manual and keep the actual identified arm length separate from any electronic setting.
This guide neither pairs a sensor nor changes a device. If a bicycle uses such equipment, note its exact model and the source of its length setting in the equipment record. Replacing an arm does not demonstrate that another device updated itself. No power measurement or calibration result is claimed here; those checks remain outside the browser’s dimensional comparison.
Position the ruler in the declared plane
Where both centres are safely visible, place a suitable measuring tool in the plane that joins them. Read along the centre line rather than around the curved surface. Reposition the tool and relocate both endpoints before recording a second observation. Note the method, units and the practical resolution. A ruler marked in whole millimetres does not establish tenths merely because the product label uses a decimal.
If a ruler is separated from the crank plane, viewing at an oblique angle can shift the apparent endpoint. A photograph adds scale and perspective questions as well. A small image should therefore remain an illustration unless its measurement method is established. The guide supplies no universal tolerance that turns an approximate reading into verified manufacture. A disagreement with the marking is a reason to resolve method and identity, not to round toward a preferred size.
Radius, diameter and circumference answer different questions
For a fictional rigid crank with length L = 170 mm, a pedal-axis point traces a circle of radius 170 mm around a fixed BB. Its diameter is 2 × 170 = 340 mm. The axis is 170 mm above the BB at the ideal top position and 170 mm below it at the ideal bottom position. Those two extreme positions are 340 mm apart; neither the radius nor the diameter is the whole visible arm length.
The circle’s circumference would be 2 × π × 170, approximately 1068.1 mm. That describes one planar circuit of this axis, not distance travelled by the foot on a moving bicycle or a prescribed stride. No shoe thickness, pedal-body extension or body segment has been included. Keeping radius, diameter and path length separately labelled prevents a factor-of-two error before any comparison is made.
A shorter arm changes both extremes
In a fictional 172.5→165 mm comparison, radius decreases by 7.5 mm and diameter decreases by 15 mm. With the BB fixed, the pedal axis is 7.5 mm lower at the top and 7.5 mm higher at the bottom. This pair of directions matters: a radius difference is not a whole-circle height difference. It does not by itself tell where the saddle, foot or knee will move.
Use the next guide to inspect the conditional saddle-compensation model. Ground clearance adds actual BB height and the lower pedal envelope; use the existing BB drop/height guide rather than reading clearance from crank length alone. A radius tells the axis orbit, not the space beneath every part of the pedal while cornering. These are connected questions with different endpoints, not extra meanings for the same length.
Do not substitute spindle length or Q-factor
Crank spindle length runs across the bicycle through the BB assembly. Crank-arm length runs from the BB axis to the pedal axis. Q-factor concerns the lateral crank pedal-attachment endpoints. A one-sided pedal offset and a shoe-centre span add their own references. Two cranksets can share arm length yet have different lateral dimensions and interface requirements. Conversely, a shared spindle family does not prove equal arm length.
The SRAM DUB chart keeps shell dimensions, spindle identification, chainline and crankset type in separate fields. Use that separation as an identification lesson, not as a compatibility answer for another family. Record the exact left and right components, bottom bracket and chainring assembly before evaluating a replacement. The interface guide explains which questions are still open even after the length is known.
Carry a baseline rather than a purchase verdict
A useful note keeps both axes, installed side, model, marking, each raw observation, ruler method and any inaccessible features together. Park Tool’s positioning chart also records crank length, saddle dimensions and pedal/shoe identity separately. You can preserve that separation on our unfilled worksheet without entering a customer record in the app. Unknown is a valid field state; it should not become zero or an invented standard size.
Opening the free crank tool does not install anything or verify the identity you enter. Its inseam-and-discipline range is a product convention, not a measured ideal length. This guide does not estimate watts, joint angles, comfort or symptoms. After resolving the baseline, compare declared dimensions only within the tool’s stated model; exact current manuals, physical compatibility and any actual riding outcome remain separate evidence.
Practical questions
Frequently asked questions
Where is bicycle crank length measured?
Between the BB rotation axis and pedal spindle axis in the crank plane. The arm silhouette and outer ends do not define that separation.
Is a crank marking a physical measurement?
It is specification evidence for the identified arm. Preserve it separately from raw observed readings and unresolved endpoints.
Does a 170 mm crank have a 170 mm pedal circle?
Its fictional fixed-BB pedal-axis circle has a 170 mm radius and 340 mm diameter. Shoe and pedal-body dimensions are not included.
Does matching crank length prove compatibility?
No. Spindle, BB shell, chainring interface, chainline, frame clearance and exact assembly approval remain separate checks.