Horizontal distance from the saddle BRP (Biomechanical Reference Point) to the bottom bracket centre. Measured along the seat tube line plus any saddle setback. Use this to set your saddle position relative to the BB (e.g. KOPS or behind-BB targets).
Fit Targets · Measure to:
Horizontal distance from the saddle BRP (Biomechanical Reference Point) to the bottom bracket centre. Measured along the seat tube line plus any saddle setback. Use this to set your saddle position relative to the BB (e.g. KOPS or behind-BB targets).
Fit Results
Geometry Diagram
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Bike FitCockpit Calculator
This tool helps you translate professional bike fit measurements into the specific components you need for a given frame, or check what fit numbers a particular setup produces. Frame geometry for many popular bikes is available via the built-in presets. For bikes not included, you can find geometry data on the geometrygeeks.bike database and enter the values manually.
How it works
All calculations use the geometric relationship between the bottom bracket (BB), saddle, and hood positions. The frame's stack, reach, seat tube angle, and head tube angle define the skeleton. Components (stem, spacers, handlebars) extend from the top of the head tube to the hood position along the steerer and stem axes using trigonometry.
Measure to: Hoods / Handlebars
Use the toggle next to the Components and Fit Targets headings to switch between measuring to the Hoods (brake hood position, accounting for handlebar reach) or the Handlebars (top of the bar at the stem clamp, ignoring handlebar reach). All fit results and the diagram update accordingly.
Mode 1: Components to Fit
Enter your frame geometry and the components you have (or are considering). The calculator shows the resulting fit numbers:
Saddle-to-Reach — horizontal distance from saddle to the measurement point
Saddle-to-Drop — how far the measurement point sits below the saddle (positive = lower)
Mode 2: Fit Targets to Components
Enter the fit targets from your bike fitter along with your frame geometry. The system of equations has three unknowns (stem length, stem angle, spacer stack) but only two constraints (reach and drop), so you fix one variable and solve for the other two:
Fix Stem Angle — choose your preferred stem angle, get the required stem length and spacer stack
Fix Spacer Stack — set your spacer height, get the required stem length and angle
Fix Stem Length — pick a stem length, get the required spacers and angle
Frame presets
Click the Presets button to open a searchable overlay with multiple bike brands and models. Each model lists its available sizes — select one to instantly load the frame geometry. Dimmed sizes are bikes we haven’t added geometry data for yet. You can always enter or adjust the geometry fields manually for any frame.
Compare Frames
Click Compare Frames to select two frames from the database and see their geometry overlaid on the same diagram with a side-by-side comparison table.
Key inputs
Stack & Reach — from the frame geometry chart (mm)
Seat Tube Angle (STA) — angle from horizontal (typically 72–76°)
Head Tube Angle (HTA) — angle from horizontal (typically 71–74°)
Stem Angle — relative to the steerer perpendicular (0° = perpendicular, negative = angled down)
Saddle Height — measured along the seat tube from BB centre to the saddle Biomechanical Reference Point (BRP). The BRP is the designed sit-bone contact point, usually marked by the manufacturer or at the deepest point of the saddle profile (mm)
Saddle Setback — horizontal offset of the saddle BRP behind the seatpost clamp (mm)
Handlebar Reach — horizontal distance from bar clamp to hoods (mm, from bar spec sheet)
Tips
Yellow warnings appear when a solved value is outside typical commercially available ranges
If the reverse solver shows negative spacers, the frame likely has too much stack for your fit targets
The diagram updates live as you adjust inputs
Use the Presets button to quickly load frame geometry for popular bikes and compare sizes
Select Frame Preset
To suggest more bikes or if any data is incorrect email jakestuchbury (at) gmail.com