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Stack and Reach Calculator — Compare Bike Geometries
Compare stack and reach between two bikes with equivalent stem calculations.
How We Calculate This
Stack and Reach Comparison
This calculator compares the geometry of two bikes and determines the stem length needed on bike B to match your hand position on bike A.
Effective Position
Effective reach and stack combine the frame geometry with stem length, stem angle, and spacer height. The stem contribution is calculated using trigonometry based on the headtube angle. Because the steerer leans back at the head angle, adding spacers raises the bar but also shortens effective reach by spacer height × cos(head angle), while increasing stack by spacer height × sin(head angle).
Frequently Asked Questions
Stack is the vertical distance from the bottom bracket centre to the top of the head tube. Reach is the horizontal distance. Together they define the fundamental frame geometry and riding position.
Frame size labels (54cm, Medium, etc.) are inconsistent between brands. Stack and reach are universal measurements that directly describe the riding position. Two bikes labelled "56cm" can have very different stack and reach.
A longer stem extends your reach. When moving between bikes with different reach values, you can often compensate by changing stem length. This calculator works out the equivalent stem for bike B to match your position on bike A.
Stack determines base handlebar height. You can adjust height with stem angle and spacers, but bikes with very different stack values will always feel different. More than 20mm stack difference is noticeable.
Spacers raise the stem along the angled steerer, so the bar moves up and slightly back toward you. On a bike with a 73 degree headtube angle, 10mm of spacers adds roughly 9.5mm of stack and shortens effective reach by about 3mm. Steeper or slacker head angles change those proportions.
Related Calculators
Frame Size Calculator
Find the right frame size based on your body measurements.
Handlebar Reach Calculator
Determine ideal handlebar reach from torso and arm length.
Stem Length Calculator
Calculate the stem length needed for your ideal reach.
Saddle Height Calculator
Calculate optimal saddle height from inseam measurement.
Crank Length Calculator
Find optimal crank length based on leg length and riding style.
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Last updated: February 2026
All calculations are estimates. Always verify results and consult a professional bike fitter where appropriate.