Stack and Reach Calculator
Compare bike geometries using stack and reach measurements. The most accurate way to compare bikes across different brands and sizes.
Results
Visualization
How It Works
The Stack and Reach Calculator determines the vertical (stack) and horizontal (reach) distances from your bike's bottom bracket to the top of the headtube, which defines your riding position and bike fit. These measurements are the most reliable way to compare how different bike models and sizes will position your body, regardless of brand or frame geometry differences.
The Formula
Variables
- Head Tube Length (HTL) — The actual length of the head tube measured along its axis in millimeters. This is the vertical tube that connects the top and bottom of the head tube where the fork steerer enters.
- Head Tube Angle (HTA) — The angle in degrees between the head tube and horizontal ground. Steeper angles (74°+) are more aggressive and responsive; slacker angles (67-72°) provide stability and comfort.
- Fork Rake/Offset — The perpendicular distance in millimeters between the fork's steerer tube axis and the center of the wheel axle. Typical values range from 40-50mm; greater rake increases stack and reach.
- Stem Length — The length of the stem in millimeters, measured from the center of the steerer tube clamp to the center of the handlebar clamp. Common lengths are 70-130mm.
- Headset Spacers — The total thickness of spacers above the stem on the steerer tube, measured in millimeters. These directly add to stack height and help fine-tune fit without changing components.
Worked Example
Let's say you're comparing two gravel bikes: a size 56cm model with a 110mm head tube, 72° head tube angle, 45mm fork rake, 100mm stem, and 20mm spacers. Stack = 110 + (45 × sin(72°)) + (100 × sin(72°)) + 20 = 110 + 42.8 + 95.1 + 20 = 267.9mm. Reach = (45 × cos(72°)) + (100 × cos(72°)) = 13.9 + 30.9 = 44.8mm. Now compare this to a competing brand's 56cm with 105mm head tube, 71° angle, 48mm rake, 100mm stem, and 10mm spacers: Stack = 105 + (48 × sin(71°)) + (100 × sin(71°)) + 10 = 105 + 45.4 + 94.5 + 10 = 254.9mm. The first bike sits 13mm higher despite similar components, suggesting a more upright riding position. This numerical comparison is far more useful than just knowing both are 'size 56cm.'
Practical Tips
- Use stack and reach to compare bikes across brands with confidence—a Trek 56cm and a Specialized 56cm can have dramatically different geometries, but stack/reach numbers tell the true story of fit differences.
- Remember that stack changes are permanent based on frame geometry, but reach can be adjusted slightly with different stem lengths (shorter stem = less reach); stack adjustment is limited to spacers only.
- When shopping online or through brand websites, look for geometry charts that list both stack and reach alongside traditional measurements like top tube length and seat tube angle—this gives you the complete picture.
- If you're between sizes, calculate stack and reach for both options: you might find a size up actually gives you less stack than a size down, which contradicts what frame size alone would suggest.
- Document your current bike's stack and reach to use as a reference point: if you loved how your previous bike fit, replicate those numbers rather than guessing based on seat tube length or other geometry numbers.
Frequently Asked Questions
What's the difference between stack and reach, and why do I need both measurements?
Stack is the vertical height from the bottom bracket to the top of the headtube, while reach is the horizontal distance. Together, they define the 3D position of your body on the bike. Stack determines how upright or aggressive your position is (higher = more upright), and reach determines how stretched out you'll be. You need both because a bike could have high stack with short reach (very upright and compact) or high stack with long reach (upright but stretched out).
Why is stack and reach better than using frame size or top tube length to compare bikes?
Frame sizes (S, M, L, XL) and top tube lengths vary inconsistently between manufacturers—a large from one brand might fit like a medium from another. Stack and reach measure the actual position your body will occupy relative to the pedals, making them objective and brand-independent. Two frames with the same top tube length can have completely different stack and reach due to head tube angle, fork rake, and geometry differences.
Can I adjust my stack and reach after I buy a bike?
Stack can be adjusted slightly by adding or removing headset spacers (typically ±10-20mm range), but reach cannot be meaningfully adjusted without changing the stem length, which only partially compensates and affects handling. Most reach adjustment requires buying a different stem, while stack adjustment is free and reversible. This is why getting close to your ideal measurements before purchase is important.
What stack and reach numbers should I aim for based on my height and flexibility?
There's no single 'correct' number—it depends on your inseam, flexibility, and riding style. Generally, cyclists 5'6"-5'10" often fit well in the 210-240mm stack range for road bikes, while gravel and endurance bikes might be 240-270mm. Shorter riders may prefer 200-220mm stack. Flexible, aggressive riders can handle lower stack numbers; taller or less flexible riders often prefer higher stack for comfort.
If I know my ideal stack and reach, how do I use this calculator to find the right bike?
Use the calculator in reverse: input the geometry specifications from a bike brand's spec sheet (head tube length, angle, fork rake, and your intended stem length and spacers) to calculate that bike's stack and reach. Compare the results to your ideal numbers. If the numbers are close, that bike will likely fit you well; if they're significantly different, consider a different size or frame.
Sources
- BikeCalcs: Bike Geometry and Fit Calculators
- UCAR (Bicycle Manufacturers Association): Bike Geometry Standards
- Competitive Cyclist: Understanding Stack and Reach
- Specialized Bikes: Geometry and Fit Guides
- Trek Bikes: Frame Geometry Specifications