Check a proposed dropper post using actual rail height, insertion limits, body length, travel and stack. Missing dimensions and physical checks stay unknown.
Published and source-reviewed November 4, 2026. A measurement worksheet, not an inseam-based recommendation or installation approval.
Use the exact diameter, travel and generation you intend to buy. Our dropper post buying guide explains the measurements to check. You can also browse the dropper post catalog. Catalog inclusion does not mean a post fits your frame; confirm each interface and use the manufacturer drawing.
Exposed body is B − I, so extended rail height is H = B − I + T + S. Required insertion is therefore I = B + T + S − H. Its lower limit is the greater of frame and post minimum insertion; its upper limit is the smaller of usable frame insertion and B. Equality passes the arithmetic check, but adds no measurement margin.
Holding B and S fixed, the geometric travel interval is from max(0, H − B − S + minimum allowed insertion) to H − B − S + maximum allowed insertion. An upper bound below the lower bound means no feasible interval. A different SKU or travel-reduction setup may change B or S. The interval does not tell you which post to buy or whether a particular reduction is supported.
Matching axial geometry cannot establish tire clearance, structural condition, frame approval, clamp torque or installation quality. Saddle shape, rail position, bags, suspension motion and other hardware can change real clearances. Follow the manufacturers’ requirements and consult a qualified mechanic when a dimension or installation step is uncertain. No shim compatibility is inferred.
These manufacturer tools are useful alternatives when considering their posts or frames. Their allowances and product dimensions belong to those systems; this checker does not turn them into cross-brand constants.
Uses seat-clamp-to-rail ride height, diameter and insertion depth, and describes a cable allowance for its calculator. No OneUp allowance is applied automatically here.
Calls for exact model/year diameter checks, minimum insertion and clearance with both suspension and post compressed. Its stated insertion and clearance requirements are not universal.
Explains collar-to-rail height, insertion and diameter measurement for Resolve posts, with configuration-specific travel-reduction options. Use its own specifications for a Resolve candidate.
Enter what you know, then check feasibility. Missing dimensions stay unknown. Results clear whenever an input changes.
Geometry: H = B − I + T + S, so I = B + T + S − H. Both frame and post minimum insertion limits apply. Saddle height is measured to rail center, never to the saddle top. No universal cable or tire allowance is applied.