Interchange Ramp Length Calculator

• Grade run length: L_run = Δh / (G/100), where Δh is elevation change and G is ramp grade (%).
• Vertical curve length (K method): L_vc = K × A, where A = |g2 − g1| is algebraic grade difference (%).
• Total (combined): L_total = L_run + L_vc + L_landing + L_extra.

K values should match your adopted roadway criteria. This tool uses your entered K directly without assuming a design-speed table.

1. Select a method suited to your current design step.
2. Choose units and rounding for the needed precision.
3. Enter elevation change, or provide start and end elevations.
4. Enter ramp grade, and optionally g1/g2 with K for curves.
5. Add landing and extra allowances to reflect site needs.
6. Press Calculate to view results above, then export.

For planning, start conservative and refine with detailed geometry.

Ramp length drivers in interchange geometry

Ramp length is primarily governed by elevation change, allowable grade, and the need for smooth grade transitions. For a 6% grade, every 1.0 m of rise requires about 16.67 m of run, while the same rise at 4% needs 25.00 m. This calculator makes the sensitivity visible so teams can quickly compare alternatives and avoid late redesigns. Shorter ramps can raise speeds and sight-distance demands at terminals.

Using grades for early feasibility checks

During corridor screening, a grade-only run gives a fast, defensible minimum length. Enter start and end elevations or a direct elevation change, then set a target grade. The output also reports a slope ratio (1:N), which helps communicate steepness to non-design stakeholders, estimators, and permitting reviewers. If grades exceed policy, consider split-level connections or additional elevation breaks within terrain limits.

Applying the K method for vertical curves

Vertical curves are commonly sized with the K method, where curve length equals K times the algebraic grade difference A. If g1 is −1% and g2 is +2%, then A is 3%. With K = 20, the curve length is 60 in your selected units. This approach aligns with typical design criteria and keeps computations transparent.

Combining run and curve lengths for planning quantities

In combined mode, total ramp length is the sum of constant-grade run, vertical curve length, and any added landing or allowance. Optional refinement can account for estimated elevation change occurring within the vertical curve, reducing the remaining elevation assigned to the constant-grade run. This can improve conceptual estimates without requiring full profile design. Use allowances to reflect superelevation runoff, gore constraints, and survey tolerances.

Export-ready results for coordination and documentation

Because ramp geometry affects earthwork, drainage, structures, and right-of-way, the calculator provides exportable results. The CSV supports quick checking in spreadsheets and cost models, while the PDF snapshot helps attach calculations to meeting minutes or submittals. Keep inputs consistent with your adopted criteria and document any project-specific constraints.

What inputs are required to get a run length?

Provide elevation change and a nonzero ramp grade. You can enter elevation change directly or derive it from start and end elevations. The calculator converts grade percent into a run length automatically.

When should I use the vertical curve option?

Use it when you only need a curve length estimate based on approach and departure grades. It is useful for checking whether a profile transition will fit between controlling points on the ramp.

How do I choose a K value?

Use the K value from your adopted roadway design criteria for the selected facility type and design speed. K is entered as length per percent grade change, matching your chosen units.

Why does combined mode show a note about curve elevation change?

A vertical curve can contribute some elevation change. When enabled, the calculator estimates that contribution and reduces the remaining elevation assigned to the constant-grade run, improving conceptual totals.

Do landing and extra allowance affect the slope ratio?

No. The slope ratio is derived from ramp grade only. Landing and extra allowance are added as flat length to reflect layout needs, transitions, and field adjustments.

Can I share results with my team?

Yes. After calculating, download a CSV for spreadsheets or a PDF snapshot for review packages. Exports help document assumptions, support quantity takeoffs, and keep coordination consistent across disciplines.

Example totals exclude landing and extra allowances for simplicity.