Marine & Navigation

CPA/TCPA Risk Calculator

Compute closest point of approach and time to CPA with risk banding.

Formula reviewed: 2026-02-14 Marine & Navigation

CPA/TCPA Risk Calculator estimates closest point of approach and time to CPA using relative motion vectors from ownship and target kinematics. It is useful for watchstanding triage of collision risk and prioritizing contacts that need early action. The tool also maps computed values to a simple risk band for quick operational interpretation. Use it as support data, not as a substitute for continuous lookout, radar/AIS cross-checking, and proper maneuvering practice.

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Input Pattern

Enter values in the left panel, keep units explicit, run the calculation, then copy or share the result. Invalid fields are highlighted immediately.

How to use this tool

Enter own course/speed and target bearing/range/course/speed.
Run computation to obtain CPA distance and TCPA minutes.
Use risk banding to prioritize contacts for closer monitoring and maneuver planning.

CPA/TCPA Inputs

Result

CPA distance: 1.604 nm

TCPA: 24.24 min

Risk level: LOW

Formula or method

CPA and TCPA use relative position and relative velocity to estimate closest approach under constant course and speed.
Risk banding is a simplified triage layer based on CPA distance and time thresholds.

Worked example

Prioritizing a crossing target

Ownship course/speed: 090 deg at 12 kn
Target bearing/range: 045 deg at 3 nm
Target course/speed: 180 deg at 10 kn

Result: The calculator estimates CPA distance, TCPA minutes, and a simple risk band from relative motion.

Use the output to prioritize monitoring and early assessment, then verify with radar/AIS, visual lookout, COLREGs, and bridge-team procedures.

How to interpret the result

CPA/TCPA output is a collision-risk planning signal, not a command decision or substitute for watchkeeping.

CPA is the estimated closest distance if both vessels maintain course and speed.
TCPA is the estimated time until that closest approach.
A low CPA with short TCPA deserves early attention, but sensor quality and vessel intentions still matter.
Risk bands are triage aids, not maneuvering instructions.

Common mistakes

Assuming target course and speed will remain constant.
Ignoring bearing drift, visual aspect, traffic density, or restricted water.
Treating AIS data as perfect and current.
Using CPA/TCPA output without applying COLREGs and good seamanship.

Review note and limitations

Method - constant-vector relative-motion CPA/TCPA estimate.

Assumes constant course and speed and does not model maneuvers, sensor error, sea state, traffic density, or COLREGs obligations.
AIS, radar, GPS, heading, and speed data can disagree or lag.

Training and planning support only. Do not use this page as a substitute for lookout, radar/AIS cross-checking, COLREGs, local rules, or professional navigation judgment.

FAQ

Does CPA/TCPA tell me what maneuver to make?

No. It estimates relative-motion risk. Maneuver decisions require COLREGs, lookout, radar/AIS cross-checking, local context, and bridge judgment.

Why can CPA/TCPA change quickly?

Small changes in course, speed, sensor data, or target maneuvering can materially change relative-motion predictions.

Is AIS enough for collision risk assessment?

No. AIS can be delayed, missing, inaccurate, or absent. Cross-check with radar, visual bearings, and navigation practice.

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