CPA/TCPA Risk Calculator estimates closest point of approach and time to closest point of approach from ownship course/speed plus target bearing, range, course, and speed. Use it as a CPA calculator, TCPA calculator, or closest-point relative-motion check when a bridge team, instructor, student, sailor, or watchkeeper wants to understand whether a contact deserves closer monitoring. The tool uses constant-vector relative motion and maps the result to a simple high/medium/low risk band for triage. It does not replace COLREGs, lookout, radar plotting, AIS/radar cross-checking, visual bearings, or professional watchkeeping.
Use this calculator as one part of a broader passage-planning workflow. The
Marine Navigation Calculators toolkit
explains how CPA/TCPA, UKC, tide windows, route distance, set and drift, fuel range, and manoeuvring checks fit together.
You can also browse the full
Marine & Navigation category.
Enter the vessel, route, tide, current, or clearance assumptions exactly as labelled, then verify the result against official data and onboard judgment.
How to use this tool
Enter ownship course and speed in degrees true and knots.
Enter target bearing and range from ownship, then target course and speed.
Click "Compute CPA/TCPA" to obtain CPA distance, TCPA minutes, and a simplified risk level.
Treat high concern or monitor results as prompts for earlier assessment, not as manoeuvring instructions.
Recheck the calculation whenever either vessel changes course or speed, sensor data updates, or visual/radar bearings change.
CPA/TCPA Inputs
Result
CPA distance: 1.604 nm
TCPA: 24.24 min
Risk level: LOW
CPA/TCPA relative-motion diagram
The sketch shows ownship and target motion as vectors. CPA is the closest predicted separation if both vessels maintain the entered course and speed; TCPA is the time to that point.
Formula or method
Relative position vector is built from target bearing and range, resolved into east/north components.
Relative velocity vector is target velocity minus ownship velocity, using course and speed for each vessel.
Signed TCPA in the relative-motion formula is positive when closest approach is still in the future and negative when it occurred in the past.
The current implementation clamps past closest-approach cases to 0 minutes for displayed triage, so rerun with current observations rather than relying on stale data.
CPA distance is the predicted closest separation if both vessels maintain the entered course and speed.
Worked example
Prioritizing a crossing target in open water
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 is a relative-motion signal; it shows what happens if the entered motion remains unchanged.
The closest point of approach is 1.6 nm with TCPA of 24.2 min.
The risk level is LOW, based on the entered relative-motion assumptions.
Maneuver decisions still require COLREGs, lookout, radar/AIS cross-checking, and bridge judgment.
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.
Focusing on one target while missing other vessels, land, depth, traffic schemes, or environmental constraints.
Assumes constant course and speed and does not model manoeuvres, sensor error, sea state, traffic density, restricted visibility, human intent, or COLREGs obligations.
AIS, radar, GPS, heading, speed, and visual bearing data can disagree, lag, or be absent.
Does not provide collision-avoidance orders or determine stand-on/give-way duties.
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.
What does negative TCPA mean?
In the signed formula, negative TCPA means the closest point of approach has already occurred. This page displays a triage value clamped at zero for past-CPA cases, so refresh the inputs from current observations.
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.
Related tools and workflows
Encounter risk belongs with drift, route distance, turning-circle, COLREGs, ETA, and fuel-reserve tools in a broader marine navigation workflow.