Tide Window Planner estimates the time window around high water when tide height may provide enough depth for a draft-limited transit. Use it for marina entries, bar crossings, shallow-channel passages, drying approaches, or harbour arrivals where charted depth, vessel draft, and safety margin determine whether there is enough water. The tool uses high tide, low tide, charted depth, vessel draft, and safety margin to estimate required tide height, clearance at low/high water, total window duration, and hours each side of high tide. It is a planning approximation only; actual tide tables, local corrections, weather setup, surge, river flow, and harbour authority guidance matter.
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 high tide and low tide heights for the relevant tide cycle, using the same chart datum as the charted depth.
Enter charted depth, vessel draft, and the safety margin you want left after draft is considered.
Click "Calculate window" and review required depth, required tide above datum, low/high tide clearance, window availability, duration, and hours each side of high tide.
Compare the result with the official tide station, local corrections, time zone, weather, surge, swell, and harbour guidance before planning arrival.
Pair the result with the under-keel clearance calculator when squat, wave allowance, or separate UKC allowances need to be explicit.
Tide Window Inputs
Estimate transit window around high tide using a semidiurnal sinusoid approximation.
The window is approximated using a smooth 12.42-hour semidiurnal tide curve, so local asymmetry is not modeled.
Worked example
Planning a shallow-channel marina entry
Charted depth: 1.8 m
Vessel draft: 1.5 m
Safety margin: 0.3 m
High tide: 2.1 m
Low tide: 0.2 m
Result: The planner estimates how much of the tide cycle provides enough water and reports the approximate window around high tide.
The calculation helps decide whether arrival should be planned near high water, but the final decision still depends on official tide predictions, observed conditions, and local access guidance.
How to interpret the result
A longer calculated tide window gives more planning flexibility, but it does not prove the transit is safe.
A longer window gives more timing flexibility, but it does not remove local hazards.
A short or marginal window should trigger a more conservative transit plan or a delay.
Tide height should use the correct station, time zone, and chart datum for the transit area.
Surge, wind setup, river flow, waves, and bar conditions can make the real window narrower than the estimate.
Common mistakes
Treating a smooth tide approximation as a local tide table.
Forgetting chart datum, time zone, or daylight-saving adjustments.
Ignoring wave troughs, squat, siltation, or local depth restrictions.
Planning to arrive at the edge of the window without operational margin.
Tide-window approximation from sinusoid phase fraction over a 12.42-hour cycle.
Assumptions
Uses a smooth semidiurnal tide approximation and does not model local asymmetry.
Does not include wave setup, surge, or pilot authority constraints.
Review note and limitations
Method - semidiurnal sinusoid tide-window approximation using required tide height.
Does not read live tide tables, apply secondary-port corrections, model asymmetric tide curves, or account for surge, bar conditions, river flow, silting, traffic, or harbour restrictions.
Does not replace official tide tables, notices to mariners, local pilotage guidance, depth sounder checks, or under-keel-clearance policy.
Navigation planning support only. Do not use this page as a substitute for official charts, tide/current sources, local rules, bridge procedures, or qualified navigation judgment.
FAQ
Is this tide window exact?
No. It is a planning approximation. Use official tide predictions, local observations, weather, and pilot guidance for actual transit decisions.
Why does chart datum matter?
Charted depths and tide heights must refer to the same datum. Mixing datums can overstate or understate available water.
How much clearance do I need?
There is no universal answer. Vessel type, route, sea state, squat, bottom type, regulations, company policy, and local harbour guidance all affect the margin.
What should I check before a shallow transit?
Check tide station relevance, weather, wave conditions, chart updates, local notices, draft, squat, safety margin, and any port or pilot requirements.
Related tools and workflows
Tide windows connect directly to under-keel clearance, drift/current planning, fuel reserve, anchoring, and route timing in the marine navigation workflow.