Tide Height and Safe Passage Window Calculator

Estimate tide height between three consecutive high and low waters, calculate the tide height required for an entered clearance, and find when enough water remains available for the complete passage duration.

An estimated window is not a safe-passage decisionUse official predictions for the correct station, datum, units, and time zone, then check observed water level, weather, currents, route, vessel response, local requirements, and a complete passage plan. This simplified interpolation can differ materially from the real tide.

Tide events and required height

Three consecutive tide events

Copy times and heights from one official source. Times are treated as station-local clock times exactly as entered; no time-zone conversion is applied.

Required tide height

Depth relative to the same datum as tide height.

Use the controlling requirement; this tool does not choose it.

User-assessed squat, motion, depth/tide uncertainty, and other effects.

Timing

Include the complete transit and any timing margin you choose.

Must fall between the first and last event.

Inputs and results stay in your browser and are not added to analytics events.

Estimated passage windows

Enter three consecutive tide events and the required height, then calculate.

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Formulas used

Required tide = loaded draft + minimum UKC + combined allowance − charted depth

For elapsed fraction r between two consecutive extrema with heights h₀ and h₁:

Estimated height = h₀ + (h₁ − h₀) × [1 − cos(πr)] ÷ 2

Threshold crossings are solved from that same curve. A feasible departure interval ends one full passage duration before the above-threshold tide window ends.

Why this is only an interpolation

Official harmonic tide predictions sum many astronomical constituents, each with its own amplitude and phase. A single smooth curve between highs and lows cannot reproduce shallow-water constituents, double highs or lows, weather effects, or every local tidal shape.

Whenever official interval predictions are available, use them directly. This tool is for transparent screening from entered high/low values, not for replacing a prediction service.

Datum, station, and time zone

  • Tide height and charted depth must refer to the same vertical datum.
  • Use three consecutive events for the same station, not values from nearby ports unless the official source supplies the applicable corrections.
  • Keep the official time zone and daylight-saving convention consistent. The calculator treats the displayed clock times as entered and does not convert zones.
  • Negative tide height is valid when the predicted level lies below the selected datum.

What to include in the allowance

The combined allowance is user supplied and conservatively increases the water-level threshold. Depending on the vessel and location, an assessed allowance may address squat, heel, roll, pitch, heave, waves, density and loading effects, chart or survey uncertainty, prediction error, and observed-versus-predicted water-level differences.

The calculator does not determine any of those values or recommend a minimum UKC.

How to use the calculator

  1. From the relevant official source, copy three consecutive alternating high/low event times and heights.
  2. Choose whether to derive the required height from charted depth and vessel values or enter a known threshold directly.
  3. Enter the complete uninterrupted passage duration, including any margin required by applicable procedures.
  4. Choose a time to check, then calculate and review the estimated height, crossing times, full tide window, and feasible departure interval.
  5. Re-check official predictions and actual conditions before the passage; recalculate whenever inputs or timing change.

Example passage-window calculation

A channel has 5.0 ft of charted depth. A boat draws 8.0 ft, the selected minimum UKC is 2.0 ft, and the combined allowance is 0.5 ft:

Required tide = 8.0 + 2.0 + 0.5 − 5.0 = 5.5 ft above chart datum

With low water at 06:00 (1.2 ft), high water at 12:15 (9.4 ft), and low water at 18:30 (1.6 ft), the calculator estimates where the smooth curve crosses 5.5 ft. A 45-minute passage must start at least 45 minutes before the falling crossing. The example values are illustrative, not suitable defaults for any real vessel or route.

Frequently asked questions

How is tide height between high and low water estimated?

The tool fits a smooth half-cosine curve to each pair of consecutive events. It exactly matches the entered event times and heights, but it does not reproduce a station's full harmonic prediction.

How is the required tide height calculated?

Required tide height equals loaded draft plus the selected minimum UKC plus the combined allowance, minus limiting charted depth. Every vertical value must use the same datum.

Why are three tide events required?

Three alternating extremes cover two adjacent segments. A low-high-low pattern usually provides the complete window around high water. A high-low-high pattern may show two partial windows at the edges of the entered period.

Does passage duration include a buffer?

Only if you include one. Enter the entire uninterrupted time required to clear the constrained part of the route, plus any timing margin specified by the operator, harbour, pilot, or applicable procedure.

Can the actual water level differ from the prediction?

Yes. Atmospheric pressure, wind, river flow, waves, and other non-tidal effects can change observed water level. Check current observations and local forecasts where available.

What if the required tide height is below zero?

A negative threshold can be valid when charted depth already provides the entered draft, clearance, and allowance at datum. The same-datum requirement still applies.

Does a displayed window mean the passage is safe?

No. It only identifies when this simplified curve meets the entered arithmetic threshold. It cannot validate the route, source data, currents, weather, vessel response, delays, obstructions, or regulatory requirements.

Are my tide and vessel details uploaded?

No. The calculation runs locally, and entered values are not sent, stored, or attached to analytics events.

Limits and safety disclaimer

  • The tool does not retrieve or verify tide predictions, observed water level, charts, surveys, notices to mariners, weather, currents, route data, or vessel particulars.
  • The curve assumes a smooth monotonic change between each entered high and low. It is unsuitable for double highs or lows, strong tidal asymmetry, river-dominated levels, Great Lakes weather-driven levels, or other irregular patterns.
  • It does not predict squat, wave response, heel, motion, loading, density effects, chart uncertainty, silting, or forecast error.
  • Windows are bounded by the first and last entered events. A window touching either boundary may continue outside the displayed period.
  • Rounding crossing times to the nearest minute can overstate or understate the estimated interval by about a minute; use conservative operational margins.

Safety disclaimer: This educational screening estimate is not an official tide prediction and is not navigation, pilotage, passage-planning, hydrographic, safety, or engineering advice. Never use it as the sole basis for operating a vessel. Follow current official information, vessel and operator procedures, local requirements, and qualified professional guidance.

Methodology and sources

Last reviewed: August 1, 2026. The simplified method and its limits were checked against official marine and tide-prediction guidance:

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