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Tidal Range Calculator

Calculate tidal range, high and low tide times, and current strength estimates. Plan sailing and anchoring around tidal conditions.

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Inputs that matter
High Tide Height, Low Tide Height, Hours After Low Tide, Tidal Period
Output to expect
Tidal Range Calculator
  • Check the units and required inputs before comparing results.
  • Keep the assumptions with a copied result so you can reproduce the calculation later.
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Reference & details

How it works

Tidal Range

Tidal range is the vertical difference between high and low tide. Spring tides (full/new moon) produce maximum range; neap tides (quarter moon) produce minimum.

Range = MHW − MLW (mean high − mean low water)

Tide Height at Time

Tidal height follows sinusoidal harmonic components (M2, S2, K1, O1). Simplified: interpolate between known high and low times using rule of twelfths for planning.

Rule of twelfths: 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 per hour of 6-hour tide phase

Tidal Current Estimation

Maximum current occurs mid-tide. Peak speed correlates with range and channel geometry. Spring tides produce roughly 1.5× neap current in many locations.

Current max ∝ tidal range × channel constriction factor

Updated: July 2026

Example Scenarios

Low tide 2 m depth at datum; high tide adds 3 m = 5 m depth. At 7:1 scope, rode need varies 21 m (low) vs 35 m (high) — set anchor for low tide depth.

Bridge 12 m clearance at MHW. Vessel air draft 10 m with 1.5 m spring tide above MHW: 12 − 1.5 = 10.5 m — passes with 0.5 m margin. Check tide table for exact hour.

Channel max flood 2 knots at 2 hours after low tide. Depart 1 hour before for slack-to-flood transition, riding favorable current through narrow pass.

Common Mistakes to Avoid

Anchoring for high tide depth only

At low tide, insufficient depth grounds the boat or steepens rode angle until anchor breaks free. Always plan scope and depth at lowest astronomical tide.

Using tide time from a distant station without correction

Secondary ports may differ 30+ minutes and different range from reference station. Apply local corrections from tidal almanac or chart.

FAQ

Approximates tide height change per hour: in a 6-hour rise, water rises 1/12 of range in hour 1, 2/12 in hour 2, 3/12 in hours 3–4, then decreasing. Useful without full harmonic data.

Spring tides occur at new and full moon when sun and moon align — largest range. Neap tides at quarter moon when forces partially cancel — smallest range.

Use official tide tables (NOAA in US, UKHO in Britain) for reference stations. Apply time and height corrections from almanac for secondary ports.

Yes. Calculate scope at lowest expected tide depth — the shallowest point determines maximum rode angle and holding. High tide adds depth margin only.

Typically midway between high and low tide at most locations. Slack water occurs near high and low tide when current reverses direction.

About Tidal Range Calculator

Tides affect anchoring depth, bridge clearance, and current strength. This calculator estimates tidal range from harmonic constituents or reference station data, and predicts high/low times for voyage planning.