Tide times approximation

How this tide estimate works

This page gives a quick, low-precision tide rhythm for selected coastal cities. It is built for casual planning questions such as “is the tide roughly rising or falling later today?” rather than marine operations. The model uses a simplified semidiurnal lunar tide with a cycle of about 12 hours 25 minutes between alternating highs and lows. For each city, the timing starts from an estimated local lunar transit based on longitude, then applies a rounded phase lag for the local harbour or waterfront.

Real tide prediction is much more detailed. Official predictions combine many harmonic constituents, station datums, shallow-water effects, weather, river flow, pressure, wind setup, bathymetry, and local harbour geometry. This tool intentionally does not attempt that level of precision. The timing should be treated as approximate to about ±60 minutes, and the displayed height is only a rough range-based shape around mean sea level.

The 20 city reference ranges are rounded from public tide tables and station datum material, including NOAA CO-OPS for United States stations and UK Hydrographic Office Admiralty tide table material where applicable. For boating, anchoring, coastal walks, swimming, flood risk, port work, or any safety-critical use, check the official national hydrographic service, harbour authority, or local tide gauge before acting.

What a one-constituent model leaves out

Knowing exactly what is missing tells you when these numbers will look wrong. First, spring and neap cycles: real tidal range swells when the sun and moon pull together near a new or full moon and shrinks near the quarter moons, while this tool shows the same rounded range every day of the month. Around spring tides it will understate the highs and overstate the lows. Second, weather: sustained onshore wind and low pressure raise actual water levels, and storm surge can overwhelm the astronomical tide entirely. Third, local shape: river flow, harbour geometry, and shallow-water distortion bend the clean cosine into the lopsided curves many real stations record, where the rise and fall take unequal amounts of time.

Range extremes across the 20 cities

The reference range attached to each city explains most of what the chart shows. A selection from the list, using the rounded values the tool works with:

The spread is no accident. Funnel-shaped estuaries such as the Thames amplify the incoming tide, which is why London's figure dwarfs the open-ocean values of Miami and Rio de Janeiro. Stockholm sits on the Baltic, a nearly enclosed sea connected to the ocean through narrow straits, so little tidal energy gets in and the water moves centimetres rather than metres.

Choosing between this page and an official table

Reach for this page when the question is casual and low-stakes: whether the water will be roughly in or out during an afternoon walk, or which half of the day looks better for beach photos. Switch to official predictions the moment timing or depth carries any consequence. Anything involving a boat, a swim in moving water, a walk to a tidal island, or work in a port belongs with the national hydrographic service or harbour authority. The ±60 minute figure quoted above is a working expectation for a model this simple; in complex estuaries the error can be larger.

Tide questions

Need daylight as well as water level? Pair this with the sunrise and sunset table.