How tides work

Tides are the periodic rise and fall of sea levels caused primarily by the gravitational pull of the Moon and, to a lesser extent, the Sun on Earth’s oceans. The Moon’s gravity creates a tidal bulge on the side of Earth facing the Moon, and a second bulge on the opposite side due to inertia. As Earth rotates, most coastal locations experience two high tides and two low tides each day (semidiurnal tides), though some locations have one high and one low per day (diurnal tides) due to local geography.

Reading a tide chart

A 24-hour tide height chart plots water level on the y-axis against time on the x-axis. The sinusoidal wave shape shows:

Peaks: high tides - maximum water level.
Troughs: low tides - minimum water level.
Tidal range: the vertical distance between high and low tide. This varies from <0.5 m in the Gulf of Mexico to over 15 m in the Bay of Fundy.
Tidal cycle: the time between consecutive high tides is approximately 12 hours 25 minutes, matching half the Moon’s orbital period.

Spring vs. neap tides

Spring tides: occur at new moon and full moon when the Sun, Moon, and Earth are aligned. The gravitational forces combine, producing the highest high tides and lowest low tides of the month.
Neap tides: occur at first and third quarter moon when the Sun and Moon are at right angles relative to Earth. Their gravitational pulls partially cancel, producing moderate tides with the smallest tidal range.

Important note on tide predictions

Accurate tide predictions require harmonics analysis based on local tidal gauge data and account for dozens of astronomical and oceanographic constituents. The estimates shown here are simplified models. For navigation, fishing, or safety-critical planning, always consult an official source such as NOAA (US), UKHO (UK), or your national hydrographic office.

Tidal range by location

Tidal range (difference between high and low water) varies enormously depending on coastal geography:

Location	Approximate tidal range
Bay of Fundy, Canada	~16 m (highest in the world)
Bristol Channel, UK	~12–14 m
Cook Inlet, Alaska	~9–12 m
Open ocean (typical)	1–3 m
Gulf of Mexico	~0.3–0.6 m (diurnal)
Mediterranean Sea	<1 m (nearly tideless)

Tidal current vs. tidal height

Tide height (what this tool shows) and tidal current (how fast water is moving) are related but distinct:

Tidal height indicates the water level relative to a datum. It is highest at high tide and lowest at low tide.
Tidal current is the horizontal flow of water. Current is generally strongest when tide height is changing fastest - around mid-tide - not at high or low water.
Slack water (minimal current) occurs slightly after high tide and slightly after low tide - not exactly at the peak/trough. The delay varies by location.

For boating, diving, and fishing, both height and current matter. For hazardous inlets and tidal races, always consult current predictions separately from height predictions.