Tide Chart

High and low water for US coastal stations, predicted from NOAA harmonic constants.

Find a station

San Francisco

CA · 37.806°, -122.466° · NOAA station 9414290

Date

A tide is a wave thousands of kilometres long, raised by the gravity of the moon and the sun and shaped by the coastline it runs into. Because the forces driving it are astronomical, and astronomy is predictable, tides can be forecast years ahead — which is exactly what this tool does. Pick one of the 1283 US coastal stations below, choose a date, and it computes high and low water from that station's own measured constants.

How the prediction works

The tide at a given place is treated as a sum of simple waves called harmonic constituents. Each one corresponds to a specific astronomical rhythm. The largest, M2, is the principal lunar semidiurnal constituent: it completes a cycle every 12 hours 25 minutes, which is half a lunar day, and it is why most coasts see two high tides a little under 12½ hours apart. S2 is its solar counterpart at exactly 12 hours. K1 and O1 are the two big diurnal constituents, cycling roughly once a day.

Every station has its own amplitude and phase lag for each constituent, measured by NOAA from years of recorded water levels. Amplitude says how much of the tide's range that rhythm contributes; phase lag says how long after the astronomical forcing the water at that particular place actually responds. A narrow bay can lag the open ocean by hours and amplify the range several times over. This tool carries 37 constituents per station.

Predicting is then a matter of evaluating each constituent's angle at the moment you asked about, scaling it by that station's amplitude, and adding the results together. Two slow corrections are applied on top: the node factor and node angle, which account for the 18.6-year wobble of the moon's orbital plane. Skipping those is a common shortcut and it costs up to about 4% of the range.

Spring tides, neap tides and king tides

When the sun and moon line up — at new and full moon — their two pulls add together and the tide range is large. These are spring tides, and the name has nothing to do with the season. About a week later, at the quarter moons, the two pull at right angles, they partly cancel, and the range is small: neap tides. The cycle repeats roughly every 14¾ days.

The very largest tides of the year happen when a spring tide coincides with the moon at perigee, its closest approach to Earth, and often near an equinox as well. Those are the ones popularly called king tides. The "Biggest tides" section above finds them by scanning the whole year's predicted curve for this station, rather than by assuming they land on the full moon — the lag between the astronomical alignment and the actual peak, known as the age of the tide, differs from place to place.

Reading the numbers

Heights are given above mean lower-low water (MLLW), the chart datum used on US nautical charts. It is the average of the lower of each day's two low waters, measured over a 19-year period. Because it is an average, roughly half of all lower-low waters fall below it — so a negative predicted height is completely normal and simply means the water is lower than the chart datum, not that something has gone wrong.

Some coasts see two nearly equal highs and lows each day (semidiurnal, common on the US East Coast). Others see one of each (diurnal, common on the Gulf Coast). Much of the West Coast is mixed: two highs and two lows, but of noticeably different heights. That difference between the two daily lows is the diurnal inequality, and it comes from the diurnal constituents K1 and O1.

What this cannot tell you

These are astronomical predictions only. The real water level on the day also depends on weather: a strong onshore wind or a deep low-pressure system can raise it well above prediction, and an offshore wind or high pressure can drop it below. Storm surge during a hurricane can add several metres. NOAA's own published tide tables carry exactly the same caveat.

Coverage is United States only, because NOAA's harmonic constants are a work of the US federal government and therefore free to use. Most other countries' tidal data is commercially licensed. This tool also covers NOAA's reference stations, which publish full harmonic constants — not the thousands of smaller subordinate stations defined by time and height offsets from a reference.

Everything runs in your browser. The station constants are bundled with the page, so once it has loaded it works with no network connection at all, and no request tells anyone which harbour you looked up.

How accurate are these tide predictions?

They are checked against 355 of NOAA's own published high and low waters, at eight stations spanning semidiurnal, diurnal and mixed coasts and a seven-year span. The predicted water level stays within about 2% of the day's tidal range at most of those stations and within 3.4% at all but one — a few centimetres on a typical coast. The exception is Cook Inlet in Alaska, where the second-largest tidal range in the world and very strong shallow-water effects push the error to about 6%. High and low water times agree within a few minutes where the tide turns sharply; on flat-topped diurnal coasts the turning point is inherently less well defined, and the time can differ by half an hour while the height still agrees to a centimetre. In practice the real limit is not the maths but the weather: wind and air pressure routinely move actual water levels by more than any of this.

Why is the predicted tide height sometimes negative?

Heights are measured from mean lower-low water, the chart datum, which is an average of the lower daily low water over 19 years. Because it is an average, about half of all lower-low waters fall below it. A reading of −0.4 ft simply means the water is 0.4 ft lower than chart datum, which is ordinary — it is most common around the spring tides near new and full moon.

Why does my location only have two tides some days and four on others?

The lunar day is about 24 hours 50 minutes, so the semidiurnal cycle drifts later by roughly 50 minutes each day. Every so often that drift pushes one of the four events across midnight into the neighbouring calendar day, leaving three on one date. On diurnal coasts such as parts of the Gulf of Mexico there genuinely is only one high and one low per day, because the diurnal constituents dominate there.

What is a king tide?

An informal name for the highest tides of the year. They occur when a spring tide — sun, moon and Earth in line at new or full moon — coincides with the moon at perigee, its closest point to Earth, which increases the tide-raising force by around 20%. Coastal flooding on a calm, clear day is usually a king tide. The "Biggest tides" section finds them for the station you have selected.

Can I use this for boating or fishing?

For planning, yes. For navigation, no — use official NOAA tide tables and nautical charts. This tool predicts the astronomical tide only and knows nothing about the weather on the day, local currents, or channel depths.

Does this work offline?

Yes. All the station data and all the maths are bundled into the page, so after the first load it works with no network connection. Nothing about which station or date you look at is ever sent anywhere.

Why are only US stations covered?

NOAA publishes its harmonic constants as a work of the US federal government, which puts them in the public domain and free to redistribute. Tidal data for most other countries is commercially licensed, so it cannot be bundled into a free, ad-free tool like this one.

Updated August 20, 2026