Plymouth, MA tide chart

Plymouth's tide is Cape Cod Bay's: a mean range of almost ten feet, semidiurnal and regular, so the flats of the harbor drain and refill on a large scale twice a day. NOAA's gauge is in Plymouth Harbor itself, a few hundred metres from the middle of town.

Moving water sets the hour here. NOAA gauge PLYMOUTH, PLYMOUTH HARBOR, 0.3 km from the middle of Plymouth, publishes 37 harmonic constants. Mean range is 9.8 ft, and the form factor — diurnal constituents over semidiurnal — is 0.16, which is semidiurnal water.

Tide times and heights in Plymouth

The curve answers a question the table cannot. A table gives the minute of high water; the curve gives the hour you can actually get out — the stretch between one turn and the next that no row describes, and on a semidiurnal coast that is most of the fishing day.

What the tide does at Plymouth

The same tide reaches Plymouth about twelve minutes after Boston at high water and twenty-five at low, by NOAA's own predictions, at nearly the same height. Round the elbow of the Cape it is a different sea: at Chatham, on the ocean side, the predictions run about thirty-eight minutes later than Plymouth's at high water and fifty-three at low, and the tide there is a good deal smaller.

Spring tides run to about 10.3 feet and neaps to about 7.5. In the middle of a big flood the water climbs close to three feet an hour, by this page's curve, so the flats that are walkable at low water go under quickly — the stage of the tide matters more here than the minute of high water.

What the heights are measured from

Every height here is feet above mean lower low water. That reference is not the sea bed and not the water under your keel — it is the average of the lower of each day's two low waters, taken over a nineteen-year epoch. A printed low water of 1.3 ft means the surface stands 1.3 feet above that line, not that there is 1.3 feet of water.

The useful consequence is that NOAA measures its charted depths from the same line. A sounding of four feet with a 1.3 ft tide is about 5.3 feet of water at that moment, and the same sounding at a −0.4 ft tide is about 3.6. Mixing the two references is the single most common way to read a tide table wrong.

Spring and neap tides in Plymouth

The moon sets the rhythm and the sun decides how hard it lands. When the two pull in line the ranges stack; when they pull across each other the range collapses. At Plymouth that puts the spring range near 10.3 ft against a neap range near 7.5 ft — a factor of 1.36 between the strongest and the weakest tides of the fortnight.

Both figures come from the station's own constants rather than from the fortnight on this page, which is why they do not move when the table is rebuilt. They are also close to the station's published great diurnal range, and that agreement is a check on the arithmetic rather than a coincidence. The same calculation at Pensacola, FL gives a mean range of 1.2 ft against 9.8 ft here — the same Moon and Sun acting on a different coast, with a different amount of water moving through it.

How close these numbers are to NOAA

These are not NOAA's published predictions. The station publishes 37 harmonic constants — amplitudes and phases of the periodic pulls that make up the tide here — and we synthesise the curve from them. Saying so is easy; the number that follows is the part that is worth reading.

Against NOAA's own predictions for August 12–19, 2026, every one of the 31 high and low waters matched, with an average difference in time of 0.26 minutes and a worst case of 0.68. The worst height difference was 0.3% of the day's range. That is the whole claim, and it is smaller than the width of the pencil line on a printed table.

Two limits belong with that number. It is a week of comparison, not a year, and it is this station: the same method run at a station with a big, awkward range does measurably worse, and our own worst case on the grid is off by minutes rather than seconds. Where that is true, the page says so, and the method behind every number on this site is written out rather than summarised.

When a tide table is not the right tool

A tide prediction is astronomy, and the water is weather as well. Wind and barometric pressure move the real level against the printed one: a hard onshore blow holds water in and makes a predicted low shallower and later, and a cold, dry high-pressure spell does the opposite. NOAA publishes observed levels alongside its predictions for exactly this reason. The numbers here are also the station's water, not the water two miles up a tidal creek, where the same tide arrives later and smaller. On rivers a spring freshet outweighs the tide outright.

Tide is the hour; it does not tell you which hour is worth taking. The reasoning behind the solunar windows — and what they are worth on a coast where moving water already sets the hour — is in best fishing times. The rest of the grid is catalogued on the fishing forecast hub. A printed page is also poor company in a creek at first light, and which fishing apps actually carry tide tables is a separate comparison.

The nearest stations this site carries are Marshfield, MA (9.08 ft), Scituate, MA (8.94 ft), Boston, MA (9.49 ft) and Charlestown, RI (3 ft), and the full list with the station behind each one is on the tide charts hub.