Tide height at a given time
Pick a coastal place, a day and a time, and we tell you how high the water will be, whether the tide is rising or falling and how long until the next high and low tide. Calculated from the harmonic constants of the nearest tide station, with the rule of twelfths alongside.
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Work out the tide height
Chosen place: Charleston · South Carolina, United States
Charleston, Wednesday, October 28 · 3:56 pm
- Next high tide
- 10:17 pm · 5.6 ftin 6 h 21 min
- Next low tide
- 4:34 pm · 0.3 ftin 38 min
- Rule of twelfths
- 0.7 fthour 6 of the falling tide · +0.1 ft
- Coefficient
- 83
Hour by hour, from one tide to the next
| Time | Rule of twelfths | Harmonic calculation |
|---|---|---|
| 10:05 am·High tide | 6.8 ft | 6.8 ft |
| 11:10 am | 6.2 ft | 6.5 ft |
| 12:14 pm | 5.2 ft | 5.5 ft |
| 1:19 pm | 3.6 ft | 3.9 ft |
| 2:24 pm | 2.0 ft | 2.2 ft |
| 3:29 pm | 0.9 ft | 0.9 ft |
| 4:34 pm·Low tide | 0.3 ft | 0.3 ft |
Station: Charleston (Customhouse Wharf). Heights above mean lower low water (MLLW), the zero of US nautical charts.
How to use it
- Search for the place by name (a town, a beach or a harbor) or tap “Near me”.
- Pick the day and the time. The time is local to the place, daylight saving time included.
- Read the height: it is measured from the place’s chart datum, the same zero as in tide tables. Compare it with your boat’s draft, a rock or the day’s low tide.
The formulas
How we calculate the height
The tide is the sum of many waves, each with a fixed period: the main lunar one (M2, 12 h 25 min), the solar one (S2, 12 h), the diurnal ones (K1 and O1, about 24 h) and dozens of smaller ones. Each tide station has measured the amplitude and lag of every one of them for years.
With them, the height at any instant is the mean level plus the sum of all the waves, corrected for the 18.6-year lunar cycle. It is the same method NOAA, the UKHO and other offices use for their official tables.
h(t) = Z₀ + Σ f·H·cos(ω·t + V₀ + u − g)
Z₀: mean level above chart datum. H and g: amplitude and phase of each wave at the station. ω: its speed. V₀, f and u: positions of the moon and sun and nodal corrections.
The rule of twelfths
To do the math in your head: on a tide of about 6 hours from low to high, the water rises 1 twelfth of the range in the first hour, 2 in the second, 3 in the third, 3 in the fourth, 2 in the fifth and 1 in the sixth. Falling works the same way.
Half of the change happens in the two middle hours, when the current is strongest. Near high and low tide the water barely moves: that is slack water.
height = low tide + range × twelfths / 12
| Hour | Rises that hour | Total so far | Percent |
|---|---|---|---|
| 1 | 1/12 | 1/12 | 8% |
| 2 | 2/12 | 3/12 | 25% |
| 3 | 3/12 | 6/12 | 50% |
| 4 | 3/12 | 9/12 | 75% |
| 5 | 2/12 | 11/12 | 92% |
| 6 | 1/12 | 12/12 | 100% |
It works well on semidiurnal coasts such as the US East Coast, the UK or Atlantic Europe. It fails on mixed or diurnal tides (Gulf of Mexico, much of the Pacific), in estuaries where the flood is shorter than the ebb, and in seas with almost no tide such as the Mediterranean.
Example: Charleston, October 28
Low tide at 3:40 am (0.0 ft) and high tide at 10:05 am (6.8 ft): the range is 6.8 ft, so one twelfth is 0.6 ft. At 6:52 am, three hours into the tide, the rule of twelfths gives 3.4 ft and the harmonic calculation 3.9 ft.
| Time | Rule of twelfths | Harmonic calculation |
|---|---|---|
| 3:40 am | 0.0 ft | 0.0 ft |
| 4:44 am | 0.5 ft | 0.5 ft |
| 5:48 am | 1.7 ft | 2.0 ft |
| 6:52 am | 3.4 ft | 3.9 ft |
| 7:56 am | 5.1 ft | 5.5 ft |
| 9:01 am | 6.2 ft | 6.5 ft |
| 10:05 am | 6.8 ft | 6.8 ft |
Where heights are measured from
- United States
- NOAA measures from MLLW, the average of each day’s lower low water. On big spring tides, lows go below zero, such as −0.5 ft.
- United Kingdom and Europe
- Chart Datum is close to the Lowest Astronomical Tide (LAT), so negative heights are rare.
- Spain and the rest of the world
- Most hydrographic offices use a datum near LAT. Our heights use the datum of the station’s own data, shown on each place’s tides page; where no station is nearby and the tide comes from the global model, they are above mean sea level.
Frequently asked questions
What is the tide height measured from?
From the chart datum of the place (MLLW in the U.S., close to LAT in most other countries); where no station is nearby and the tide comes from the global model, from mean sea level. It is not the depth: to know how much water there is, add the tide height to the charted depth at that spot.
What is the rule of twelfths?
A way to estimate the height between low and high tide in your head: the water rises 1, 2, 3, 3, 2 and 1 twelfths of the range in each of the six hours. Halfway through the tide it is at half the range.
How accurate is the rule of twelfths?
On a regular semidiurnal tide it is off by less than 5% of the range: a few inches on a 10 ft tide. On mixed tides, in estuaries or with double high waters it can be far off, which is why we show the harmonic calculation next to it.
Why is the water not at the height the calculator says?
Because this is the astronomical tide. Air pressure and wind change it: each hectopascal (0.03 inHg) below normal raises the water about 0.4 inch (1 cm), and an onshore storm can add a foot or more.
Can I use it for navigation?
It is a prediction for fishing and leisure. For navigation, always use the official publications and keep a safety margin.
More tools
- Tide coefficient calculatorThe tide coefficient for any place and day, or from the tidal range, with the 20–120 scale explained.
- Solunar calendarMajor and minor periods and the activity rating for every day of the month.
- Nautical unit converterKnots, Beaufort, mph, fathoms, nautical miles, inHg and °F, with tables.
- Moon calendarEvery moon phase of the year, eclipses, supermoons and the best fishing days. Printable too.
All of this, already worked out for your spot
Every coastal town, beach and harbor has its own page with today’s tide, the yearly tide table, sun, moon and the best time to fish.
Where the data comes from
Our own calculation, using the @slackwater/engine library and the harmonic constants of tide stations: NOAA in the U.S., the official services of Japan, South Korea, the United Kingdom, Ireland, New Zealand, Norway and Queensland, and TICON-4 in the rest of the world; where no station is nearby, the global FES2022 and EOT20 models. Each place uses its nearest station; the distance and source are on the place’s tides page.
Spotted a mistake? Tell us · How we calculate tides, sun and moon