What MLLW means

Every height on a US tide chart is measured from the same zero: MLLW, mean lower low water. Knowing what that zero is explains why tides can be "minus".

The definition

Most US coasts get two low tides a day, and one is usually lower than the other. NOAA takes the lower of the two each day, averages it over a 19-year period called the National Tidal Datum Epoch, and calls that average 0.0 ft. It is set separately for every station.

So a tide of 4.0 ft is four feet above that long-run average low, not four feet deep. Water depth on the beach depends on the slope of the sand, which a tide table cannot know.

Minus tides

Because zero is an average, many lower lows fall below it. Those are minus tides, shown with a minus sign: −0.4 ft is about five inches below the average lower low. Minus tides come around new and full moon (spring tides) and uncover more beach, flats and rock than usual.

The lowest predicted tide of 2027 at Springmaid Pier, Myrtle Beach in Myrtle Beach is −1.4 ft, on Saturday, January 23, 2027 at 1:31 am.

Minus tides in the 21 days from Sunday, November 15, 2026

Days with at least one low below 0.0 ft, by town:

Other datums you may see

  • MLW, mean low water: the average of all lows, so slightly above MLLW where the two daily lows differ.
  • MHHW, mean higher high water: the average of the higher of the two daily highs. Coastal flood thresholds are often given relative to it.
  • NAVD88: a land survey datum used for elevations and flood maps, not tied to the local tide.

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