How to read a tide chart

A tide chart answers three questions: how high is the water now, when does it turn, and how big is today's swing. Everything else — when to launch, when to dig, when the current runs — falls out of those three.

The curve and the zero line

The curve traces predicted water height through the day. The vertical scale is measured from MLLW(mean lower low water) — the long-term average of the lower of each day's two low tides. Zero is deliberately set near the bottom of the normal range, so most of the time the water sits above it.

That choice makes negative tidesmeaningful: a “−1.2 ft” low means the water will drop over a foot below an already-low baseline. Those minus tides expose flats, tidepools, and clam beds that stay covered the rest of the month — they're the days to circle.

Highs, lows, and the shape of the day

Most of the US Atlantic and Pacific coasts are semidiurnal: two highs and two lows a day, roughly 6 hours 12 minutes apart, sliding about 50 minutes later each day as the moon does. The West Coast and much of Alaska run mixed— two highs and two lows of noticeably different sizes (hence “higher high” and “lower low”). Parts of the Gulf of Mexico are diurnal, with a single high and low per day.

The vertical distance between a high and the following low is the range, and it changes through the month with the moon — spring and neap tides can double or halve it at the same station.

The water doesn't move at one speed

The tide is a wave, so the level changes slowly near each high and low and fastest in the middle hours. The old sailor's rule of twelfths captures it: of the total range, the water moves roughly 1/12 in the first hour after the turn, then 2, 3, 3, 2, and 1 twelfths in the following hours. Practical translation: mid-tide is when the current rips and the beach disappears fastest; the hour around slack high or low is the gentle window.

Reading it for a purpose

• Fishing — moving water moves bait. Many anglers fish the two hours either side of a tide change rather than slack itself.
• Paddling and small boats— time trips to ride the tide, not fight it, and remember mid-tide currents in narrow passages are the day's strongest.
• Clamming and tidepooling — arrive an hour before a minus low; the window closes fast once the flood starts.
• Beach walks— know whether the tide is coming or going before you round a headland you can't re-round at high water.

Prediction vs. observation

The published curve is the astronomical tide; wind and pressure move the real water above or below it. Perigee's station pages draw the live six-minute observations on top of the prediction, so you can see today's error bar for yourself. And the standing caveat applies: tide charts are planning tools, not navigation charts.

Try it on a real gauge: browse stations by state or start from the one nearest you, and check sun and moon timesfor the same spot while you're there.