A tide chart answers one question: when will the water be high, and when will it be low? Everything else — the heights, the dates, the curve — is there to help you judge how much the water will move and whether that movement fits your plans. This guide walks through a real chart the way you'd read it on our location pages.
The two numbers that matter
Every tide chart is built from tide events: the moments when the water reaches its highest point (high tide) and its lowest point (low tide). Each event has a time and a height, like "High 9:15 AM, 5.2 ft." Most of the US coast gets a pattern calledsemidiurnal tides — roughly two highs and two lows every day, spaced about 6 hours and 12 minutes apart. Parts of the Gulf of Mexico get diurnal tides: just one high and one low a day. Some areas get an uneven mix, where the two daily highs are noticeably different heights.
When you open a chart for today, scan it in this order: find the next event after now, then the one after that. "Low tide 3:33 PM, high tide 9:29 PM" tells you the water is currently going out (falling) and will turn around mid-afternoon. If you only remember one habit, make it this: plan around the next event, not the clock.
Tidal range: big, small, and why you care
The tidal range is the difference between the day's highest high and lowest low. A 1-foot range in the Gulf means the water barely moves all day; a 9-foot range in Maine means the shoreline can look completely different six hours later. Range drives almost everything practical:
- Current speed. Bigger range means more water has to move in and out, so currents run faster, especially in inlets and channels.
- Beach access. On a big range, a sandbar or tide pool that is exposed at low tide can be several feet underwater at high.
- Shallow-water boating. A ramp or channel usable at mid-tide may be dry at low.
Range isn't fixed — it grows around new and full moons (spring tides) and shrinks around quarter moons (neap tides). The word "spring" has nothing to do with the season; spring tides happen every two weeks all year.
Datums: what "0 ft" actually means
Tide heights are measured against a reference level called a datum, and in the US the standard is MLLW — Mean Lower Low Water, the average of the lower of each day's two lows over a 19-year period. Two practical consequences:
- Most low tides read slightly above 0 ft, because 0 is an average, not a floor.
- Negative numbers are normal. A low of −0.5 ft just means "half a foot below the average lower low" — that's often the best tidepooling of the month.
Never mix datums when comparing sources. A chart in MLW (Mean Low Water) reads lower than the same tide in MLLW — MLLW uses the lower of the two daily lows as its reference, so heights measured from it come out higher. Every page on this site states its datum: MLLW.
Why the times slide every day
Tides follow the moon, and the moon runs about 50 minutes behind the sun each day. That's why tomorrow's high tide comes roughly 50 minutes later than today's, and why a beach that's perfect at 4 PM this Saturday will look the same at about 5 PM next Saturday. Over a week, the whole schedule slides by nearly six hours. It's also why static printed tide tables go stale so fast — and why this site rebuilds every page four times a day.
Reading the 7-day curve
Between the printed events, the water follows a smooth curve. On the 7-day curve on our location pages, the shaded vertical bands mark night (sunset to sunrise), so you can see at a glance which lows fall in daylight. A few things to practice spotting:
- Steepness = current. The steepest part of the curve is mid-tide, when water moves fastest. The flattest moments are the highs and lows themselves — "slack tide."
- Unequal days. Some days show one big high and one small high. That's the "mixed" pattern, normal on much of the West Coast.
- The weekly slide. Watch any feature — say the morning low — drift later each day across the week.
Putting it together for a trip
Say you want to fish the two hours before high tide on Saturday. Open the chart, find Saturday's highs, and work backwards two hours. Say you want to walk the tide pools: look for a low tide below about 1.5 ft that falls in daylight, and arrive an hour before it. Boaters do the reverse: they look for enough water over their draft, plus a margin. The chart is the same chart — the skill is anchoring your plan to the event that matters to you, then watching the range to know how much margin you have.
One last warning worth repeating: tide charts are astronomical predictions. Wind and barometric pressure can push actual water levels a foot or more off the prediction on any given day. On calm days the chart is eerily accurate; in a storm, treat it as a schedule for the moon's pull, not the water you'll actually meet.
Sources
- NOAA Tides & Currents (CO-OPS) — the prediction data behind every page on this site
- NOAA: Datums explained — MLLW and friends, from the source
- NOAA: What are tides?