Estimate high and low tide times.
Semi-diurnal tides repeat every 12 hours 25 minutes on average — half a lunar day — so the next high tide is one interval after the last and low water falls halfway between. The rule of twelfths describes how the range is distributed across the six hours of the cycle: a twelfth in the first hour, then two, three, three, two and one, which makes the third and fourth hours the fastest at roughly a third of the range each.
Tide timing and rate
Next high = last high + tidal interval (12 h 25 min typical); next low = last high + interval / 2; fastest hourly change = range / 6 x 2 (rule of twelfths)
Tidal predictions here are simplified averages and must not be used for navigation or any activity where water level or timing affects safety. Consult official tide tables for your location.
Because a lunar day is about 24 hours 50 minutes and most coasts see two tides per lunar day. Some locations are diurnal or mixed and have very different intervals.
An approximation that splits the tidal range across the six hours between high and low water as 1, 2, 3, 3, 2 and 1 twelfths, giving a quick estimate of depth at any point in the cycle.
No. Local topography, weather and barometric pressure shift real tides significantly. Use official tide tables for your specific port for anything safety-critical.