Tidal Range Predictor

Spring tides occur whenever the sun and moon line up along the same axis relative to Earth, which happens twice per lunar month — once when they're on the same side (new moon) and once when they're on opposite sides (full moon). In both alignments their combined gravitational pull reinforces the tidal bulge, producing a larger range than at the in-between quarter-moon phases.

As the sun's angle relative to the equator increases toward the solstices, tides tend to show more diurnal inequality — a bigger difference between the day's two high tides — which this simplified model represents as a modest reduction in the overall semidiurnal range being predicted. It's a secondary effect compared to the lunar spring-neap cycle, which is why its influence here is capped at a 10% adjustment.

The calculator converts lunar phase into a position on the spring-neap cycle using the same cosine relationship that drives the range calculation, then classifies that position as Spring, Moderate, or Neap based on how close it falls to the cycle's peak or trough. A lunar phase near 0 or roughly 14.75 days lands near spring tide, while a phase near roughly 7.4 or 22 days lands near neap tide.

No — both are measured relative to mean sea level, a standard reference datum representing the average water level over time, not the seabed itself. High Water is half the predicted range above that average line, and Low Water is half the predicted range below it, so actual water depth at any point also depends on the depth of the seabed beneath mean sea level.