This tool uses harmonic analysis with 4 primary tidal constituents (M2, S2, K1, O1) to approximate tide heights. Each constituent represents a periodic gravitational influence — primarily from the Moon and Sun. The predicted height at any time is the sum of cosine waves with known amplitudes and phase angles (harmonic constants) specific to each port. Note: This is a simplified approximation using only 4 of the 37+ constituents used in official tide predictions, so times may differ by 10–30 minutes from published tables.

Harmonic constants are pairs of numbers — amplitude (H) and phase lag (g) — that describe how each tidal constituent oscillates at a specific location. Amplitude is the half-range (in feet or meters), and the phase lag determines when the peak occurs relative to a reference. These constants are derived from long-term tide gauge observations and are unique to each port. The four major constituents used here: M2 (principal lunar semidiurnal, ~12.42h cycle), S2 (principal solar semidiurnal, 12h), K1 (lunisolar diurnal, ~23.93h), and O1 (principal lunar diurnal, ~25.82h).

Tides shift later by approximately 50 minutes each day because the Moon orbits Earth in the same direction as Earth's rotation. A lunar day (the time for the Moon to return to the same position in the sky) is about 24 hours and 50 minutes. The M2 constituent, which dominates most tides, follows this rhythm. The tool accounts for this daily progression through the phase accumulation of each constituent over successive days.

Spring tides occur around new and full moons when the Sun, Moon, and Earth align (syzygy). The gravitational pulls combine, producing the largest tidal range (highest highs and lowest lows). Neap tides occur around the first and third quarter moons when the Sun and Moon are at right angles relative to Earth, producing the smallest tidal range. Spring-neap cycle repeats approximately every 14.8 days. This tool's moon phase indicator helps you anticipate these patterns.

Semidiurnal tides have two high and two low waters each tidal day, with similar heights (e.g., New York, London). Diurnal tides have only one high and one low per day (rare, occurs in parts of the Gulf of Mexico and Southeast Asia). Mixed semidiurnal tides have two highs and two lows but with noticeably different heights — a higher high and a lower low (e.g., Los Angeles, San Francisco, Seattle on the US West Coast). The ratio (K1+O1)/(M2+S2) determines the type.

No. This tool provides approximate predictions for general reference and educational purposes only. Official tide tables from NOAA (US), UKHO (UK), or other hydrographic offices use 37+ harmonic constituents and are verified against real-time observations. Always consult official tide tables or apps like NOAA Tide Predictions, UKHO EasyTide, or Navionics for navigation, fishing, or any safety-critical activities.

Using only 4 constituents, predictions are typically accurate to within ±15–30 minutes for timing and ±0.3–0.6 ft (±0.1–0.2 m) for height at most semidiurnal ports. Accuracy degrades in shallow-water ports or areas with complex bathymetry where overtides (M4, M6) and compound tides are significant. For critical applications, always refer to official predictions incorporating 37+ constituents.

These are standard names for major tidal constituents: M2 = principal lunar semidiurnal (Moon's gravitational effect, twice daily), S2 = principal solar semidiurnal (Sun's effect, twice daily), K1 = lunisolar diurnal (combined Sun-Moon declination effect, once daily), O1 = principal lunar diurnal (Moon's declination effect, once daily). Together they account for about 80–90% of tidal variance at most ports.