Tides are caused by the differential gravitational pull of the Moon (and to a lesser extent the Sun) across the Earth's diameter, stretching the ocean into two bulges that appear as high tides as Earth rotates beneath them.

Why does the Moon dominate tides despite the Sun being far more massive?

Tidal force is a differential gravitational effect that falls off with the cube of distance, not the square like ordinary gravity. The Moon's much shorter distance more than compensates for its smaller mass, giving it roughly 2.2x the Sun's tide-raising force.

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Tides Calculator

Tidal range, high/low tide predictions, spring & neap tide cycles, and the gravitational mechanics behind ocean tides — based on harmonic tide theory.

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Educational Model — Not for Navigation

This tool uses a simplified harmonic model (superposed M2/S2 or K1/O1 tidal constituents) scaled to a mean range you provide. Official station predictions use dozens of gauge-calibrated harmonic constants. For navigation, fishing, or coastal safety, always use official NOAA (US) or UKHO (UK) tide tables for your specific location.

Tide Type

Mean Tidal Range (meters)

Typical: 0.3-1m (micro), 1-2m (meso), 4-6m+ (macro, e.g. Bay of Fundy)

Reference High Tide Hour (0-23)

Days Since New Moon (0-29.5)

0 days (0 = new moon, ~14.8 = full moon)

Semidiurnal Tide

🌑 New Moon

🔺 Spring tide — larger range

1.8 m

Effective range today

📏Mean range

1.5 m

4.92 ft

🌙Moon illumination

⚖️Moon:Sun force

2.18:1

Tide-generating force ratio

🔄Cycles/day

Semidiurnal

Predicted Tide Curve (48 hours)

Harmonic model using M2 + S2 constituents.

Predicted High & Low Tides

Time	Type	Height
06:00	High	0.9 m
12:00	Low	-0.9 m
18:00	High	0.89 m
+1d 00:15	Low	-0.89 m
+1d 06:30	High	0.88 m
+1d 12:30	Low	-0.87 m
+1d 18:30	High	0.85 m

About the Tides Calculator

This free tidal range and tide prediction calculator models ocean tides using harmonic tide theory — the same underlying mathematics (superposed sinusoidal tidal constituents) that official agencies like NOAA use, simplified to an educational level. It answers questions like "what causes spring and neap tides", "how much stronger is the Moon's tidal pull than the Sun's", and "how does a semidiurnal tide differ from a diurnal tide" — with interactive charts, not just text explanations.

How harmonic tide prediction works

Real tides are the sum of dozens of periodic "tidal constituents," each corresponding to a specific astronomical cycle (Earth's rotation, the Moon's orbit, the Sun's apparent motion). The method was formalized by Arthur Doodson in 1921 for the British Admiralty and remains the basis of modern tide prediction. This calculator uses the two dominant constituents for your selected tide type — M2 (principal lunar semidiurnal, 12.42-hour period) and S2 (principal solar semidiurnal, 12-hour period) for semidiurnal/mixed tides, or K1 and O1 (diurnal constituents) for diurnal tides — superposed and scaled to your mean tidal range.

Spring tides vs. neap tides

Spring tides (larger tidal range) occur at new moon and full moon, when the Sun, Earth, and Moon are roughly aligned and their gravitational tide-raising forces add together. Neap tides (smaller tidal range) occur at the first and third quarter moon, when the Sun and Moon pull at roughly right angles to each other and partially cancel. This 14.8-day spring-neap cycle follows the lunar phase, tracked here via days since the last new moon.

Why the Moon dominates tides despite the Sun's mass

The Sun is about 27 million times more massive than the Moon, yet the Moon generates roughly 2.2× the tide-raising force. That's because tidal force is a differential gravitational effect that falls off with the cube of distance (1/r³), not the square (1/r²) like ordinary gravity. The Moon's much shorter distance more than compensates for its far smaller mass — a classic result in tidal geophysics (Doodson & Warburg, Admiralty Manual of Tides, 1941).

Note on accuracy: Official tide tables use station-specific harmonic constants derived from years of tide-gauge measurements and account for local bathymetry, coastline shape, and dozens of additional constituents. This calculator is a simplified two-constituent educational model and should not be used for navigation, fishing, or coastal safety planning.

Frequently Asked Questions

What causes tides?▼

Tides are caused by the differential gravitational pull of the Moon (and to a lesser extent the Sun) across the Earth's diameter. The side of Earth closest to the Moon is pulled more strongly than the far side, stretching the ocean into two bulges — one facing the Moon, one facing away — which appear as high tides as the Earth rotates beneath them.

What is the difference between semidiurnal, mixed, and diurnal tides?▼

Semidiurnal tides have two nearly equal high and low tides per lunar day (~24h 50m) — common on the U.S. East Coast. Mixed semidiurnal tides also have two highs and two lows, but with noticeably unequal heights — common on the U.S. West Coast. Diurnal tides have only one high and one low per day — found in parts of the Gulf of Mexico and Gulf of Tonkin. The pattern depends on local coastline geometry and which tidal constituents dominate.

How often do spring and neap tides occur?▼

Spring tides occur twice per lunar month, at new moon and full moon (roughly every 14.8 days). Neap tides also occur twice per lunar month, at the first and third quarter moon, roughly midway between spring tides.

Does "spring tide" refer to the season?▼

No — despite the name, spring tides have nothing to do with the spring season. The term comes from the tide "springing forth" with greater range, and occurs monthly year-round at new and full moon.

Where can I get official tide predictions for my location?▼

In the United States, use NOAA Tides & Currents (tidesandcurrents.noaa.gov), which publishes station-calibrated predictions using dozens of harmonic constants. In the UK, use the UK Hydrographic Office (UKHO). This calculator is an educational model and is not a substitute for official station data.

Sources & References

Doodson AT. “The harmonic development of the tide-generating potential.” Proceedings of the Royal Society A 100(704):305-329 (1921).
Pugh DT, Woodworth PL. Sea-Level Science: Understanding Tides, Storm Surges, Tsunamis and Mean Sea-Level Changes. Cambridge University Press (2014).
Doodson AT, Warburg HD. Admiralty Manual of Tides. HMSO (1941).
NOAA Tides & Currents — harmonic constituent reference data. National Oceanic and Atmospheric Administration.