Tide Height Calculator

How Tides Actually Work

Tides are the ocean's slow breathing, and the lungs are the Moon and the Sun. The Moon's gravity pulls water toward it, creating a bulge on the near side of the Earth — and because the Earth–Moon system rotates around a shared center of gravity, a second bulge forms on the far side too. As the Earth spins beneath these two bulges, most coastlines pass through two high tides and two low tides roughly every 24 hours and 50 minutes. That 50-minute lag is the lunar day: while the Earth completes one rotation, the Moon has moved a little further along its orbit, so the Earth must spin a bit extra to bring the same point back under the Moon.

The result is the semidiurnal tide cycle: high tide, falling water, low tide, rising water, repeating every 12 hours and 25 minutes (12.4 hours in the simplified model this tool uses). The Sun contributes its own smaller tidal bulges. When the Sun, Moon, and Earth line up at new and full moon, the solar and lunar bulges add up and you get spring tides — the biggest highs and lowest lows of the month. At the quarter moons the bulges partly cancel and you get neap tides, with a much smaller range. None of this is academic: a spring tide can expose or flood a beach by meters more than a neap tide on the very same shore.

The Simplified Tide Curve Behind This Tool

Real tide prediction is harmonic analysis — dozens of sine waves (astronomers call them constituents, like M2 for the principal lunar tide) summed with local phase corrections measured over years at a specific gauge. TideTime uses a deliberate simplification: one cosine wave with a 12.4-hour period, anchored to the time you enter as the last high tide. The height at h hours after high water is cos(h × 2π / 12.4), scaled from +1 (high) to −1 (low).

This model nails the two things that matter most for planning: the timing of highs and lows and the direction of the water (rising or falling). What it cannot do is tell you absolute water depth in meters, account for shallow-water distortion (which makes real tide curves asymmetric — the flood often runs faster than the ebb), or handle diurnal tides (one high and one low per day, common in the Gulf of Mexico and parts of Southeast Asia) and mixed tides. For navigation, harbour entry, or anything where grounding or clearance matters, always use your national hydrographic office's official tide tables. For deciding when to fish, surf, beachcomb, or photograph the shore, the simplified curve is genuinely useful.

Reading the Dial: Rising, Falling, Slack, and Stand

The dial on this tool shows where you are in the cycle right now. A few terms worth knowing:

• Flood tide: water rising toward high tide. Currents push shoreward; channels fill.
• Ebb tide: water falling toward low tide. Currents run seaward; flats and reefs emerge.
• High-water stand / slack water: the brief pause around high tide when the water barely moves. Low-water stand is the same at the bottom of the cycle.
• Tide rip / race: where a strong tidal current squeezes through a narrow gap, the water can stand up in visible waves even on a calm day.

The fastest current usually runs about three hours after high or low water — halfway through the flood or ebb — while the hour around each extreme is the gentlest. That single fact organizes most coastal activity: divers and kayakers often plan around slack water, while anglers hunt the moving water in between.

Why Anglers Obsess Over Tide Changes

Ask a dozen surfcasters when to fish and most will say the same thing: around the change of tide. Moving water carries scent, dislodges baitfish and crustaceans, and concentrates predators at current edges, gutters, and drop-offs. The first two hours of the flood push baitfish up onto flats; the first two hours of the ebb drain them back off, and predators line up at the exits. Slack water at the exact top or bottom is often the slowest bite of the cycle.

TideTime's fishing window finder highlights the two hours straddling each tide change — the periods when the water is moving fastest. It is a simplification (real feeding also depends on light, moon phase, water temperature, and bait presence), but 'fish the moving water' is the highest-percentage rule in shore angling. Combine it with dawn or dusk and you have covered the two biggest variables in one plan.

Spring Tides, Neap Tides, and the Monthly Rhythm

The spring–neap cycle runs about 14.8 days. If you fish the same mark regularly, you will notice the pattern: spring tides bring stronger currents and more dramatic feeding windows but can make small boats and wading uncomfortable, while neap tides are forgiving and predictable. Serious anglers keep a log of catches against tide state — after a season, your own data beats any generic advice.

Tide Safety: What the Curve Cannot Tell You

Tides kill people every year, almost always through the same handful of mistakes:

• Cut off by the flood: walking out on a causeway, sandbar, or rocky ledge at low water and finding the return route underwater an hour later. Check how fast the tide floods at your specific spot — some estuaries rise faster than you can walk.
• Underestimating spring tides: a beach that is fine at neap high water can be fully submerged at spring high water, trapping you against cliffs.
• Rip currents on the ebb: outgoing tidal flow combines with wave-driven rips. Swim parallel to shore to escape, never against.
• Mud and quicksand flats: exposed tidal flats can be soft enough to trap boots or legs; the incoming tide does not wait.

A simplified model like this one is a planning aid, not a safety instrument. For unfamiliar coasts, check official tide tables, ask locals, and give yourself a generous margin — being back an hour early beats being rescued.

Coastal Activities Timed to the Tide

Surfing

Most beach breaks work best on a rising tide: the extra water over the sandbar shapes the wave up instead of dumping it. Reef breaks vary — some need the low-tide push over the reef. Watch your spot across a full cycle once and you will know its window forever.

Beachcombing and rock-pooling

Go on a falling tide toward a spring low. The lowest water exposes the richest ground: rock pools at their fullest variety, shells and sea glass freshly deposited, and the retreating water leaves everything visible. Start two hours before low water and head back as it turns.

Photography

Long exposures of water over rocks want mid-tide movement — enough flow to blur, not so much that your tripod is swimming. Reflections in wet sand are best just after the ebb exposes fresh, glassy flats.

Kayaking and paddleboarding

Plan with the current, not against it: ride the flood in, ride the ebb home. Fighting a 3-knot tidal stream in a kayak is a losing proposition, and many coastal rescues start exactly this way.

Tides Around the World: Not Everywhere Is Semidiurnal

The 12.4-hour model fits the Atlantic coasts of Europe and North America beautifully, but the ocean is full of exceptions. The Bay of Fundy in Canada sees the world's largest range — over 16 meters — because the bay's natural sloshing period resonates with the tidal period, amplifying it like a pushed swing. The Mediterranean barely has tides at all (often under half a meter) because it is nearly enclosed. Southampton famously gets a 'double high water' from interacting tidal waves in the English Channel. And in parts of the Gulf of Mexico, the Philippines, and Vietnam, tides are diurnal — one high and one low per lunar day — where this tool's cosine curve simply does not apply. Knowing which regime your coast follows is step one of reading any tide tool sensibly.

From Tide Tables to Your Phone: A Short History

Predicting tides is one of humanity's oldest applied sciences. Tide tables were being printed for London Bridge by the 13th century; Newton's Principia (1687) gave the gravitational explanation; and in the 1860s William Thomson (Lord Kelvin) built a mechanical tide-predicting machine — brass gears summing harmonic constituents, accurate enough for harbour planning, with some machines still working into the 1960s. Modern harmonic prediction uses 30–60 constituents fitted to years of gauge data. Your phone's tide app is the direct descendant of Kelvin's gears — and this tool's single cosine is the great-great-grandchild simplified to one wave. Use the right tool for the job: harmonic tables for navigation, a quick curve for a fishing trip.