Tide Height Interpolator – Find the Water Level at Any Time Between Tides

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What this tool does

Tides do not jump from high to low in one step. The water rises and falls on a smooth curve, and this tide height interpolator estimates the level at any time between two known tides. You type in the times and heights of two tides next to each other, for example a high tide in the morning and the low tide that follows it, and then you pick a time in between. The tool gives you an estimated height for that time, so you can judge whether a shallow channel, a slipway, or a drying mooring will have enough water.

This is a manual-input tool on purpose. It does not fetch live tide data, and it never claims to know the tide at your location. Real tide times and heights come from an official tide table or a harbour authority for your exact place and date. You copy those two numbers into the tool, and the tool only does the maths in between. That keeps the result honest: the quality of the answer depends on the quality of the tide table you use.

How to use it

First, find two tides that sit next to each other in your tide table, with your target time between them. Enter the time and height of the first tide, then the time and height of the second tide. Times use the 24-hour clock, written as hours and minutes, for example 06:00 or 18:45. Heights should both use the same unit, normally metres above chart datum, exactly as printed in your table. Then enter the time you want to know about and press the button.

The tool offers two methods. The smooth-curve method is the recommended one. It follows a cosine curve, which rises slowly near high and low water and fastest in the middle of the tide, much like a real tide does. The straight-line method simply draws a line between the two heights. It is easier to check by hand, but it overstates the change near high and low water. Compare both results if you want to see how much the method matters for your times.

The maths behind the estimate

The straight-line result is simple proportion: the height moves from the first height to the second height in proportion to how much of the time gap has passed. The smooth-curve result uses a cosine shape instead. Halfway between the two tides, the cosine result equals the average of the two heights, just like the straight line. But near the ends of the gap, the cosine curve flattens out, which matches the way a real tide slows down as it approaches high or low water and then turns.

The tool also shows the average rate of change, in metres per hour, between the two tides. That number is useful on its own. A fast rate means the water level is changing quickly and your window of enough depth may be short. A slow rate near high or low water means the level stays close to the extreme for longer, which is why the hour around high water is often the most forgiving time to cross a shallow bar.

A worked example

Suppose your tide table gives high water of 3.2 metres at 06:00 and low water of 0.8 metres at 12:15, and you want the height at 09:00. The gap is 6 hours and 15 minutes, and 09:00 sits 3 hours into it, just under halfway. The straight-line estimate is about 2.05 metres. The smooth-curve estimate is a little higher, about 2.08 metres, because the fall starts slowly after high water. Either way, you now have a number to compare with the depth your boat needs, plus the charted depth of the channel, to judge whether you can pass.

Remember to add the charted depth and subtract your draught before you decide anything. The tide height alone is not the depth of water under your keel. Depth of water equals charted depth plus tide height, and clearance equals that total minus your draught. If the result is tight, wait for more tide or choose another route. This tool gives you the tide part of that sum, and nothing more.

Limits and safety

An interpolated tide is an estimate, not a measurement. Wind can push water in or out of an estuary, air pressure changes the level, and rivers in flood add their own effect. A strong onshore wind can raise the actual level above the estimate, and a strong offshore wind or high pressure can leave it lower. That is why this tool must never be your only source for a navigation decision. Always check the official tide table, the weather forecast, and local knowledge for the place you are sailing.

This tool is an educational estimator only. It is not professional navigation advice, and it does not replace official charts, official tide tables, weather information, or the judgement of the person in charge of the boat. If the estimate is close to a limit that matters, treat it as a warning to get better information, not as permission to proceed. When in doubt, wait for the tide.

Common questions

Can I use times that cross midnight? Yes. If the second tide is after midnight, for example a low water at 00:40 following a high water at 18:30, the tool handles the wrap correctly, as long as the two tides are next to each other. Can I use feet instead of metres? Yes, as long as both heights use the same unit, the result comes out in that unit. Just keep the unit consistent and compare it with depths in the same unit.

Versión en español: Interpolador de Altura de Marea