Tide Window Planner - Safe Departure Windows Above Your Required Depth

Can your boat leave, or is the water too shallow? Paste your tide heights, enter the depth you need, and this planner lists the windows when the water is deep enough for a safe departure. You supply the heights from your own tide table, so the results match your harbour, not a generic forecast.

What the Tide Window Planner does

Many harbours, estuaries and river mouths are only deep enough to use for part of the tidal cycle. A boat that needs 1.5 metres of water cannot cross a bar that has only 1 metre at low tide, however good the weather looks. Tide tables give heights at high and low water, but the question a skipper actually asks is different: between which clock times is the depth at least what I need? This planner answers that question. It takes the heights you paste in, fills in the gaps by straight-line interpolation, and lists every window where the height stays at or above your required depth, with start, end, duration and maximum height for each window.

How the calculation works

Tide heights between published points are estimated by interpolation, which draws a straight line between each pair of neighbouring heights. Real tides follow a smooth curve, so interpolation is close but not exact, especially near high and low water where the curve flattens. The planner scans the whole span of your data in five-minute steps and marks each step as safe or not, then joins consecutive safe steps into windows. The required depth you enter should be your boat’s draft plus a safety margin for waves, squat and soft ground: many skippers add at least half a metre. The planner compares heights and required depth in the same units, so metres and feet both work as long as you are consistent.

How to use the planner

Copy heights from your official tide table into the box, one per line, as time and height separated by a space, for example 06:30 1.2. Include enough points to cover the whole period you care about, ideally every hour or every high and low water. Enter your required depth in the same units as your heights. Press the button and read the windows table. Plan to arrive at the critical shallow point inside a window, not at its edge, because interpolation error and real-world waves can eat into the margin. If no window appears, either the tide range is too small for your draft that day, or you need more data points.

Planning a departure around a window

A window tells you when the water is deep enough, but a good departure plan adds more. Allow time to reach the shallow point from your berth, and allow for other traffic and lock or bridge opening times. If the window is short, plan to be ready before it opens so you use its full length. If the window straddles high water, remember that the depth falls after the peak, so a return trip may face a different window. Keep a note of the maximum height in each window: a window with a high peak gives more margin for error than one that barely clears your requirement. When in doubt, wait for the next window rather than squeezing through a marginal one.

Common mistakes to avoid

The most common mistake is mixing units, such as entering a draft in feet against heights in metres. Check both before you press the button. Another mistake is forgetting the safety margin and entering the bare draft, which leaves no room for waves or a soft grounding. A third mistake is pasting heights for the wrong day or the wrong port, which gives confident-looking but wrong windows. Always check the date, the port name and the time zone of your tide table. Finally, remember that published heights are predictions: strong winds can push real water levels above or below the forecast, so check the actual gauge or harbour report on the day.

Limits and safety warning

This planner is a planning aid, not a navigation authority. It interpolates between your points, it does not know about dredged channels, shifting sandbanks, or local harbour rules, and it cannot see the weather. Always confirm with the official tide table, the harbour master, and your own charts before sailing. If the planner and the official sources disagree, trust the official sources. The tool runs entirely in your browser, and your tide data is never uploaded or stored.

Tide Window Planner – Safe Departure Windows Above Your Required Depth