We’ve developed a free tide height calculator to help you work out key depths and clearances. It’s easy to use, and works for HW to LW and LW to HW. You can download your copy, and the related documentation, below.
The calculator works best with Excel, on a desktop, laptop or mobile device. As the user, you need input only the chart datum, highest astronomical tide, tide data for the day (heights and times) and your vessel data (draught, air draught and safety margins – see the documentation for more).
The calculator will then use the rule of twelfths model to give you information about:
Depth of water for each of the six hours before and after high water.
Length of anchor chain, line, or chain/line combination for each hour.
Length of mooring line required to tie up to a wall for each hour.
Safe clearance below power cables, bridges and other overhead obstructions.
The results are colour-coded, so that you can see at a glance when there is enough water for you sail, and when it is safe to pass under an overhead obstruction. You can even see clearly when your boat will be aground – a useful feature if you have a bilge-keeler or similar which is capable of taking the ground.
As with most modern electronic aids to navigation, the tide calculator is clearly marked as not for navigation. This is the same message that you might see when you boot up your onboard chart plotter – how you approach that is up to you.
We strongly recommend that you download both the tide calculator and the documentation, and that you read and understand the documentation before using the calculator. Use the links below: