🌊 Tide Height Calculator
Enter the high and low water heights and the hours into a falling tide to estimate the current water level using the Rule of Twelfths.
🧭 High & Low Water to Current Height
What is a Tide Height Calculator?
It estimates how high the water is at any point through a falling tide, using the time-honoured Rule of Twelfths. Between high and low water the tide doesn't drop evenly — it eases away slowly near the turns and races through the middle. Splitting the range into twelfths of 1, 2, 3, 3, 2, 1 per hour captures that shape, and subtracting the fallen twelfths from high water gives the current height.
Use it for a quick clearance check over a bar or a shoal, to time a departure, or to judge whether you'll float off a drying berth. It's an approximation for a regular tide and takes no account of weather or local quirks, so for navigation always confirm against official tide tables and a full tidal curve.
❓ Frequently Asked Questions
What is the Rule of Twelfths?
It's a sailor's shortcut for estimating tide height without a full tide curve. Over the roughly six hours from high to low water, the tide falls in twelfths of its total range: 1/12 in the first hour, then 2/12, 3/12, 3/12, 2/12, and 1/12 in the last hour. The tide moves slowest near high and low water and fastest at mid-tide.
How do I use it to find the current depth?
Take the tidal range (high water height minus low water height), work out how many twelfths have fallen for the hours since high water, and subtract that fall from the high water height. This calculator does it for you — enter the high and low water heights and the hours into the tide, and it returns the current height and how much has fallen so far.
How accurate is the Rule of Twelfths?
It's a good approximation for a regular semi-diurnal tide where the rise and fall is roughly sinusoidal, and it's widely taught for quick mental checks. It's less reliable where tides are strongly asymmetric, in estuaries, or in areas with double high waters, and it doesn't account for weather-driven surge. For navigation, always cross-check with official tide tables and a proper tidal curve.
Does this work for a rising tide too?
This calculator models a falling tide, from high water down to low water, since that's when clearance under the keel is the pressing question. A rising tide follows the same twelfths in reverse — 1, 2, 3, 3, 2, 1 added on each hour — so you can mirror the logic by counting up from low water instead of down from high.