Rule of Twelfths Tide Calculator
Fill in the values and the answer updates as you type.
How the Rule of Twelfths Tide Calculator works
The rule of twelfths splits the tide range into 12 parts and assumes the water moves by 1, 2, 3, 3, 2, and 1 parts in the six equal hours of the tide. The tide runs fastest in the third and fourth hours. Heights are measured from the same datum as your low and high tide figures, usually chart datum. Enter the hours since the tide turned, counting from low water for a rising tide or from high water for a falling tide. It is an approximation, so use official tide tables where depth is critical. Example: with Low tide height 1.5 ft, High tide height 9.5 ft, Hours from low to high 6.2 h, Hours since the tide turned 2.5 h, and Tide direction rising, the result is Tide height at that time = 4.34; Share of the range already covered = 35.48%; Tidal range = 8.
Inputs and results
The Rule of Twelfths Tide Calculator takes 5 inputs and gives 5 results.
What you enter
- Low tide height (ft) — starts at 1.5.
- High tide height (ft) — starts at 9.5.
- Hours from low to high (h) — starts at 6.2.
- Hours since the tide turned (h) — starts at 2.5.
- Tide direction — starts at rising. Choose from: rising and falling.
What you get
- Tide height at that time
- Share of the range already covered
- Tidal range
- Rate of change now (per hour)
- Hour of the tide (1 to 6)
Example: Rule of Twelfths Tide Calculator with the starting values
With low tide height 1.5 ft, high tide height 9.5 ft, hours from low to high 6.2 h, hours since the tide turned 2.5 h and tide direction rising, the calculator returns:
| Tide height at that time | 4.34 |
|---|---|
| Share of the range already covered | 35.48% |
| Tidal range | 8 |
| Rate of change now (per hour) | 1.94 |
| Hour of the tide (1 to 6) | 3 |
Change any value above and the results update straight away.
Rule of Twelfths Tide Calculator FAQ
How do I use the Rule of Twelfths Tide Calculator?
Enter low tide height, high tide height, hours from low to high and hours since the tide turned in the fields, then read tide height at that time, share of the range already covered, tidal range and rate of change now (per hour) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Rule of Twelfths Tide Calculator calculate?
Estimate the tide height at any time between low and high water. It returns tide height at that time, share of the range already covered, tidal range and rate of change now (per hour) from the 5 values you provide.
What values does the Rule of Twelfths Tide Calculator start with?
It opens with low tide height 1.5 ft, high tide height 9.5 ft, hours from low to high 6.2 h and hours since the tide turned 2.5 h so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Rule of Twelfths Tide Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Rule of Twelfths Tide Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.