Tide Height — Rule of Twelfths

Estimate tide heights and stream rates hour-by-hour from HW/LW values, with a curve chart.

HW height (m)LW height (m)HW time (24h)Period (minutes)

Tidal curve (Rule of Twelfths)

Range: 4.40 m

Offset	Time	Height (m)	Phase
-6h	06:17	0.80	Flooding
-5h	07:19.166666666666742	1.17	Flooding
-4h	08:21.333333333333485	1.90	Flooding
-3h	09:23.5	3.00	Flooding
-2h	10:25.666666666666515	4.10	Flooding
-1h	11:27.833333333333485	4.83	Flooding
HW	12:30	5.20	High water
+1h	13:32.166666666666515	4.83	Ebbing
+2h	14:34.333333333333485	4.10	Ebbing
+3h	15:36.5	3.00	Ebbing
+4h	16:38.666666666666515	1.90	Ebbing
+5h	17:40.83333333333303	1.17	Ebbing
+6h	18:43	0.80	Low water

Bonus: tidal stream rate (Rule of 50/90/100)

Max spring rate at this location:kn

Window	Phase	Rate (kn)
12:30 – 13:32.166666666666515	Hour 1 after HW	1.25
13:32.166666666666515 – 14:34.333333333333485	Hour 2 after HW	2.25
14:34.333333333333485 – 15:36.5	Hour 3 after HW	2.50
15:36.5 – 16:38.666666666666515	Hour 4 after HW	2.25
16:38.666666666666515 – 17:40.83333333333303	Hour 5 after HW	1.25
17:40.83333333333303 – 18:43	Hour 6 after HW	0.00

⚠ For educational use only. Always use official tide tables and admiralty data for navigation.

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About Tide Height — Rule of Twelfths

The Tide Height — Rule of Twelfths tool estimates how the tide rises or falls hour by hour between a known high water and low water. Enter your HW and LW heights and times, and it works out the water level at each hour of the cycle using the classic 1/12, 2/12, 3/12, 3/12, 2/12, 1/12 distribution, plus the approximate stream rate at each stage. A curve chart plots the full tidal cycle so you can see the level at a glance.

It is built for sailors, kayakers, divers, anglers, and coastal walkers who need a quick clearance or depth check without a full tide table app. It is completely free and runs entirely in your browser, so the heights and times you enter never leave your device and nothing is uploaded to a server.

How to use

Enter the high water height and time for the cycle.
Enter the low water height and time.
Read off the estimated tide height at each hour from the table.
Check the curve chart to see the full rise and fall.
Use the per-hour stream rate to judge the strongest flow.

Features

Hour-by-hour tide height estimates
Built-in rule of twelfths calculation
Visual tidal curve chart
Approximate stream rate per hour
Works from any HW and LW pair
Runs privately, no data uploaded

Frequently asked questions

Is my tide data kept private?

Yes. The tool runs fully client-side in your browser, so the heights and times you type stay on your device and are never sent to a server. It is free to use with no sign-up.

What is the rule of twelfths?

It is a rule-of-thumb that splits the rise or fall of the tide across the six hours between high and low water in the proportions 1, 2, 3, 3, 2, 1 twelfths. It assumes a roughly six-hour, sinusoidal semidiurnal tide.

How accurate are the results?

The rule of twelfths is an approximation that works best for regular semidiurnal tides with a near six-hour interval. For navigation in shallow or critical situations, always cross-check against official tide tables and charted datums.

Can I use it where the tide is not exactly six hours?

The method assumes a six-hour cycle, so results are least reliable where the interval differs greatly, such as diurnal or strongly distorted tides. Treat it as a rough guide in those locations.

What does the stream rate tell me?

It estimates how fast the tidal stream is running at each hour, peaking around the middle of the cycle and slackening near high and low water, helping you plan around the strongest flow.

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Offset

Time

Height (m)

Phase

-6h

06:17

0.80

Flooding

-5h

07:19.166666666666742

1.17

Flooding

-4h

08:21.333333333333485

1.90

Flooding

-3h

09:23.5

3.00

Flooding

-2h

10:25.666666666666515

4.10

Flooding

-1h

11:27.833333333333485

4.83

Flooding

12:30

5.20

High water

+1h

13:32.166666666666515

4.83

Ebbing

+2h

14:34.333333333333485

4.10

Ebbing

+3h

15:36.5

3.00

Ebbing

+4h

16:38.666666666666515

1.90

Ebbing

+5h

17:40.83333333333303

1.17

Ebbing

+6h

18:43

0.80

Low water

Window

Phase

Rate (kn)

12:30 – 13:32.166666666666515

Hour 1 after HW

1.25

13:32.166666666666515 – 14:34.333333333333485

Hour 2 after HW

2.25

14:34.333333333333485 – 15:36.5

Hour 3 after HW

2.50

15:36.5 – 16:38.666666666666515

Hour 4 after HW

2.25

16:38.666666666666515 – 17:40.83333333333303

Hour 5 after HW

1.25

17:40.83333333333303 – 18:43

Hour 6 after HW

0.00

About Tide Height — Rule of Twelfths

Frequently asked questions

Is my tide data kept private?

Yes. The tool runs fully client-side in your browser, so the heights and times you type stay on your device and are never sent to a server. It is free to use with no sign-up.

What is the rule of twelfths?

How accurate are the results?

Can I use it where the tide is not exactly six hours?

The method assumes a six-hour cycle, so results are least reliable where the interval differs greatly, such as diurnal or strongly distorted tides. Treat it as a rough guide in those locations.

What does the stream rate tell me?

It estimates how fast the tidal stream is running at each hour, peaking around the middle of the cycle and slackening near high and low water, helping you plan around the strongest flow.