Tide Height at Time Calculator

Enter your high and low tide heights and times, then pick a time of interest to estimate the tide height using both the Rule of Twelfths and the cosine method.

Created by Luis GonzalezLast updated: May 9, 2026

How to Use This Calculator

1
Enter High Tide Height
Input the height of the high tide in feet above chart datum.
2
Specify Low Tide Height
Enter the height of the low tide in feet above chart datum.
3
Provide High Tide Time
Input the time of high tide in 24-hour format (HH:MM).
4
Enter Low Tide Time
Input the time of low tide in 24-hour format (HH:MM).
5
Indicate Time to Estimate
Enter the specific time (HH:MM) for which you want to estimate the tide height.
6
Review your results
The calculator will display the estimated tide height using both the Rule of Twelfths and the Cosine Method, along with the tidal range and rate of change.

Example Calculation

A boater needs to know the tide height at 09:00, given a high tide of 12.5 ft at 06:30 and a low tide of 2.0 ft at 12:45.

High Tide Height (ft)

12.5

Low Tide Height (ft)

2.0

High Tide Time (time)

06:30

Low Tide Time (time)

12:45

Time to Estimate (time)

09:00

Results

8.83 ft

Tips

Use Reliable Tide Data

Always source your high and low tide data from official, up-to-date hydrographic charts or government-issued tide tables. Unreliable data will lead to inaccurate predictions and potential navigational hazards.

Understand Tidal Period

The accuracy of these methods depends on knowing the true duration between high and low tide (half-period). For most locations, this is approximately 6 hours, but local factors can cause variations.

Account for Local Anomalies

In complex estuaries, narrow inlets, or areas with river outflow, tidal behavior can be highly irregular and deviate from simple models. In such cases, prioritize local knowledge or real-time sensors over general calculations.

Accurate tide height prediction is a cornerstone of safe and efficient marine navigation. The Tide Height at Time Calculator provides mariners, coastal planners, and recreational boaters with the ability to estimate the exact water level at any given moment, using both the Rule of Twelfths and the Cosine Method. This dual-method approach ensures robust predictions for planning passages, anchoring, and accessing tide-restricted areas, critical for operations in 2025.

Why Knowing Tide Height is Essential for Boaters

Knowing the exact tide height at any given moment is essential for boaters because it directly dictates the navigable water depth. Misjudging the tide can lead to vessels running aground, propeller damage, or being stranded for hours, incurring costly repairs and delays. For a vessel with a 6-foot draft, a 1-foot error in predicting a falling tide could mean the difference between clearing a shallow bar and being stuck until the next high tide. This precise knowledge allows for optimal route planning, safe access to marinas, and confident navigation through dynamic coastal environments, protecting both vessel and crew.

The Dual Approach to Tide Height Calculation

The Tide Height at Time Calculator employs two widely recognized methods for estimating tidal heights: the Rule of Twelfths and the Cosine Method. The Rule of Twelfths offers a segmented, empirical approximation, while the Cosine Method provides a smoother, more mathematically continuous curve. Both methods use the provided high and low tide heights and times to interpolate the water level at a specific query time.

Rule of Twelfths Logic: This method divides the 6-hour period between high and low tide into hourly segments, applying fractions of the total tidal range to each.

tide_dropped_or_risen = (cumulative_twelfths_fraction / 12) × tidal_range
estimated_height = (is_ebbing ? high_tide_height - tide_dropped_or_risen : low_tide_height + tide_dropped_or_risen)

Cosine Method Logic: This method models the tide as a simple harmonic motion, using a cosine function to smooth the curve.

elapsed_fraction = elapsed_minutes / total_tidal_period_minutes
cosine_height = low_tide_height + (tidal_range / 2) × (1 - cos(π × elapsed_fraction))  // (for rising tide)

(A different cosine function is used for falling tide).

💡 For estimating other critical operational parameters on a boat, our Watermaker Output Calculator helps plan fresh water production.

Estimating Mid-Tide Height for Passage Planning

Consider a boater needing to determine the tide height at 09:00, given the following data from official tide tables:

High Tide Height: 12.5 ft (at 06:30)
Low Tide Height: 2.0 ft (at 12:45)
Time to Estimate: 09:00

Here’s the calculation:

Tidal Range: 12.5 ft - 2.0 ft = 10.5 ft.
Time from High Tide to Low Tide (half-period): 12:45 - 06:30 = 6 hours 15 minutes = 375 minutes.
Time from High Tide to Query Time: 09:00 - 06:30 = 2 hours 30 minutes = 150 minutes.
Tidal Phase: Since 09:00 is between high tide (06:30) and low tide (12:45), the tide is ebbing (falling).

Applying the calculator's internal logic for the Rule of Twelfths, after 2 hours and 30 minutes into the ebb:

The tide would have fallen approximately 4.2 twelfths of the total range.
Tide Dropped: (4.2 / 12) × 10.5 ft = 3.675 ft.
Estimated Tide Height: 12.5 ft (High Tide) - 3.675 ft = 8.825 ft.

The Estimated Tide Height at 09:00 is approximately 8.83 ft. This provides crucial information for assessing under-keel clearance.

💡 To understand potential hazards from vessel movement, our Wake & Wash Hazard Distance Calculator helps assess impact on other boats or shorelines.

Optimizing Boat Operations with Accurate Tide Predictions

Knowing the precise tide height at any given time is critical for optimizing boat operations, from planning departures to anchoring in shallow areas or transiting over bars. Commercial mariners, for instance, utilize detailed tide predictions to maximize cargo loads, ensuring vessels draw the absolute maximum without grounding. A difference of just 1 foot in predicted tide height can be the deciding factor for whether a deep-draft vessel can safely pass a particular obstacle or enter a marina, especially for vessels with drafts exceeding 6 feet (around 1.8 meters). Accurate tidal data allows for strategic route planning, minimizing delays and preventing costly incidents, making it an indispensable part of modern marine navigation.

Comparing Rule of Twelfths with the Cosine Method for Tide Prediction

The Rule of Twelfths and the Cosine Method represent two distinct approaches for estimating tide heights, each with its own advantages. The Rule of Twelfths provides a simplified, segmented approximation, dividing the tidal cycle into hourly increments and applying fixed fractions of the total range. This method is often favored for quick, manual calculations on the water due to its ease of use. In contrast, the Cosine Method models the tidal curve as a smooth, continuous sinusoidal wave, offering a more mathematically precise and often more accurate representation of actual tidal behavior. It is particularly effective in locations with regular, semi-diurnal tides. While the Rule of Twelfths might provide a general idea, the Cosine Method typically yields results within 0.1-0.3 feet of actual predictions in regular tidal regimes, making it preferred for more critical navigation where higher precision is required.

Frequently Asked Questions

What is the Rule of Twelfths for tide prediction?

The Rule of Twelfths is a simplified empirical method used to estimate tidal height at various points during a 6-hour tidal cycle between low and high tide. It posits that the tide rises or falls by 1/12, 2/12, 3/12, 3/12, 2/12, and 1/12 of its total range during each successive hour. While an approximation, it is a practical tool for quick manual estimations, especially for mariners needing to gauge water depth for safe navigation in coastal waters.

How does the Cosine Method estimate tide height?

The Cosine Method estimates tide height by modeling the tidal curve as a sinusoidal wave, using a cosine function to approximate the smooth, continuous rise and fall of the tide. It takes into account the high and low tide heights and the duration of the tidal period. This method often provides a more accurate and smoother prediction than the Rule of Twelfths, particularly when the tidal cycle closely resembles a simple harmonic motion, making it a valuable tool for detailed navigation planning.

Why is knowing the tide height at a specific time important for mariners?

Knowing the tide height at a specific time is crucial for mariners to ensure safe navigation, especially in shallow harbors, inlets, and channels. It directly impacts under-keel clearance, allowing boaters to determine if there is sufficient water depth to pass without grounding or damaging their vessel. This information is also vital for planning docking maneuvers, anchoring strategies, and estimating transit times, as currents associated with rising and falling tides can significantly affect a vessel's speed over ground.