🌊 Tide Height Calculator (Rule of Twelves)
Calculate tide height at any time using the Rule of Twelves method. Perfect for planning harbor entries, anchoring, and shallow water navigation.
Understanding the Rule of Twelves
What is the Rule of Twelves?
The Rule of Twelves is a simple method for estimating tide height at any time between high and low water. It divides the six-hour tidal cycle into hourly segments and assumes the tide rises or falls by predictable fractions of the total range:
- Hour 1: 1/12 of the tidal range
- Hour 2: 2/12 of the tidal range
- Hour 3: 3/12 of the tidal range
- Hour 4: 3/12 of the tidal range
- Hour 5: 2/12 of the tidal range
- Hour 6: 1/12 of the tidal range
This creates the characteristic S-curve of tidal movement, with slower changes at high and low water (slack water) and fastest movement at mid-tide.
When to Use This Method
The Rule of Twelves works best for:
- Semi-diurnal tides: Areas with two high and two low tides per day (most locations)
- Regular tidal curves: Locations without significant tidal anomalies
- Quick estimates: When you need a reasonable approximation without complex calculations
- Passage planning: Determining safe times to enter harbors or cross shallow areas
Limitations and Considerations
Be aware of these limitations:
- Less accurate for areas with strong tidal streams or river influences
- Doesn't account for atmospheric pressure effects (add 0.3m safety margin in low pressure)
- Wind can significantly affect actual water levels (onshore winds raise levels)
- Spring tides (new/full moon) have larger ranges than neap tides (quarter moon)
- Some locations have diurnal (once daily) or mixed tides where this method doesn't apply
Practical Applications
Harbor Entry Planning
Use the calculator to determine the earliest safe time to enter a shallow harbor. Add your draft plus a safety margin (typically 0.5-1.0m) to find the minimum required depth, then calculate when the tide will reach this height.
Anchoring Calculations
When anchoring, calculate the tide height at low water to ensure you won't ground. Also check high water height to ensure you have enough scope (anchor rode length) and swinging room.
Bridge Clearances
For vertical clearance under bridges, calculate high water height and add your air draft (height from waterline to highest point). Many bridges show clearance at Mean High Water Springs (MHWS).
Safety Tips
- Always add a safety margin to calculations (minimum 0.5m under keel)
- Use official tide tables for accurate high/low times and heights
- Consider weather effects - low pressure reduces heights by ~1cm per millibar below 1013mb
- Strong onshore winds can raise water levels by 0.5-1.0m
- In unfamiliar waters, observe a full tidal cycle before attempting shallow passages
- Keep a tide table app or almanac as backup to calculations
Understanding Tidal Terms
- Chart Datum (CD):
- The reference level for depths on charts, usually close to the lowest astronomical tide (LAT)
- Height of Tide:
- Vertical distance between sea level and chart datum at any given time
- Tidal Range:
- Difference between high and low water heights
- Springs and Neaps:
- Spring tides (largest range) occur at new and full moon; neap tides (smallest range) at quarter moons
- Drying Height:
- Height of seabed above chart datum in areas that dry out at low tide
Frequently Asked Questions
How accurate is the Rule of Twelves?
The Rule of Twelves is typically accurate to within 10-15% of actual tide height for regular semi-diurnal tides. It's most accurate at hours 1, 3, and 6, with slightly less precision at intermediate times. For critical decisions like grounding risks, always add a safety margin of at least 0.5m. The method works well for standard locations but is less accurate in areas with complex tidal patterns, strong river influences, or unusual topography. For maximum accuracy, use official tidal prediction software or tide tables with harmonic constants.
What safety clearance should I use under my keel?
Minimum safe clearance depends on conditions: Calm conditions, soft bottom: 0.3-0.5m minimum. Moderate conditions: 0.5-1.0m recommended. Rough conditions or hard bottom: 1.0-1.5m minimum. Unfamiliar waters: Add extra 0.5m to normal clearance. Areas with swell: Add expected swell height to clearance. Consider that boats pitch and heel while sailing, effectively increasing draft. In anchorages, allow for boat settling in soft mud and potential squat effect when motoring in shallow water.
How do I account for atmospheric pressure and wind?
Weather significantly affects actual water levels: Atmospheric pressure: For every millibar below standard pressure (1013mb), add approximately 1cm to tide height. In a deep low (980mb), this adds about 33cm. Wind effects: Strong onshore winds can raise levels by 0.5-1.0m, while offshore winds lower them. Storm surge: Severe weather can cause surges of 1-3m above predictions. Swell and waves: In exposed locations, add maximum expected wave height to calculations. Always check local weather forecasts and add appropriate safety margins during unsettled conditions.