Understanding Tides: Springs, Neaps and the Rule of Twelfths
- Reece Smith-ward
- Jul 27
- 4 min read
The tide is one of the most predictable rhythms in nature, yet it can feel mysterious the first time you try to read it. Whether you are a swimmer timing a calm entry, an angler waiting for the turn, or a photographer hoping for a low-water shot, understanding why the water rises and falls makes planning far easier.
This study guide covers the well-established basics: what causes high and low water, why spring tides differ from neap tides, and how the rule of twelfths gives a quick way to estimate the height of tide between high and low water. It is learning material rather than a set of operational instructions, so treat it as background reading to build your understanding.
What causes the tide
Tides are driven mainly by the gravitational pull of the Moon, with a smaller contribution from the Sun. The Moon draws the ocean into gentle bulges, and as the Earth rotates beneath them, most coasts pass through a repeating pattern of high and low water. Many places see two highs and two lows across roughly a day, though local geography shapes the exact timing and size of the swing.
Spring and neap tides
The size of the tide changes over the month as the Sun and Moon shift relative to each other. When they line up, around the new and full Moon, their pull combines to give a larger range known as a spring tide, with higher highs and lower lows. When they sit at right angles, around the first and last quarter, their effects partly cancel to give a smaller range called a neap tide. Recognising where you are in this springs-to-neaps cycle helps explain why some days show far more movement than others.
The rule of twelfths
The rule of twelfths is a long-standing way to estimate how the height of tide changes between high and low water. It assumes the total range is covered in six roughly equal hours, with the water moving one twelfth of the range in the first hour, two twelfths in the second, three twelfths in the third, three twelfths in the fourth, two twelfths in the fifth, and one twelfth in the last. In other words the tide moves fastest around the middle of the cycle and slowest near high and low water. It is an approximation for study and rough planning, and it works best where the tide follows a regular twice-daily pattern.
Seeing tides in TideDeck
TideDeck is a tide times and charts app for iPhone and iPad that computes the tide for anywhere on Earth on your device. The home screen shows the current water height, whether the tide is rising or falling, and the next high and low with a live countdown, so you can take in the state of the water at a glance. You can scrub the day's curve with a finger and feel a gentle haptic tick at each high and low, which makes the shape of the day's rise and fall much easier to picture.
It is built for people who watch the water, from swimmers and paddlers to anglers, rock-poolers, and dog walkers on tidal sands, and it works as a planning and interest tool rather than a navigation tool. Along US coasts, tides are computed from published harmonic constants for 1,302 measured NOAA stations and badged as exact, while everywhere else they are modelled from global and regional ocean models and clearly labelled as modelled, with the distance to the nearest measured station shown. A built-in Tide School, week and month charts that show the swing from springs to neaps, and sun and moon data give you plenty to explore as you learn. TideDeck is free, and an optional TideDeck Pro upgrade extends the forecast horizon to a full year ahead, available as an annual subscription or a one-time lifetime unlock.
Rule of twelfths in CalcDeck
If you would rather let the numbers do the work, Maritime Master CalcDeck includes a rule of twelfths calculator as part of its tides section, alongside tools for secondary ports, under-keel clearance, and squat. CalcDeck brings together a large library of maritime calculators for the whole ship, with clear inputs that show the method behind each result, so you can see how an answer is reached rather than just reading it off. It is available for iPhone and iPad, works offline, and has no accounts, ads, or tracking. CalcDeck is free, with a large set of calculators free to use including all twelve unit converters, and a single one-time CalcDeck Pro purchase unlocks the full library, with no subscription.
For learning and planning only
TideDeck predicts the height of the tide above a reference level. It does not know the depth of the water or the level of the seabed, it does not include storm surge, wind, or barometric effects, and it does not predict tidal streams. It is a planning and interest tool, not a nautical publication. For navigation, always use official tide tables, official charts, and your harbour authority. Predictions are exact at the 1,302 bundled US stations and modelled (approximate) elsewhere.
CalcDeck results are for training and reference and should always be verified against approved instruments, official tables, and the vessel's documentation before any operational use. Both apps are study, learning, and planning aids only, not tools for real-world navigation. For anything you do on or beside the water, official tide tables, official charts, and your harbour authority always govern the decision.
