How WalkHilbre calculates the crossing window
WalkHilbre takes NTSLF tide predictions for Hilbre, opens a window 3 h after high water and ends it 3½ h before the next high water, following the RNLI advice. It is an estimate — it cannot see the sands.
Method last updated 20 October 2026 · version crossing-gate/1
The calculation, step by step
Fetch predictions
High and low water times and heights for Hilbre from NTSLF, fetched at most every 12 hours.
Window opens
3 h after an NTSLF predicted high water: HW + 3 h.
Window ends
3½ h before the next high water: next HW − 3½ h (RNLI advice). That is the latest time to be back on the mainland at Dee Lane. It is measured from the earlier of the NTSLF and harmonic times for that high water.
Leave-by and start times
Calculated leave-by time = window end − 60 min (the walk back). Latest calculated start from Dee Lane = window end − 2 h.
Warnings and adjustments
Forecast wind, visibility and rain warnings are shown alongside a window and never change it. Adjustments can only make a window shorter.
When do I need to leave Hilbre?
The RNLI advises being back on the mainland at least 3½ hours before high water. WalkHilbre reads that cautiously: “back on the mainland by” is the latest time to be at Dee Lane, off the sands, and it is the end of the calculated window. The calculated leave-by time is earlier, by 60 minutes, to allow for the 45 to 60 minute walk back.
Example: if the next high water is 15:00, be back on the mainland by 11:30, so the calculated leave-by time is 10:30. RNLI advice: do not walk to the islands in the dark or fog. An estimate — not permission to cross. Check the Dee Lane noticeboard before setting out.
What it can’t account for
- Storm surge, strong wind or low pressure raising water above prediction
- Channels and sandbanks moving after storms
- What you can see at Dee Lane on the day
- Your own walking speed, fitness and stops
- Fog, darkness or sudden weather changes
Wind, low pressure and surge are covered in weather and surge.
The rule
- A calculated window opens 3 h after an NTSLF predicted high water.
- It ends 3½ h before the next high water. That is the latest time to be back on the mainland at Dee Lane, off the sands. The end is measured from the earlier of the NTSLF and harmonic times for that high water.
- The calculated leave-by time, the latest time to start back from Hilbre, is 60 min before the window ends, allowing for the 45 to 60 minute walk.
- The latest calculated start from Dee Lane is 2 h before the window ends.
- The RNLI advises walking out from three hours after high tide and returning at least three and a half hours before the next high tide. WalkHilbre applies this cautiously, as the latest time to be back on the mainland at Dee Lane, off the sands. Read the RNLI's West Kirby and Hilbre safety advice.
- With 2 h or less left the status reads “Past calculated start · return only”. With 1½ h or less left it reads “Window ending soon”. After the end, or before the start, it reads “Outside calculated window”; a window opening within 3 h is shown as starting soon.
- Margins are elapsed time. Times are shown in UK time (GMT or BST).
Tide size
A day is labelled a spring tide when its highest predicted high water at Hilbre is 9.0 m above Chart Datum or more, and a neap tide when it is below 7.7 m; anything between is moderate. The size adds caution copy only and never changes a window.
When no window is shown
Every page, the widget and the public API get windows from one gate. The gate shows no window, only “No planning window available. Check the Dee Lane noticeboard before setting out.”, whenever any of these is true:
- NTSLF predictions could not be fetched, or no NTSLF high water covers the period.
- The NTSLF predictions were fetched more than 36 hours ago.
- The stored predictions do not cover the time from now to the next high water.
- A high water height is outside 5–11 m above Chart Datum, or a low water height is outside -1–5 m.
- Events are out of time order, or high and low waters do not alternate.
- Two consecutive high waters are less than 11h 56m or more than 13h 25m apart.
- The harmonic cross-check has no valid calibration.
- An NTSLF high water bounding a window differs from the calibrated harmonic high water by more than 30 minutes, or the model has no high water nearby.
Your browser checks again on every update. It hides every window if the page's data is more than 3 minutes old or the NTSLF predictions behind it are more than 36 hours old. A cached page or an embedded widget left open therefore stops showing times rather than showing old ones.
Adjustments only make a window shorter
Surge, weather and inundation adjustments can only delay the start or bring the close earlier. They can never widen a window or extend the close. An adjustment whose inputs are more than 3 hours old is dropped. No adjustments are currently applied; windows follow the rule above.
Warnings
Warnings are shown next to a window. They never change it.
- Darkness
- The end of the window, when visitors should be back on the mainland, is before sunrise, or within 60 minutes of sunset or later.
- Large tide
- A high water bounding the window is 9.0 m above Chart Datum or more.
- Strong wind
- Forecast hourly wind during the window reaches 25 mph.
- Poor visibility
- Forecast visibility during the window is 1,000 m or less, or fog is forecast.
- Heavy rain
- Forecast rain during the window reaches 4 mm per hour, or heavy rain or thunderstorms are forecast.
- Weather not checked
- The weather forecast is missing or more than 3 hours old.
More about the data
- NTSLF Hilbre predictions (primary)
- High and low water predictions for Hilbre Island from the National Tidal and Sea Level Facility, © National Oceanography Centre, Liverpool. Fetched at most every 12 hours. They are the only source of planning windows and of the high and low water times shown. They are shown no more than 7 days ahead. Heights are above Chart Datum, which at Hilbre is 4.93 m below Ordnance Datum Newlyn (NTSLF datum table).
- Predicted tide curve
- The chart's predicted tide passes exactly through every NTSLF high and low water. Between them it follows the shape of a harmonic tide model, calibrated each night against Environment Agency gauge readings and stretched to fit the NTSLF times and heights. Like NTSLF, it excludes surge and weather. Where NTSLF predictions are not available, no curve is shown.
- Harmonic model (cross-check)
- The same model cross-checks each NTSLF high water before a window is shown. It never produces a window or a displayed high or low water time.
- Environment Agency readings
- Observed water levels at the Liverpool Gladstone Dock gauge, in metres above Ordnance Datum Newlyn and converted to Chart Datum for display. They calibrate the model, give the surge estimate shown on the dashboard (not added to the predicted tide), and are drawn on the tide chart as a separate dashed line. The gauge is not at Hilbre, so its heights differ from Hilbre predictions. This uses Environment Agency tide gauge data from the real-time data API (Beta).
- Open-Meteo weather
- Forecast wind, visibility, rain, sunrise and sunset, used for warnings. Warnings never change a window.
- LiDAR inundation overlays
- Precomputed map overlays showing which ground is below a given water level. They illustrate the tide on the map and never change a window.
How it is checked
- Automated tests cover each condition above that stops a window being shown, using real NTSLF predictions as well as constructed data, including both clock changes.
- A randomised test checks that adjustments never produce a window wider than the rule allows.
- The high-to-high limits come from a full-year run of the calibrated model, with 15 minutes added either side.
- When the cross-check was introduced on 15 October 2026, NTSLF and harmonic high waters differed by a median of 10 minutes and at most 17 minutes over 54 high waters.
- Every 15 minutes the site records exactly what it would display and the data behind it, in an append-only log kept for at least six years.
Today's first calculated window
- High water
- + 3 h → window opens
- Next high water
- next day
- − 3½ h → back on the mainland by
- Calculated leave-by time (back − 60 min)
- Latest calculated start (back − 2 h)
Calculated at 02:40 UK time from NTSLF predicted high waters. An estimate — not permission to cross.
Data sources and updates
Tide predictions
NTSLF, © National Oceanography Centre, Liverpool. Fetched at most every 12 hours.
Weather
Open-Meteo forecast (CC BY 4.0), credited as “Weather data by Open-Meteo.com”. A forecast older than 3 hours is not used.
Safety margin
RNLI advice for West Kirby and Hilbre. Last checked 28 Aug 2026.
Times
Shown in Europe/London time (GMT or BST).
Change log
- 20 Oct 2026 The calculated leave-by time (60 min before the window end) is shown separately from “back on the mainland by”, the window end. Earlier versions showed only the window end as the time to leave.
- 16 Oct 2026 The window end is read cautiously, as the latest time to be back on the mainland. The calculated leave-by time (60 min earlier) and the latest calculated start (2 h earlier) are shown.
- 15 Oct 2026 Method page published.
- Oct 2026 Use of NTSLF Hilbre predictions confirmed with the National Oceanography Centre.
- 28 Aug 2026 Safety guidance re-checked against RNLI, Wirral Council and Friends of Hilbre sources.
Made by Page & Place
The studio that designed and built WalkHilbre.
WalkHilbre fetches the Hilbre high and low water predictions shown here directly from the National Tidal and Sea Level Facility (NTSLF), © National Oceanography Centre, Liverpool. NTSLF publishes times in GMT and heights in Chart Datum without meteorological effects; WalkHilbre displays Europe/London local time and keeps weather context separate.