Air clearance calculator
Check whether your mast or aerial will clear a bridge at the current tidal height.
Related tools
Worked example
A bridge is charted at 8.0 m above Chart Datum. Current tidal height is 2.5 m. Mast is 14.5 m above waterline. Safety margin 0.5 m:
- Actual clearance: 8.0 − 2.5 = 5.5 m
- Clearance with 0.5 m margin: 5.0 m
- Mast height: 14.5 m
- Result: will not clear — 14.5 m mast vs 5.5 m clearance
- Wait for lower tide: need tidal height ≤ 8.0 − 14.5 − 0.5 = −7.0 m — impossible. Use an alternative route.
Use the tidal height calculator to find when clearance will be greatest (near low water).
How it works
Charted bridge clearances on Admiralty charts are given above Chart Datum (CD), which is approximately the Lowest Astronomical Tide. The actual clearance at any moment is the charted figure minus the current tidal height — because a higher tide means less space under the bridge.
Actual clearance = Charted clearance − Tidal height
Your mast height above water (not above the keel or overall length) is what matters. Check your boat’s specification sheet or measure from the waterline to the masthead, including any aerials or wind instruments that may be higher.
Frequently asked questions
Where do I find a bridge’s charted clearance?
Charted bridge clearances are shown on Admiralty (UKHO) charts as a number in metres at the bridge location, typically with an underline to indicate it’s a clearance figure. They’re also listed in the Reeds Almanac and in harbour authority sailing directions. Some bridges have signs showing clearance at the current state of tide.
Should I use high water or low water tidal height?
Use the current tidal height for the time you plan to pass. Clearance is least at high water (highest tidal height) and greatest near low water. If timing is flexible, pass as close to low water as possible for maximum clearance. Use the tidal height calculator to estimate height at your planned passage time.
What safety margin should I use?
0.5 m is a common minimum — enough to account for small errors in tidal prediction and mast measurement. For uncertain conditions, use 1.0 m or more. Some skippers apply a larger margin when tidal prediction is based on interpolation rather than a standard port table.
My mast height — where do I measure from?
Always from the waterline (with your actual load aboard, not the light ship specification) to the highest fixed point — usually the masthead fly, wind instrument, or VHF aerial. Measure with your boat loaded as it will be on passage. A lightly loaded boat rides higher in the water, so the mast base is further from the waterline — meaning effective mast height above water is slightly less.