Bridge Clearance and Tide Calculator
Fill in the values and the answer updates as you type.
How the Bridge Clearance and Tide Calculator works
Charted bridge clearance is measured from a high water level, such as mean high water springs. At a lower tide the water is further down, so clearance now = charted clearance + (high water height − current tide height). Spare room = clearance now − air draft. The highest safe tide = high water height + charted clearance − air draft − safety margin. Include antennas, radar, and wind instruments in the air draft, and allow for waves, wakes, and a rising or falling tide. A tidal gauge on the bridge is more reliable than any calculation. Example: with Boat air draft (waterline to highest point) 62 ft, Charted clearance at high water 65 ft, Tide height of the high water the chart uses 6 ft, Current tide height 2.5 ft, and Safety margin 3 ft, the result is Clearance under the bridge now (ft) = 68.5; Spare room above your mast (ft) = 6.5; Spare room after the safety margin (ft) = 3.5.
Inputs and results
The Bridge Clearance and Tide Calculator takes 5 inputs and gives 5 results.
What you enter
- Boat air draft (waterline to highest point) (ft) — starts at 62.
- Charted clearance at high water (ft) — starts at 65.
- Tide height of the high water the chart uses (ft) — starts at 6.
- Current tide height (ft) — starts at 2.5.
- Safety margin (ft) — starts at 3.
What you get
- Clearance under the bridge now (ft)
- Spare room above your mast (ft)
- Spare room after the safety margin (ft)
- Highest tide you can pass under (ft)
- Verdict
Example: Bridge Clearance and Tide Calculator with the starting values
With boat air draft (waterline to highest point) 62 ft, charted clearance at high water 65 ft, tide height of the high water the chart uses 6 ft, current tide height 2.5 ft and safety margin 3 ft, the calculator returns:
| Clearance under the bridge now (ft) | 68.5 |
|---|---|
| Spare room above your mast (ft) | 6.5 |
| Spare room after the safety margin (ft) | 3.5 |
| Highest tide you can pass under (ft) | 6 |
| Verdict | Clear with your safety margin |
Change any value above and the results update straight away.
Bridge Clearance and Tide Calculator FAQ
How do I use the Bridge Clearance and Tide Calculator?
Enter boat air draft (waterline to highest point), charted clearance at high water, tide height of the high water the chart uses and current tide height in the fields, then read clearance under the bridge now (ft), spare room above your mast (ft), spare room after the safety margin (ft) and highest tide you can pass under (ft) in the result panel. The answer updates as you type, and you can press Calculate or Enter to refresh it. Use “Copy link” to share your exact numbers with someone else.
What does the Bridge Clearance and Tide Calculator calculate?
Check if your mast fits under a bridge at the current tide level. It returns clearance under the bridge now (ft), spare room above your mast (ft), spare room after the safety margin (ft) and highest tide you can pass under (ft) from the 5 values you provide.
What values does the Bridge Clearance and Tide Calculator start with?
It opens with boat air draft (waterline to highest point) 62 ft, charted clearance at high water 65 ft, tide height of the high water the chart uses 6 ft and current tide height 2.5 ft so you can see a working result straight away. Replace them with your own numbers, or press Reset to return to these starting values.
Is the Bridge Clearance and Tide Calculator free and private?
Yes. It is free, needs no account, and runs in your browser, so nothing you type is sent to or stored on our servers. See the Privacy Policy for details.
Can I rely on the Bridge Clearance and Tide Calculator for important decisions?
Treat the result as a well-rounded estimate. Real situations often have details a general calculator cannot know, so confirm important decisions with a qualified professional. See the Disclaimer for more.