Bridge Clearance Calculator
Calculate the available clearance between your boat's highest point and a bridge using bridge clearance, tide level, boat air draft, and a safety margin.
Bridge Clearance Result
How do you calculate bridge clearance?
Start with the published bridge clearance, adjust it for the actual water level, and subtract the boat's effective air draft.
$$Available\ Clearance = Bridge\ Clearance - Tide\ Adjustment$$
$$Boat\ Clearance = Available\ Clearance - Air\ Draft$$
Bridge clearance formula
When the published bridge clearance is referenced to the same water level being used:
$$Clearance = Bridge\ Height - Tide - Air\ Draft$$
A higher water level reduces the clearance beneath the bridge.
What is boat air draft?
Air draft is the vertical distance from the boat's waterline to its highest point. For a sailboat, this can include the mast and other equipment. For a powerboat, it may include an arch, radar, antenna, or hardtop.
Why does tide affect bridge clearance?
A bridge is physically fixed, but the water surface moves. When the tide or water level rises, less vertical space remains between the boat and the underside of the bridge.
$$Higher\ Water\ Level \rightarrow Less\ Bridge\ Clearance$$
$$Lower\ Water\ Level \rightarrow More\ Bridge\ Clearance$$
Bridge clearance example
Suppose a bridge has 30 ft of published clearance, the current water level is 2 ft higher than the reference level, and the boat has an 18 ft air draft.
$$Available\ Clearance = 30 - 2 = 28\ ft$$
$$Boat\ Clearance = 28 - 18 = 10\ ft$$
The boat would have approximately 10 ft of vertical clearance under the simplified assumptions.
What safety margin should I use for bridge clearance?
There is no single margin that applies to every boat or bridge. Consider measurement uncertainty, changing water level, boat loading, waves, wake, boat motion, and the accuracy of the published clearance.
A conservative margin is especially important when the clearance is close to the boat's air draft.
What can make a boat's air draft change?
- Different loading condition
- Fuel and water tank levels
- Fresh versus salt water
- Boat trim
- Equipment mounted on the boat
- Adjustable antennas or radar equipment
- Mast or rig configuration
Does boat weight affect bridge clearance?
It can. A heavier boat may sit lower in the water, reducing its air draft relative to the waterline in some loading conditions. The relationship depends on the boat's hull and weight distribution.
Does freshwater change bridge clearance?
Yes. A boat generally sits slightly deeper in fresh water than in seawater because fresh water is less dense. That can reduce the vertical clearance between the boat's highest point and a bridge.
Bridge clearance chart
The following simple chart shows the air-draft limit when a 2 ft safety margin is maintained.
| Bridge Clearance | Water Level Above Reference | Maximum Air Draft With 2 ft Margin |
|---|---|---|
| 20 ft | 0 ft | 18 ft |
| 25 ft | 0 ft | 23 ft |
| 30 ft | 0 ft | 28 ft |
| 35 ft | 0 ft | 33 ft |
| 40 ft | 0 ft | 38 ft |
What is the difference between bridge clearance and air draft?
Bridge clearance is the available vertical space beneath the bridge. Air draft is how tall the boat is above the waterline. The difference between them is the actual vertical clearance above the boat.
$$Bridge\ Clearance - Air\ Draft = Available\ Clearance$$
Can I use charted bridge clearance directly?
Not always. Verify the reference datum used for the published clearance and determine whether the value represents a minimum, mean, high-water, or other specified condition. The actual clearance can change with tide, river level, storm surge, and other water-level changes.
Bridge clearance FAQ
Subtract the boat's effective air draft from the actual water-level-adjusted bridge clearance.
Yes. When the water level rises, the distance between the boat and the bridge becomes smaller.
The required margin depends on the boat, bridge, water-level accuracy, boat motion, and local conditions. A conservative margin should be maintained rather than relying on a zero-clearance calculation.
Air draft is the vertical distance from the waterline to the highest point of the boat that must pass under the bridge.
Yes. Tide directly changes the water level and therefore the available vertical clearance between the mast and the bridge.