How to Measure a Boat's Width (Beam)

A boat's beam is its maximum width. Knowing this exact measurement is critical when ordering a properly fitting boat cover, verifying highway trailering restrictions, or selecting an appropriately sized marina slip.

Overview diagram highlighting a boat's beam

1. Understanding the Beam

In boating terminology, the beam is the single widest point of the hull. Unlike length overall (LOA), which is measured from bow to stern, the beam is a side-to-side (port to starboard) measurement.

2. Preparing to Measure

Before you begin, make sure you have the right tools to get an accurate reading:

  • A reliable steel tape measure (at least 25 feet long).
  • A helper to hold the opposite end of the tape measure (or painter's tape to secure it to the gunwale).
  • If measuring from the ground, two straight edges or plumb bobs are highly recommended.

3. The Deck Measurement Method

If you have easy access to the inside of the boat, this is the most common way to measure the beam:

  • Locate the Widest Point: Walk around the boat and identify the absolute widest section. On most standard V-hull boats, this is typically located near the center (midship) or slightly aft of center.
  • Measure Outside to Outside: Place one end of the tape measure on the absolute outside edge of the port side, and run it across to the absolute outside edge of the starboard side.
  • Include the Rub Rail: Ensure your measurement includes the rub rails, as they represent the true outermost physical boundary of the vessel.
  • Keep the Tape Straight: Pull the tape measure tight and perfectly straight in the air. Do not let the tape drape down into the boat or arch over consoles, windshields, or seating. A bent tape will give an artificially large measurement.
Diagram showing the correct straight-line measurement across a boat's beam

4. The Ground Measurement Method (Trailer/Dry Dock)

If your boat is on a trailer or dry dock and you cannot easily pull a tape measure straight across the deck, use the plumb bob method:

  • Locate the widest point of the boat along the rub rail on the port side.
  • Drop a plumb bob (or hold a straight edge perfectly vertically level) from that point down to the ground. Mark this exact spot on the floor with tape or chalk.
  • Repeat this process on the starboard side at the identical point.
  • Measure the straight-line distance between your two marks on the ground. This gives you a highly accurate beam measurement without having to fight consoles or windshields.