More Free Maritime Training Tools
Everything you need to master maritime visual communications — all free, all browser-based.
Learn the International Code of Signals quickly. Test your knowledge of flag hoists and their exact meanings.
Start Training →Compare US Inland rules against International COLREGs side-by-side in seconds.
Compare Rules →Master every semaphore arm position with a dynamic, interactive visual study aid.
Launch Animator →Generate crisp, scalable vector graphics of any maritime signal flag for your projects.
Generate SVGs →Why Maritime Flag Size Calculator Is the Reference Every Mariner Needs
Correct vessel flag sizing follows a precise international convention — one inch of fly per foot of LOA for the national ensign, with separate ratios for courtesy flags, signals, and secondary pennants. The NauticalCore Maritime Flag Size Calculator applies the right formula for your vessel class automatically. Enter your vessel length, get the correct ensign dimensions mapped to commercially available standard sizes. No manual arithmetic, no chart lookups.
This tool handles the critical distinction between powerboat and sailboat sizing conventions, supports both imperial and metric inputs, and covers courtesy flag proportions for vessels entering foreign waters. It’s built for real maritime use — not a generic flag aspect-ratio tool designed for graphic designers.
Built for Maritime Flag Sizing Standards, Not General Use
Every result is grounded in accepted maritime flag etiquette and vessel flag sizing rules used by deck officers, maritime cadets, and port authorities. 100% free, browser-based, no login required.
Built for Every Level of Maritime Training
Lock in the one-inch-per-foot rule before your oral assessments. This tool reinforces how fly-to-hoist ratios shift by vessel type and flag category — tested knowledge in most officer certification programs.
Confirm ensign and courtesy flag dimensions during pre-departure checks, especially when rotating between vessel classes. Correct flag display is part of proper seamanship and port etiquette on every watch.
Use this as a live reference in flag etiquette lessons. It gives students immediate, accurate output and reinforces the underlying sizing calculation they’ll be expected to apply independently at sea.
Frequently Asked Questions
How do I calculate the correct maritime flag size for my vessel?
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The standard is one inch of flag fly per foot of overall vessel length (LOA) for the national ensign. Secondary flags on powerboats use five-eighths of an inch per foot; on sailboats, secondary signals are sized at half an inch per foot of mast height above the waterline. The calculator applies the correct formula automatically based on your vessel type and input.
What is the difference between flag fly and flag hoist?
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The fly is the horizontal measurement from the hoist edge to the free outer edge — the dominant visible dimension at distance. The hoist is the vertical edge attached to the halyard or pole. Maritime sizing rules specify fly length, and the calculator rounds results to the nearest commercially available standard size.
Are there legal requirements for maritime flag size?
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The U.S. Coast Guard does not mandate specific dimensions for recreational vessels, but accepted sizing conventions are part of maritime etiquette and port authority expectations in most jurisdictions. Commercial vessels operating under SOLAS may face stricter flag state requirements depending on their registry.
Do sailboats use the same flag sizing rules as powerboats?
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No — for sailboats, the reference dimension shifts from hull length to mast height above the waterline, and the ratio for secondary flags is half an inch per foot of mast height. This distinction is a common source of error on oral examinations and is one reason the calculator asks for vessel type before returning results.
How big should a courtesy flag be relative to the national ensign?
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A courtesy flag should be approximately half an inch on the fly for every foot of vessel length — visibly smaller than the national ensign in all dimensions. It is typically flown from the starboard spreader when operating in foreign waters.
Where should flags be displayed on a vessel?
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The national ensign takes the aftmost position of honour — on the flagstaff at the stern or the gaff on sailing vessels. Courtesy flags fly from the starboard spreader. Burgees and club pennants go to the masthead or bow pulpit. Position and sizing are both part of correct flag etiquette examined in officer certification programs.
Can I use this for ICS signal flag sizing as well?
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This calculator covers ensign and courtesy flag sizing by vessel LOA and type. For International Code of Signals flag training — individual flag meanings, combined hoists, and flag identification drills — use the NauticalCore ICS Signal Flag Trainer.
You’ve Sized the Flags.
Now Train on a Real Bridge.
The free tools cover the reference knowledge. NauticalCore Pro Simulators put it under pressure — the Free Ship Handling Simulator (Whitehorse v1.2), NavLights 3D Visualizer, and Acoustic Whistle Analyzer. Beta access is limited to the first 500 users.