Hull Speed Calculator
Fill in the values and the answer updates as you type.
How the Hull Speed Calculator works
Hull speed in knots = 1.34 × the square root of the waterline length in feet. The speed-length ratio is your speed divided by that same square root, so a ratio of 1.34 means you are at hull speed. A displacement boat makes a bow wave and a stern wave as long as its own waterline, and past this speed it has to climb its own wave, which costs a lot of extra power. Planing hulls and long slender hulls can go beyond it. Use the waterline length, not the overall length. Example: with Waterline length 30, Length unit ft, and Your cruising speed 6 kt, the result is Hull speed (kt) = 7.3; Hull speed (km/h) = 13.6; Hull speed (mph) = 8.4.
Inputs and results
The Hull Speed Calculator takes 3 inputs and gives 6 results.
What you enter
- Waterline length — starts at 30.
- Length unit — starts at ft. Choose from: ft and m.
- Your cruising speed (kt) — starts at 6.
What you get
- Hull speed (kt)
- Hull speed (km/h)
- Hull speed (mph)
- Speed-length ratio at your speed
- Your speed as a share of hull speed
- What this means
Example: Hull Speed Calculator with the starting values
With waterline length 30, length unit ft and your cruising speed 6 kt, the calculator returns:
| Hull speed (kt) | 7.3 |
|---|---|
| Hull speed (km/h) | 13.6 |
| Hull speed (mph) | 8.4 |
| Speed-length ratio at your speed | 1.1 |
| Your speed as a share of hull speed | 81.75% |
| What this means | Displacement mode, below hull speed |
Change any value above and the results update straight away.
Hull Speed Calculator FAQ
How do I use the Hull Speed Calculator?
Enter waterline length, length unit and your cruising speed in the fields, then read hull speed (kt), hull speed (km/h), hull speed (mph) and speed-length ratio at your speed 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 Hull Speed Calculator calculate?
Maximum displacement speed of a boat from its waterline length. It returns hull speed (kt), hull speed (km/h), hull speed (mph) and speed-length ratio at your speed from the 3 values you provide.
What values does the Hull Speed Calculator start with?
It opens with waterline length 30, length unit ft and your cruising speed 6 kt 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 Hull Speed 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 Hull Speed 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.