What does RMS stand for? In front of a ship's name, as in RMS Titanic, it stands for Royal Mail Ship, sometimes given as Royal Mail Steamer. The prefix showed that the vessel held a contract to carry mail for the British postal service.
The same letters mean something very different in a math class or on an audio amplifier: root mean square, a way to measure the typical size of a value that keeps changing, like household voltage or sound power. Below you will find both meanings, plus a few others from police and business work.
Quick facts
Stands for (ships)
Royal Mail Ship or Royal Mail SteamerStands for (math, science)
Root mean squareOther meanings
Records management systemType
InitialismHow to say it
R M S, one letter at a timeRelated terms
HMS, USS, SS
What does RMS stand for in Titanic?
The Titanic's full name was RMS Titanic. The letters show it was a Royal Mail Ship: the British postal service paid the shipping line to carry letters and parcels across the Atlantic. Ships that earned the contract were allowed to use the prefix, and it was a mark of status, because the mail had to arrive on schedule.
| Letter | Word |
|---|---|
| R | Royal |
| M | |
| S | Ship (or Steamer) |
The Titanic did carry thousands of sacks of mail and had a sorting room on board staffed by postal clerks. It sank on its first voyage in April 1912. Another famous example is the RMS Lusitania, also an ocean liner on the Atlantic route.
RMS vs HMS, USS and SS
Ship prefixes look alike but carry different meanings:
- RMS: Royal Mail Ship, a passenger or cargo ship with a British mail contract. It was not a navy ship.
- HMS: His or Her Majesty's Ship, a vessel of the British Royal Navy.
- USS: United States Ship, a commissioned vessel of the US Navy.
- SS: steamship, a general label for a steam powered ship.
A common mistake is to think the Titanic was a royal or military ship. It was a privately owned liner. The "Royal" only refers to the Royal Mail.
Root mean square in math and electricity
In science, RMS stands for root mean square. To find it, you square each value, take the mean (average) of the squares, and then take the square root. The steps match the words, read backward: square, mean, root.
This matters when a value swings above and below zero. A plain average of household alternating current would come out near zero, which tells you nothing. The RMS value gives the useful size instead. When people say US outlets supply about 120 volts, that is an RMS figure. Learn more about the current types on the AC and DC pages.
On an amplifier spec sheet: "50 watts RMS per channel." This is the steady power the amp can deliver, a more honest number than a short peak rating.
Other RMS meanings by field
| Field | Stands for | Example |
|---|---|---|
| History, ships | Royal Mail Ship | RMS Titanic |
| Math, physics, audio | Root mean square | 120 volts RMS |
| Police and public safety | Records management system | An officer files a report in the RMS |
| Offices and IT | Records management system | Storing contracts for a set number of years |
In a police or office setting, a records management system is software that stores reports and documents so they can be searched later.
How to tell which RMS you are reading
- Followed by a ship's name: Royal Mail Ship.
- Next to volts, watts, amps or a formula: root mean square.
- In a police report, job posting or IT ticket: records management system.
All of them are said letter by letter. When you write the ship prefix, put it before the name with no periods: RMS Titanic, not R.M.S. Titanic, although older printed sources sometimes used periods. For more short forms that show up in history and the news, see the famous acronyms list.
Questions people ask
Why was it called RMS Titanic?
The ship had a contract to carry British mail, which earned it the Royal Mail Ship prefix. The letters signal that mail duty, not royal ownership.
Does any ship still use RMS?
A small number of ships have kept the prefix as a tradition. It is far less common now, since most international mail travels by air.
What does RMS mean in electricity?
It means root mean square, the effective value of an alternating current or voltage. It lets you compare AC to steady DC power on equal terms.
Is RMS Titanic the same as SS Titanic?
People sometimes write SS Titanic, but the ship's proper prefix was RMS. SS simply means steamship and was not its official designation.
What does RMS stand for in police work?
Usually records management system, the database where an agency stores reports, case files and incident records.