What WPM means in Morse code, and the PARIS standard
In Morse code, words per minute means how many times you could send the standard word PARIS, with its proper spacing, in one minute. PARIS is exactly 50 units long, so 20 WPM is 1,000 units a minute and every unit, the length of one dit, lasts 60 milliseconds. A WPM figure is really a way of naming how long a dit is.
That is why two operators can agree on “15 WPM” without sending the same text. The number describes the clock the Morse runs on, not how many real words went past.
Why Morse needs a standard word
In print, a word is a word. In Morse, words have wildly different lengths, because characters do. E is a single dit, 1 unit. Q, --.-, is 13 units. A word made of short letters flies by, and a word made of long ones drags.
Here is how much four ordinary words differ, each counted with its letter gaps and the 7-unit word gap that follows it:
| Word | Units, with the gap after it |
|---|---|
| THE | 24 |
| PARIS | 50 |
| HELLO | 56 |
| QUICK | 62 |
If speed were measured by counting real words, a minute of THE THE THE would score more than twice as high as a minute of QUICK QUICK QUICK at exactly the same dit length. So Morse picks one word and measures everything against it. The standard word is PARIS. Whatever the reasons it was first chosen, its 50 units make the arithmetic easy. (MORSE, as it happens, is also exactly 50 units.)
PARIS, counted out
You can count any character in units without drawing it out. Every dit is 1 unit, every dah is 3, and there is a 1-unit gap between each pair of elements. That comes to a neat shortcut: twice the dits, plus four times the dahs, minus one.
| Letter | Code | Dits | Dahs | Units |
|---|---|---|---|---|
| P | .--. |
2 | 2 | 4 + 8 minus 1 = 11 |
| A | .- |
1 | 1 | 2 + 4 minus 1 = 5 |
| R | .-. |
2 | 1 | 4 + 4 minus 1 = 7 |
| I | .. |
2 | 0 | 4 minus 1 = 3 |
| S | ... |
3 | 0 | 6 minus 1 = 5 |
The letters add up to 31 units. Between five letters there are four letter gaps of 3 units each, which is 12 more. Then comes the word gap of 7, because in a steady stream of .--. .- .-. .. ... / .--. .- .-. .. ... each PARIS is followed by a gap before the next one. 31 + 12 + 7 = 50.
The word gap is the part people forget. Leave it out and PARIS is 43 units, and every speed you work out from it comes out wrong.
From WPM to milliseconds
Once you know PARIS is 50 units, the rest is one line of arithmetic. At W words per minute you send 50 × W units every 60,000 milliseconds, so one unit lasts 60,000 ÷ (50 × W), which simplifies to 1200 ÷ W milliseconds.
Some worked examples:
| Speed | Units per minute | One unit (a dit) |
|---|---|---|
| 5 WPM | 250 | 240 ms |
| 13 WPM | 650 | about 92 ms |
| 20 WPM | 1,000 | 60 ms |
| 25 WPM | 1,250 | 48 ms |
It runs backwards just as easily: WPM = 1200 ÷ the dit in milliseconds. If you measure a recording and find its dits are 75 ms long, it was sent at 16 WPM. A dah is 3 units, so a 240 ms dah means an 80 ms dit and 15 WPM. How long is a dit? Working out Morse timing from WPM has the full table from 5 to 40 WPM, and the 1, 3, 7 rule covers how the gaps fit around the dits and dahs.
A real message is not PARIS
Because the standard only fixes the unit, real text sent “at 15 WPM” rarely moves at exactly 15 words a minute. Take HELLO WORLD:
- HELLO: H 7, E 1, L 9, L 9, O 11, plus four letter gaps of 3, makes 49 units.
- WORLD: W 9, O 11, R 7, L 9, D 7, plus four letter gaps, makes 55 units.
- With the 7-unit gap between the words, the message is 111 units.
At 15 WPM a unit is 80 ms, so HELLO WORLD takes 111 × 80 = 8,880 ms, just under 9 seconds. Two PARIS words with their gaps would be 100 units, or 8 seconds. Sent over and over, HELLO WORLD at 15 WPM would get through only about 12.7 words a minute, because its letters are longer than PARIS’s.
None of this means the sender was slow. The dits, dahs and gaps were all exactly 15 WPM lengths. WPM tells you the tempo; how many words fit in a minute depends on which words they are.
This is also why a speed you measure by counting words copied in a minute will drift around with the text. If you want a fair number, count units, or measure the length of a dit. How to measure your Morse code speed walks through both.
Two speeds at once: character speed and effective speed
Farnsworth spacing uses two WPM numbers, and both are still measured with PARIS.
- The character speed sets the length of the dits, dahs and the gaps inside each character.
- The effective speed is how many PARIS words actually fit in a minute once the gaps between characters and words have been stretched.
Take 18 WPM characters with an effective speed of 8 WPM. Each dit lasts 1200 ÷ 18, about 67 ms, exactly as at plain 18 WPM. But one PARIS now has to fill 60 ÷ 8 = 7.5 seconds instead of 3.3. The five letters themselves, at 31 units of about 66.7 ms, take about 2.07 seconds. The remaining 5.43 seconds are shared out over the gaps: four letter gaps and one word gap, which in standard timing are 12 and 7 units, 19 in all. Keeping that 3 to 7 proportion, each letter gap becomes about 858 ms and the word gap about 2 seconds.
That is the usual way of working out Farnsworth gaps, and the check is easy: the letters plus four letter gaps plus one word gap add back up to 7.5 seconds, so PARIS still runs at exactly 8 WPM. Farnsworth spacing: what it is and when to drop it covers why you would learn this way and when to close the gaps.
If you practice on a Mac, Dot & Dash handles the timing for you. It follows the PARIS standard, with a dit lasting 1.2 ÷ WPM seconds, and Settings sets the speed from 5 to 40 WPM with Farnsworth spacing as a separate, slower effective speed. After each letter you key in a lesson it shows the speed you actually keyed it at, in WPM.
A footnote: CODEX and code groups
PARIS is the standard, but speeds for random code groups are sometimes quoted against a different word: CODEX, which is 60 units. Its letters are C -.-. 11, O --- 11, D -.. 7, E . 1 and X -..- 11, which is 41 units, plus 12 of letter gaps and 7 of word gap.
A longer word suits random groups, because they use every letter equally often and many of the rarer letters are long ones. Averaged over all 26 letters, a letter is about 8.2 units, against 6.2 for PARIS’s letters and exactly 8.2 for CODEX’s. Five random letters with their gaps come to about 60 units, which is what CODEX is.
So the same dit can carry two labels. A 50 ms dit is 24 WPM by PARIS (1200 ÷ 50) but 20 WPM by CODEX (1000 ÷ 50, since a CODEX unit is 60,000 ÷ 60W). When someone quotes a speed for five-letter groups, it is worth asking which word they meant. Unless it says otherwise, assume PARIS.
Questions
Why is PARIS used to measure Morse speed?
Real words vary so much in length that counting them would give a different speed for every text. PARIS is one fixed word of exactly 50 units, including the gap after it, so it pins down how long a unit lasts at any WPM.
How do I turn WPM into the length of a dit?
Divide 1200 by the WPM to get the dit in milliseconds: 60 ms at 20 WPM, 100 ms at 12 WPM. A dah and a letter gap are three times that, and a word gap seven times.
Does 20 WPM mean I am sending 20 real words a minute?
Not exactly. 20 WPM fixes the length of every dit, dah and gap; how many real words fit in a minute depends on whether they are made of short letters or long ones.