How long is a dit? Working out Morse timing from WPM
A dit lasts 1200 divided by the speed in words per minute, in milliseconds. At 20 WPM that is 60 ms, at 12 WPM 100 ms, and at 5 WPM 240 ms. Every other part of Morse is a whole number of dits: a dah and the gap between letters are 3, the gap between words is 7, and the gap inside a letter is 1.
So once you know the dit, you know everything. The rest of this page works that division out for every speed you are likely to meet, and turns it around.
Where 1200 comes from
Morse speed is measured with the standard word PARIS, which is exactly 50 units long when you count its dits, dahs, inside gaps, letter gaps and the word gap after it. At W words per minute you send 50 × W units in 60,000 milliseconds. Divide one by the other and a unit, which is one dit, lasts 60,000 ÷ 50W, or 1200 ÷ W.
The full count of PARIS is in What WPM means in Morse code, and the PARIS standard. The formula:
- Dit = 1200 ÷ WPM milliseconds
- Dah = 3 dits, and so is the gap between letters
- Word gap = 7 dits
- Gap between the elements of one letter = 1 dit
Dit, dah and gap lengths from 5 to 40 WPM
Rounded to the nearest millisecond (the dit to one decimal place where it does not divide evenly):
| Speed | Dit | Dah | Letter gap | Word gap |
|---|---|---|---|---|
| 5 WPM | 240 ms | 720 ms | 720 ms | 1680 ms |
| 8 WPM | 150 ms | 450 ms | 450 ms | 1050 ms |
| 10 WPM | 120 ms | 360 ms | 360 ms | 840 ms |
| 12 WPM | 100 ms | 300 ms | 300 ms | 700 ms |
| 13 WPM | 92.3 ms | 277 ms | 277 ms | 646 ms |
| 15 WPM | 80 ms | 240 ms | 240 ms | 560 ms |
| 18 WPM | 66.7 ms | 200 ms | 200 ms | 467 ms |
| 20 WPM | 60 ms | 180 ms | 180 ms | 420 ms |
| 25 WPM | 48 ms | 144 ms | 144 ms | 336 ms |
| 30 WPM | 40 ms | 120 ms | 120 ms | 280 ms |
| 35 WPM | 34.3 ms | 103 ms | 103 ms | 240 ms |
| 40 WPM | 30 ms | 90 ms | 90 ms | 210 ms |
A few things stand out once it is laid out like this.
Double the speed and every length halves. A dit at 10 WPM is 120 ms, at 20 WPM 60 ms, at 40 WPM 30 ms. The proportions never change, so a correctly sent letter has the same shape at every speed, only shorter.
The steps shrink as you go up. Going from 5 to 10 WPM takes 120 ms off every dit. Going from 30 to 40 WPM takes off only 10 ms. An error of 10 ms that would pass unnoticed in a 240 ms dit at 5 WPM is a third of a 30 ms dit at 40 WPM, which is why a fast operator’s timing has to be so much more precise than a beginner’s.
In standard timing, a dah and a letter gap are the same length. One is sound and one is silence, which is easy to forget when you are sending by hand. If your dahs are a healthy 3 dits but your letter gaps are 1 or 2, your letters will run together. The 1, 3, 7 rule goes through what each wrong gap does to the text.
Working backwards: the speed from a dit
The same formula turned around gives the speed from a measured dit: WPM = 1200 ÷ the dit in milliseconds. A 100 ms dit is 12 WPM; a 75 ms dit is 16 WPM; a 50 ms dit is 24 WPM.
One dit is hard to measure well, because the edges of a tone are fuzzy. It is more reliable to measure a run of elements and divide. If you can open a recording in an audio editor and select from the start of a character to its end:
- Pick a character whose length you know in units. The figure 5,
....., is five dits and four inside gaps: 9 units. The figure 0,-----, is five dahs and four gaps: 19 units. - Measure the selection in milliseconds.
- Divide by the units to get the dit, then divide 1200 by the dit.
So a 5 that measures 720 ms has an 80 ms dit, and was sent at 15 WPM. A 0 that measures 1,140 ms has a 60 ms dit: 20 WPM.
The units of any character are easy to work out: twice the number of dits plus four times the number of dahs, minus one. B, -..., is 6 + 4 minus 1 = 9 units. Q, --.-, is 2 + 12 minus 1 = 13.
How long a whole message takes
Add up the units, multiply by the dit, and you have the time. SOS is a good example, because it is a prosign: sent as ...---..., one continuous character of six dits, three dahs and eight inside gaps of 1 unit, with no letter gaps. That is 6 + 9 + 8 = 23 units.
| Speed | SOS as a prosign (23 units) |
|---|---|
| 5 WPM | 5.52 seconds |
| 12 WPM | 2.3 seconds |
| 20 WPM | 1.38 seconds |
Sent instead as three separate letters, ... --- ..., the two 3-unit letter gaps make it 27 units. The extra 4 units come from those two gaps, each 2 units longer than the 1-unit gap the prosign has in its place, and they are what turn one signal into three ordinary letters.
What Farnsworth spacing changes, and what it does not
Farnsworth spacing does not change the length of a dit. The characters are sent at the full character speed: dits, dahs and the 1-unit gaps inside each letter keep the lengths in the table above. Only the gaps between letters and between words get longer, until PARIS takes as long as it would at the slower effective speed.
Here is what that looks like with characters at 18 WPM, using the usual way of sharing out the extra time between the 19 units of letter and word gaps in PARIS, which keeps them in their 3 to 7 proportion:
| Effective speed | Dit | Letter gap | Word gap |
|---|---|---|---|
| 18 WPM (no stretch) | 66.7 ms | 200 ms | 467 ms |
| 15 WPM | 66.7 ms | 305 ms | 712 ms |
| 12 WPM | 66.7 ms | 463 ms | 1081 ms |
| 10 WPM | 66.7 ms | 621 ms | 1449 ms |
| 8 WPM | 66.7 ms | 858 ms | 2002 ms |
| 5 WPM | 66.7 ms | 1568 ms | 3660 ms |
Compare the 5 WPM row with plain 5 WPM in the first table. In both, PARIS takes 12 seconds. With plain timing, the dit is 240 ms and each letter is drawn out. With Farnsworth timing, the dit is 66.7 ms and each letter is quick and whole, with long silences around it to think in. That is the reason the method exists. Farnsworth spacing: what it is and when to drop it covers how to shrink those gaps as your copying improves.
Feeling the length without a stopwatch
Numbers in milliseconds are hard to feel. At slow-motion speeds a metronome helps: set it to 50 × your WPM beats per minute and it ticks once per unit. That is 150 beats a minute for 3 WPM and 200 for 4 WPM; 5 WPM is already 250, at or past the top of many metronomes, so treat this as a drill for proportions, not speed. Tap a pencil on the table in time: a dit is one tick, a dah fills three, a letter gap is three silent ticks and a word gap seven. A few minutes of that makes the proportions physical in a way a table cannot. Then put the metronome away; at real speeds you want to hear rhythm, not count.
If you would rather have the arithmetic done for you on a Mac, Dot & Dash times its dits by the same formula, 1.2 ÷ WPM seconds, at any speed you set from 5 to 40 WPM. With the straight key it reads your keying at your own speed, learning the length of your dit as you go. After each letter in a lesson it draws each of your dits and dahs against the length it should have been and shows the speed you keyed it at.
To choose where on this table to start, see What speed should you learn Morse code at?.
Questions
How long is a dit at 20 WPM?
60 milliseconds: 1200 divided by 20. A dah and a gap between letters are 180 ms, and a gap between words is 420 ms.
How long is a dah?
Three dits. At 5 WPM a dah lasts 720 ms, at 20 WPM 180 ms and at 40 WPM 90 ms.
Does Farnsworth spacing change the length of a dit?
No. With Farnsworth spacing the dits, dahs and the gaps inside each character stay at the character speed. Only the gaps between characters and between words get longer.
How do I work out the speed of a recording?
Measure a character whose length you know in units, such as the figure 5 at 9 units, divide by the units to get the dit in milliseconds, then divide 1200 by the dit. The answer is the speed in WPM.