DTMF (dual-tone multi-frequency) is the signalling a push-button phone uses for each key: two tones at once, one from a low group (697, 770, 852 or 941 Hz) and one from a high group (1209, 1336, 1477 or 1633 Hz). Press a key above to hear it, type a sequence to play or download it, or use the decoder to read keys from a recording.
How DTMF works
ITU-T Recommendation Q.23 defines 16 signals: the ten digits plus six more (*, # and A–D). Each is made of two frequencies sent together, one from each group of four. Because no frequency in one group is a harmonic of another, and speech rarely contains exactly one strong tone from each group at once, an exchange can tell key presses apart from voice. Q.23 requires each transmitted frequency to be within ±1.8% of nominal, with distortion products at least 20 dB below the tones.
Timing
ITU-T Q.24 lists the values used by several telephone administrations for receiving DTMF. All of them accept a tone lasting at least 40 ms. They ignore tones shorter than about 20–25 ms, and need a pause of at least 30–70 ms between keys. Their minimum dialling speeds are 93–125 ms per digit. Our default of 100 ms tones with 100 ms gaps sits comfortably inside all of them; the generator will not go below 40 ms tones or 30 ms gaps.
How our decoder works
The decoder looks at 25 ms slices of sound every 10 ms and measures the eight DTMF frequencies with the Goertzel algorithm, allowing for the ±1.8% tolerance in Q.23. A slice counts as a key when one low and one high tone stand clearly above the rest of their group, are within 10 dB of each other, and carry most of the sound. A key is logged after about 40 ms and must be followed by a 30 ms pause before it can repeat.
In our tests, it decoded all 16 keys from generated WAVs at 8, 44.1 and 48 kHz. It decoded keys with 40 ms tones and 30 ms gaps, ignored 20 ms and 25 ms bursts, and bridged a 10 ms break inside a key. It accepted tones 2.5% off frequency, rejected tones 3.5% off, and ignored a synthetic voice.
What are the A, B, C and D keys?
Q.23 calls the six signals beyond the ten digits "reserve" signals. * and # appear on ordinary phones; A–D exist in the standard but are rarely fitted to consumer phones. They are used in some radio and private systems.
Uses
- Testing phone menus and voicemail without a phone: play the digits into a call.
- Radio: many two-way radios and repeaters use DTMF for control and calling.
- Decoding a recording: find out which number was dialled in an audio clip, or check that a system sends the right tones.
- Teaching and electronics: hear what each pair sounds like, and compare against a decoder chip's output.
Downloading tones
Choose 8 kHz for a telephone-style file (the classic telephone sample rate), or 44.1/48 kHz for media. Files are 16-bit mono WAV, with 5 ms fades on each tone so they do not click.
For any other frequency, use the tone generator.
Common questions
What are DTMF tones?
The tones a push-button phone sends for each key. Each key is two tones played together, one from 697–941 Hz and one from 1209–1633 Hz, as set out in ITU-T Recommendation Q.23.
What frequencies are used for DTMF?
Low group 697, 770, 852 and 941 Hz; high group 1209, 1336, 1477 and 1633 Hz. Each key uses one of each, for example 1 is 697 + 1209 Hz and 0 is 941 + 1336 Hz.
How long should a DTMF tone be?
At least 40 ms, with at least 30–70 ms between keys, depending on the network, according to the receiving values listed in ITU-T Q.24. 100 ms tones with 100 ms gaps work everywhere.
Can I decode DTMF from a recording?
Yes. Use Open an audio file in the decoder. It reads WAV, MP3 and other formats your browser supports, entirely on your device.
What are the A, B, C and D keys?
Four of the six extra signals defined in ITU-T Q.23 beyond the ten digits. They are rarely on consumer phones but used in some radio and private systems.
Last reviewed 14 November 2026 by Sri Murugan Systems. Found a mistake? Tell us.