BPM & Delay Calculator

The tempo counts quarter notes per minute, the way DAWs count it. Standard MIDI Files store it the same way, as microseconds per quarter note. Every other number follows from the length of one quarter note.

quarter note (ms) = 60,000 ÷ BPM

1/n note = quarter note × 4 ÷ n

dotted = plain note × 1.5

triplet = plain note × 2/3

bar = quarter note × 4 × top number ÷ bottom number

Hz = 1,000 ÷ ms

samples = ms × sample rate ÷ 1,000

At 120 BPM a quarter note is 60,000 ÷ 120 = 500 ms, and an eighth is half that, 250 ms. A dotted eighth is 250 × 1.5 = 375 ms. An eighth-note triplet is 250 × 2/3 = 166.67 ms. A bar of 6/8 holds 6 × 4 ÷ 8 = 3 quarter notes, so it lasts 1,500 ms.

Every value is worked out as one exact fraction, then rounded once for display, half away from zero. Milliseconds show 2 decimals (3 below 1 ms) and Hz 4 significant digits. Samples show as a whole number when the note lasts a whole number of samples, and with 2 decimals when it doesn’t, so you can see when a delay falls between samples. The tempo keeps up to 3 decimals. The CSV download keeps every digit.

You can type a decimal with a point or a comma, so 0.125 and 0,125 are the same time. A number such as 1,000 can mean one thousand or exactly one. Where both would fit the field, the page asks which you mean and shows no result until you choose. When it reads your number in a different form from the one you typed, a line under the field says so, for example “Read as 0.125 ms.”

To go from a delay time back to the tempo, turn the formula around: BPM = 60,000 × quarter notes ÷ ms. 375 ms is a quarter note at 160 BPM, an eighth at 80 BPM and a dotted eighth at 120 BPM.

Reverb starting points

The reverb sizes follow a common rule of thumb. The whole reverb, pre-delay plus decay, lasts one note value at your tempo. The pre-delay is a much shorter note value. The tail then fades out in time with the song.

Size	Total	Pre-delay
Hall	2 bars	1/32 note
Large room	1 bar	1/64 note
Small room	1/2 note	1/128 note
Tight ambience	1/4 note	1/512 note

Decay = total − pre-delay. Treat these as starting points: a busy mix often wants a shorter decay, and a sparse one can take more. The character of the reverb matters as much as its length. Bars follow your time signature. When a bar is a half note or shorter (2/4, 3/8, 5/16), the Large room lasts the fewest whole bars longer than a half note and the Hall twice that, so a larger room is always longer.

Compressor timing

A release that matches a note value lets the gain recover in time with the groove: a 1/16 note for punch, a 1/8 note to breathe with the beat, a dotted 1/8 note to pump and come back just before the next quarter-note kick, and a 1/4 note for smooth glue on a bus. Attack shapes the transient more than the groove. A 1/64 note (31.25 ms at 120 BPM) lets the hit through and a 1/256 note (7.81 ms) catches more of it. These are starting points; set the final values by ear and on the gain-reduction meter.

LFO rates

An LFO synced to a note value completes one cycle per note. Its rate in Hz is 1,000 ÷ the note length in ms, and cycles per bar = bar length ÷ note length. The preview starts each cycle at its peak, on the beat.

Swing

Swing is shown as a percentage, as many drum machines and DAWs show it. It is the share of each pair of steps given to the first step. 50% is straight, 66.67% is a triplet shuffle (2 : 1) and 75% a dotted feel (3 : 1).

offbeat delay = (swing − 50%) × 2 × step length

With 16ths at 120 BPM a step lasts 125 ms. At 60% swing, every second 16th lands 0.10 × 2 × 125 = 25 ms late.

Tempo changes and pitch

Speeding audio up by resampling, the way a turntable or a sampler without time-stretch does, raises the pitch along with the tempo. One equal-tempered semitone is a speed ratio of 2^(1/12) ≈ 1.0595.

semitones = 12 × log₂(new BPM ÷ old BPM)

one semitone up = BPM × 2^(1/12)

From 120 to 128 BPM the ratio is 16/15, so the pitch rises 12 × log₂(16/15) = 1.12 semitones (111.73 cents). Time-stretching changes the tempo and keeps the pitch.

Video frames and song length

Frames = seconds × frame rate. The NTSC rates are exact fractions: 23.976 fps is 24,000 ÷ 1,001 and 29.97 fps is 30,000 ÷ 1,001. So a beat at 120 BPM lasts 14.985 frames at 29.97 fps, not 15. To fit N bars into T seconds, the tempo is N × quarter notes per bar × 60 ÷ T.

Speed of sound

c = 331.3 × √(1 + T ÷ 273.15) m/s

T is the air temperature in °C. A temperature typed in °F is converted first.

This is the speed of sound in dry air: 343.21 m/s at 20 °C, so 1 ms of delay matches about 34.3 cm. Humid air is very slightly faster. To line up a speaker or a second mic, delay the closer one by the time sound needs to cover the difference in distance.

The previews

Delay previews play a short note on the first beat of two bars, over a click on every quarter note. Each echo is as loud as the feedback setting says, 50% of the sound before it by default (6 dB quieter), and comes from the left and the right in turn. Echoes stop once they are 60 dB down or the preview ends; a delay of a bar or longer gets one note. Compare feels plays the selected note straight, then dotted, then as a triplet, each with its echoes. LFO previews play a held tone whose volume rises and falls at the LFO rate. Reverb previews send a short burst of noise through a reverb made on the spot: random noise that fades by 60 dB over the decay time, after the pre-delay. A decay longer than 10 seconds is shortened to 10, and the page says so. Swing previews play two bars of hi-hats on the steps you chose, with a kick and a snare; the swing counts from each bar line. Hear both tempos plays a four-note phrase at your tempo, then the same phrase resampled to the new tempo, so you hear the pitch move. Previews last 12 seconds at most, peak between about −18 and −21 dBFS at the default volume and stay below 0 dBFS at full volume and any feedback.

Sources: Tempo, Dotted note, Tuplet, Cent (music), Frame rate and Speed of sound on Wikipedia; the Standard MIDI File 1.0 specification for the quarter-note tempo (see the Library of Congress Standard MIDI File format description).