{
  "id": "reverb",
  "version": "053645e201b0",
  "status": "published",
  "name": "Reverb Calculator",
  "question": "What reverb time fits my tempo?",
  "summary": "Works out reverb pre-delay and decay times in milliseconds that fit a song’s tempo in BPM, for a bar, half note, quarter note or eighth note, straight, dotted or triplet.",
  "category": "everyday-life",
  "subcategory": "fun",
  "url": "https://www.acalculator.org/everyday-life/reverb-calculator",
  "markdown": "https://www.acalculator.org/everyday-life/reverb-calculator.md",
  "kind": "function",
  "method": "Beat = 60,000 ÷ BPM ms; total = beat × beats in the length × feel (1, 1.5 dotted, 2/3 triplet); pre-delay = beat × 4 ÷ N for a 1/N note; decay = total − pre-delay.",
  "assumptions": [
    "The beat is a quarter note and a bar is four beats, as in 4/4 time.",
    "Pre-delay plus decay fill the chosen musical length, so the tail ends on the beat.",
    "The pre-delay keeps its straight length; the feel changes only the reverb length."
  ],
  "inputs": {
    "$schema": "https://json-schema.org/draft/2020-12/schema",
    "type": "object",
    "properties": {
      "bpm": {
        "title": "Tempo (BPM)",
        "description": "The song’s tempo in beats per minute.",
        "type": "number",
        "minimum": 1,
        "maximum": 1000
      },
      "len": {
        "title": "Reverb length",
        "description": "How long the whole reverb should last: a bar for a hall, shorter for a room.",
        "type": "string",
        "enum": [
          "4",
          "2",
          "1",
          "0.5"
        ]
      },
      "feel": {
        "title": "Note feel",
        "description": "Straight, dotted (1.5 times as long) or triplet (2/3 as long).",
        "type": "string",
        "enum": [
          "straight",
          "dotted",
          "triplet"
        ]
      },
      "pre": {
        "title": "Pre-delay",
        "description": "The gap before the reverb starts, as a short note.",
        "type": "string",
        "enum": [
          "32",
          "64",
          "128",
          "none"
        ]
      }
    }
  },
  "outputs": {
    "decay": {
      "label": "Decay time",
      "description": "The length less the pre-delay.",
      "format": "quantity",
      "unit": "ms"
    },
    "predelay": {
      "label": "Pre-delay",
      "description": "One beat × the pre-delay note’s part of a beat.",
      "format": "quantity",
      "unit": "ms"
    },
    "total": {
      "label": "Total reverb time",
      "description": "One beat × beats in the length (× 1.5 dotted, × 2/3 triplet).",
      "format": "quantity",
      "unit": "ms"
    },
    "beat": {
      "label": "One beat",
      "description": "60,000 ÷ BPM ms, a quarter note.",
      "format": "quantity",
      "unit": "ms"
    },
    "table": {
      "label": "Decay for each length",
      "description": "The decay time for a bar, a 1/2, a 1/4 and a 1/8 note, with the same feel and pre-delay.",
      "format": "text"
    }
  },
  "defaultAnswer": {
    "inputs": {
      "bpm": 120,
      "len": "2",
      "feel": "straight",
      "pre": "64"
    },
    "outputs": {
      "decay": 0.96875,
      "predelay": 0.03125,
      "total": 1,
      "beat": 0.5,
      "table": "1 bar: 1,968.75 ms; 1/2 note: 968.75 ms; 1/4 note: 468.75 ms; 1/8 note: 218.75 ms"
    },
    "text": "At 120 BPM, set a pre-delay of 31.25 ms and a decay of 968.75 ms."
  },
  "examples": [
    {
      "given": {
        "bpm": 120,
        "len": "2",
        "feel": "straight",
        "pre": "64"
      },
      "expect": {
        "beat": 0.5,
        "predelay": 0.03125,
        "total": 1,
        "decay": 0.96875
      },
      "source": "TuneLens, Reverb Time Calculator (pre-delay about a 1/64 note; decay = the musical length less the pre-delay; a bar of 4/4 is 240,000 ÷ BPM ms), https://tunelens.com/reverb-time-calculator.html (retrieved 2026-10-03); SI Brochure (BIPM, 9th edition, 2019), Table 8: 1 min = 60 s; hand calculation in content.mdx: beat 60,000 ÷ 120 = 500 ms; pre-delay 500 ÷ 16 = 31.25 ms; 2 × 500 − 31.25 = 968.75 ms"
    },
    {
      "given": {
        "bpm": 90,
        "len": "4",
        "feel": "dotted",
        "pre": "128"
      },
      "expect": {
        "beat": 0.6666666666666666,
        "total": 4,
        "predelay": 0.020833333333333332,
        "decay": 3.9791666666666665
      },
      "source": "TuneLens, Reverb Time Calculator (pre-delay about a 1/64 note; decay = the musical length less the pre-delay; a bar of 4/4 is 240,000 ÷ BPM ms), https://tunelens.com/reverb-time-calculator.html (retrieved 2026-10-03); SI Brochure (BIPM, 9th edition, 2019), Table 8: 1 min = 60 s; hand calculation in content.mdx: beat 666.67 ms; 4 × 1.5 × 666.67 = 4,000 ms; pre-delay 666.67 ÷ 32 = 20.83 ms; decay 3,979.17 ms"
    },
    {
      "given": {
        "bpm": 128,
        "len": "1",
        "feel": "triplet",
        "pre": "none"
      },
      "expect": {
        "total": 0.3125,
        "decay": 0.3125,
        "predelay": 0
      },
      "source": "SI Brochure (BIPM, 9th edition, 2019), Table 8: 1 min = 60 s; hand calculation in content.mdx: beat 468.75 ms × 2/3 = 312.5 ms"
    }
  ],
  "sources": [
    "TuneLens, Reverb Time Calculator (a pre-delay of about a 1/64 note; the decay fills the rest of the musical length; a bar of 4/4 lasts 240,000 ÷ BPM ms). https://tunelens.com/reverb-time-calculator.html (retrieved 2026-10-03)",
    "BIPM, The International System of Units (SI Brochure), 9th edition, 2019, Table 8 (1 min = 60 s). https://www.bipm.org/en/publications/si-brochure (retrieved 2026-10-03)"
  ],
  "related": [
    "bpm"
  ],
  "changelog": []
}