Reverb Time Calculator: Formula and Room Example

Calculate reverberation time (RT60) with the Sabine formula: enter a room’s length, width, height, and average absorption coefficient to estimate how long sound takes to decay by 60 decibels. The calculator returns seconds, using metric dimensions in metres and a coefficient from 0 to 1.

Formula

The calculator uses the Sabine equation for a room with a reasonably diffuse sound field:

RT60 = 0.161 × V ÷ (ᾱ × S)

• RT60 is estimated reverberation time in seconds.
• V is room volume in cubic metres (m³), calculated as length × width × height.
• S is the total area of the six room surfaces in square metres (m²): 2 × (length × width + length × height + width × height).
• ᾱ is the average sound absorption coefficient of those surfaces, a dimensionless value from 0 (idealized total reflection) to 1 (idealized total absorption).
• 0.161 is the Sabine constant for metric units, with the result expressed in seconds.

For the simple calculator, one average coefficient represents the combined room surfaces. A more detailed calculation uses the absorption coefficient and area of each material, summed as equivalent absorption area: A = Σ(αᵢ × Sᵢ). Then RT60 = 0.161 × V ÷ A. The average coefficient in this calculator is effectively A ÷ S.

Worked Example

Using the defaults—length 8 m, width 6 m, height 3 m, and average absorption coefficient 0.2:

1. Find the volume: V = 8 × 6 × 3 = 144 m³.
2. Find the total surface area: S = 2 × (8 × 6 + 8 × 3 + 6 × 3) = 2 × 90 = 180 m².
3. Calculate: RT60 = 0.161 × 144 ÷ (0.2 × 180) = 23.184 ÷ 36 = 0.644 seconds.

Rounded to two decimal places, the estimated reverb time is 0.64 seconds.

What Reverb Time Means

Reverberation is the persistence of sound in a space after its source stops. Sound reflects from floors, ceilings, walls, and objects; each reflection loses some energy through absorption or transmission. RT60 is the time a sound level would take to drop by 60 dB under a suitable decay measurement. It is a standardized description of a room’s decay, not the time until every reflection becomes literally inaudible.

A shorter RT60 generally means sound dies away more quickly; a longer one means more lingering reflections. The useful value depends on the room’s purpose and the sound being considered. Speech intelligibility often benefits from controlled decay, while some musical uses may call for a more reverberant character. There is no single ideal RT60 for every room, and the calculator does not determine a target.

How to Choose the Inputs

Measure the room’s internal length, width, and height, using the same unit for all three. This version accepts metres. For US customary measurements, convert feet to metres by multiplying each dimension by 0.3048; alternatively, use a consistent imperial form of the Sabine equation rather than mixing constants and units.

The harder input is ᾱ. It is not a universal property of a room: it depends on the materials, their areas, furnishings, and the sound frequency. A bare, hard room tends to have lower absorption than one with substantial absorptive furnishings, but assigning a reliable number requires material absorption data. If data are available, calculate A = Σ(αᵢ × Sᵢ), then use that total directly in RT60 = 0.161V/A. Do not enter the total absorption area in the coefficient field.

Absorption coefficients are frequency-dependent. A material may absorb low frequencies differently from high frequencies, so a single average can hide important differences. For a useful estimate, use coefficients for the frequency range of interest and account for major surfaces and furnishings rather than guessing one number.

When to Use the Estimate—and Its Limits

The Sabine estimate is useful for a first-pass comparison of room shapes or absorption changes. If dimensions and absorption remain otherwise similar, increasing absorption reduces the calculated RT60. Adding absorptive treatment can therefore change the estimate, but this calculator cannot tell you how much treatment to install unless its absorption and area are known.

Real rooms do not always meet the assumptions behind Sabine’s equation. It works best when sound energy is reasonably spread through the room and absorption is not extremely high or uneven. Small rooms, rooms with strong echoes or unusual shapes, and spaces with highly irregular absorption can produce measured decay that differs from the estimate. Air absorption, openings, seating, people, and other contents can also matter, especially in larger spaces or at higher frequencies.

For practical decisions, measurements are more informative than a geometry-only estimate. Room acoustics measurement can report decay across frequency bands, revealing whether a room’s low- and high-frequency behavior differs. The calculator is a planning aid, not a substitute for a measured result when precision matters.

Common Calculation Mistakes

• Mixing units: Using feet for dimensions with the metric constant 0.161 gives a wrong result. Convert dimensions or use a consistent imperial formula and units.
• Confusing coefficient and absorption area: The input ᾱ is dimensionless. A is measured in square metres of equivalent absorption; they are related by A = ᾱS but are not interchangeable.
• Assuming one coefficient fits all frequencies: Absorption changes with frequency, so a single value is only a broad approximation.
• Overreading decimal places: The output is rounded to two decimals, but uncertain material data may make the actual estimate much less precise.
• Assuming RT60 is a universal quality score: A longer or shorter decay is not automatically better; suitability depends on the room’s use and the frequency range.

Frequently Asked Questions

What is the formula for reverberation time?
For the Sabine estimate in metric units, RT60 = 0.161V/A, where V is room volume in m³ and A is equivalent absorption area in m². If using an average coefficient, A = ᾱS.

What does RT60 stand for?
RT60 is the time associated with a 60-decibel sound-level decay after the source stops. It describes the room’s reverberant decay rather than a guaranteed time for sound to become inaudible.

Can I calculate reverb time from room dimensions alone?
Dimensions give the room volume and surface area, but not how much sound those surfaces absorb. You also need an absorption coefficient or equivalent absorption area for a meaningful estimate.

Does adding carpet always reduce reverb time?
Absorptive materials can reduce decay, but the amount depends on their area, construction, placement, and frequency-dependent absorption. A small area of treatment may have little effect on the room’s overall estimate.

Is Sabine’s formula accurate for every room?
No. It is a simplified estimate and works best when sound is reasonably diffuse and absorption is not unusually high or uneven. Measurement is preferable when a dependable room-specific result is needed.

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in l | Room length | m | 8 | 0
in w | Room width | m | 6 | 0
in h | Room height | m | 3 | 0
in a | Average absorption coefficient | - | 0.2 | 0
out rt | Reverb time RT60 | s | 0.161 * l * w * h / (a * (2 * (l * w + l * h + w * h))) | 2
out area | Total surface area | m² | 2 * (l * w + l * h + w * h) | 2
out volume | Room volume | m³ | l * w * h | 2