Reverb time can be estimated from a room’s volume and the sound energy its surfaces absorb. A common starting point is the Sabine equation: T = 0.161 × V ÷ A, when volume is in cubic metres and total equivalent absorption area is in square-metre sabins. The result is an estimate in seconds, not a measurement of the room.
To calculate it, find the room volume, estimate the absorption contributed by each surface, add those contributions, and use the total in the equation. The steps below show how.
What does reverb time measure?
Reverberation time describes how quickly sound decays in an enclosed space after the source stops. It is commonly discussed as the time for the sound level to fall by 60 dB, often written as RT60. The actual measurement procedure is separate from a formula-based estimate.
This guide is about room reverberation time. It does not calculate a reverb plug-in’s decay setting or automatically measure a room through a microphone. For a quick room estimate, use the Reverb Calculator or the RT60 Calculator.
What information do you need?
Before starting, gather:
- The room’s length, width, and height, measured in the same unit.
- The area of each surface type, such as walls, floor, ceiling, doors, or large treated sections.
- An absorption coefficient for each surface at the frequency band you want to estimate.
- A consistent unit system for room volume, surface area, and the formula constant.
Absorption is frequency-dependent. A single coefficient produces a simplified estimate; it cannot show how the room behaves across the full frequency range. The Absorption Coefficients guide explains how to use coefficient data, and the Absorption Calculator can help total surface contributions.
How to calculate reverb time step by step
Step 1: Calculate room volume
For a rectangular room, multiply its internal dimensions:
V = length × width × height
For example, a room that is 6 m long, 5 m wide, and 3 m high has a volume of:
V = 6 × 5 × 3 = 90 m³
Use internal dimensions. If the room has a sloped ceiling, alcoves, or an irregular shape, a simple length-by-width-by-height calculation may only approximate its volume. Divide a complex room into simpler sections and add their volumes when that is practical.
Step 2: List the surfaces and their areas
Record the area of each surface that contributes to the room’s absorption. Separate surfaces when their materials or treatments differ. For instance, do not combine an untreated wall with a large absorptive panel if their coefficients are different.
For a rectangular room, the combined wall area is:
Wall area = 2 × (length × height) + 2 × (width × height)
The floor and ceiling areas are each:
Floor or ceiling area = length × width
Adjust these areas for openings or distinct surface sections when you have reliable measurements. Avoid counting the same area twice.
Step 3: Estimate total sound absorption
For each surface, multiply its area by its absorption coefficient. Then add the contributions:
A = α₁S₁ + α₂S₂ + … + αₙSₙ
Here, A is the total equivalent absorption area, α is a surface’s absorption coefficient, and S is that surface’s area. In metric calculations, A is expressed in square-metre sabins.
Use coefficients measured for the relevant frequency band and a mounting condition that reasonably matches the installation. A published value for a material may not match the result after it is installed in a particular way. If the source gives values by octave band, calculate a separate estimate for each band rather than mixing values from different bands.
Step 4: Apply the Sabine equation
With metric units, the common Sabine form is:
T = 0.161 × V ÷ A
- T = estimated reverberation time in seconds
- V = room volume in cubic metres
- A = total equivalent absorption area in square-metre sabins
Keep units consistent. The coefficient changes when using an imperial-unit form. Do not enter room volume in cubic feet while using the metric constant.
Worked example
Suppose a rectangular room is 6 m × 5 m × 3 m. Its volume is 90 m³. For a simple illustration, assume these frequency-specific coefficients have already been selected for the example:
| Surface | Area | Example coefficient | Absorption contribution |
|---|---|---|---|
| Walls | 66 m² | 0.05 | 3.30 m² sabin |
| Floor | 30 m² | 0.02 | 0.60 m² sabin |
| Ceiling | 30 m² | 0.30 | 9.00 m² sabin |
| Total | 12.90 m² sabin |
These coefficients are illustrative inputs, not universal values for those materials. Real values depend on the specific surface, frequency, and installation.
Now substitute the example totals:
T = 0.161 × 90 ÷ 12.90 ≈ 1.12 seconds
The result is an estimate for the assumptions entered. It does not mean every position or frequency in the room will have exactly the same decay time.
Should you use Sabine or Eyring?
The Sabine equation is a common estimate for rooms with relatively lower average absorption. The Eyring equation accounts differently for repeated sound-energy loss and is often considered when average absorption is higher. Neither formula removes the need to check whether its assumptions fit the room.
See the RT60 Formula guide for a comparison, and the separate Sabine Formula and Eyring Formula pages for each method. The Ideal RT60 guide can help you interpret a result against the room’s intended use.
Why a calculated result may differ from a measured result
A formula uses simplified room information. A real room may have unevenly distributed absorption, non-rectangular geometry, openings, furniture, or frequency-specific behavior that a single average value does not describe. A calculated value is useful for planning and comparison, but it is not a substitute for a standardized acoustic measurement.
ISO 3382-2 describes methods for measuring reverberation time in ordinary rooms. This page’s equation-based example is not an ISO measurement. Read Why Actual Reverb Differs from a Calculator for more on the limits of an estimate.
Common calculation mistakes
- Mixing metric dimensions with an imperial formula constant.
- Using a coefficient from the wrong frequency band or installation condition.
- Treating one material’s coefficient as the value for an entire mixed surface.
- Counting surface area twice or forgetting a large area with a different treatment.
- Reading an estimated RT60 as a measured result or as a complete description of room sound.
Frequently asked questions
What is the formula for calculating reverb time?
A common metric estimate uses the Sabine equation: T = 0.161 × V ÷ A, where V is room volume in cubic metres and A is total equivalent absorption area in square-metre sabins.
Can I calculate reverb time without measuring equipment?
You can estimate it from room dimensions and absorption data. A formula-based estimate does not measure the room’s actual decay; measurement requires an appropriate sound source, measurement setup, and analysis procedure.
How do I calculate total absorption?
Multiply each surface area by its absorption coefficient and add the results: A = Σ(αᵢSᵢ). Use data that corresponds to the surface and frequency band you are estimating.
Does reverb time change with frequency?
It can. Materials absorb different amounts of sound at different frequencies, so estimates made with frequency-specific data may differ from one band to another.
Is RT60 the same as a plug-in’s reverb decay setting?
They both describe time, but they refer to different tasks. RT60 is a room-acoustics reverberation-time measure or estimate. A plug-in decay control sets the behavior of an audio effect. For production settings, see DAW Reverb Settings.
Noah Bennett is an audio engineering writer and acoustics specialist at Reverb Calculator. He focuses on reverb time calculation, room acoustics, studio sound design, and music production tools for producers, audio engineers, musicians, and home studio creators.