RT60 Calculator – Sabine Reverberation Time
Enter the room volume and the absorption of its surfaces (area × absorption coefficient) to estimate the reverberation time with the Sabine equation. Lengths, volumes and areas can be entered in meters or feet, and you can also find the total absorption needed for a target reverberation time.
The Sabine equation
The reverberation time (RT60) is the time it takes for the sound pressure level to fall by 60 dB after the sound stops. For a room it can be estimated from the volume and the amount of absorption with the Sabine equation:
T60 = 0.161 × V ÷ A
- V: room volume in cubic meters (m³)
- A: total absorption in square meters, called metric sabins
- A = Σ (area × absorption coefficient) + other absorption
The reference read for this page (Wikipedia, "Reverberation") gives a constant of about 0.1611 s/m, derived from the speed of sound at 20 °C. Another page (Wikipedia, "Sabin (unit)") gives 0.161. This calculator uses the widely used 0.161 (the difference is about 0.06%). The sum over surfaces (Σ) follows from the definition of the average absorption coefficient; it is this calculator's own working and is not written in the references.
Units: how meters and feet are handled
All calculations are done in SI units (meters, square meters, cubic meters) with the constant 0.161. Values you enter in feet are converted first with exact definitions, with no rounding:
| Unit | In meters |
|---|---|
| 1 ft | 0.3048 m (defined) |
| 1 ft² | 0.09290304 m² |
| 1 ft³ | 0.028316846592 m³ |
When you change a unit, the numbers you have already entered are converted so that they still describe the same physical quantity. For example, 120 m³ becomes about 4,237.76 ft³. The calculator keeps the exact value behind the scenes, and the number shown in the box is only rounded for display. The rounded number is never used to calculate again, so switching units back and forth, or many times, does not change the result. If you type in the box, the number you type is used.
The reverberation time does not depend on the unit system. The total absorption and the volume are always shown in both: metric sabins (m²) and sabins (ft²) for absorption, and m³ and ft³ for volume.
About the constant 0.049
Some US sources write the Sabine equation as T60 = 0.049 × V ÷ A with V in cubic feet and A in square feet. This calculator does not use a separate 0.049 formula. Converting the 0.161 formula exactly gives 0.161 × 0.3048 = 0.0490728, so a result from a formula with 0.049 is about 0.15% smaller. The references read for this page do not state an imperial constant, so 0.0490728 is a value derived here, not one quoted from them.
What is a sabin?
The sabin is a unit of sound absorption. One square foot of a perfectly absorbing surface is one (imperial) sabin, and one square meter of a perfectly absorbing surface is one metric sabin (Wikipedia, "Sabin (unit)"). So 1 metric sabin = 1 m² = about 10.7639 sabins. Total absorption in metric sabins is the same quantity as the A in the equation above.
Absorption coefficient
The absorption coefficient is a number from 0 to 1: the share of the sound that hits a surface and is absorbed instead of reflected back into the room. A value near 0 means a hard, reflective surface; 1 means the surface absorbs everything (an open window is the classic example).
- It changes with frequency. It is usually reported for octave bands centered at 125, 250, 500, 1000, 2000 and 4000 Hz. To find the reverberation time at several frequencies, calculate once for each band with that band's coefficients.
- This calculator takes values from 0 to 1. Coefficients measured in a reverberation room can come out above 1 because of edge effects, according to the reference. If you have such a value, check the manufacturer's data and how it was measured.
- Typical coefficients for materials are not listed here. Use the data from the manufacturer of the material.
Assumptions and limits
What the references say:
- The Sabine equation does not take room shape into account, nor the losses from sound traveling through the air (which matter in large spaces).
- It tends to over-predict the reverberation time of small rooms with a lot of absorption. For such rooms the Eyring equation is more accurate; the two become practically the same in very live rooms.
- At higher frequencies, absorption by the air itself becomes significant.
A general assumption that the references read do not state: the equation is an idealization in which the sound is spread evenly through the room (a diffuse sound field). Real measurements can differ, and the error grows as the room becomes more absorptive. No numeric threshold is given because the references do not give one.
This is a rough estimate. Do not use it as a basis or a guarantee for the design of a studio or a concert hall; use a professional and measurements.
When the total absorption is 0
If the total absorption is 0 (every entered absorption is 0), the denominator of the equation is 0 and the reverberation time would be infinite, so the result is shown as not available.
Required total absorption
"Required total absorption" works the equation backward: A = 0.161 × V ÷ target reverberation time. It only calculates the absorption needed. It does not check whether a real room can reach it (for example, whether the average absorption coefficient would exceed 1). In rooms with a lot of absorption the error of the equation is larger.
Examples
| Input | Result |
|---|---|
| 100 m³; 10 m² × 0.5 and 30 m² × 0.5 (A = 20 m²) | About 0.81 s (0.161 × 100 ÷ 20 = 0.805) |
| The same room in feet: 3,531.47 ft³; 107.64 ft² × 0.5 and 322.92 ft² × 0.5 (A = 215.28 ft²) | The same, about 0.81 s |
| 100 m³, target 0.8 s | Required total absorption 20.125 m² (about 216.62 ft²) |
What is not covered
The Eyring and Millington equations, absorption by the air, automatic calculation for every frequency band, T20 and T30 measurements, room shape and diffusion, and a list of material coefficients are not covered. For the resonant frequencies of a room, see the Room Modes Calculator. For how sound weakens with distance, see the Sound Distance Attenuation Calculator. For wavelength and frequency, see the Sound Wavelength Calculator.
Frequently asked questions
Where do I find absorption coefficients?
The data sheets of absorbing materials and building products list them, usually per octave band. No typical values are given here.
The result does not match my room.
The Sabine equation is an estimate under idealized conditions. Room shape, how the sound spreads, absorption by the air and uneven placement of absorbers all make real values differ.
How do I get the reverberation time for each frequency?
Calculate once per frequency band, using the absorption coefficients for that band.
Why does the number in the box change when I switch units?
So that it keeps describing the same quantity: 120 m³ becomes about 4,237.76 ft³. The exact value is kept internally, so switching back gives 120 m³ again.