The reverberation time calculator above estimates RT60, the time a sound takes to decay by sixty decibels after its source stops, from the geometry of a room and how absorbent its surfaces are. It runs the classic Sabine equation and the Norris-Eyring refinement side by side, reports the total absorption in sabins, and works out how much extra absorbing area you would need to reach a target decay time. It handles both metric and imperial rooms, occupants and free-standing absorbers.
Arb Digital builds free calculators that show their working rather than a single unexplained number. Reverberation prediction is famously approximate, so this page reports the intermediate quantities — volume, surface area, sabins, mean absorption — that let you see where an implausible answer came from. It also explains the specific conditions under which the Sabine formula stops being trustworthy, which is the part most reverberation pages leave out.
What This Reverberation Time Calculator Does
The tool builds a rectangular room from your three dimensions, computes its volume and its six interior surfaces, and assigns an absorption coefficient to the floor, the ceiling and the walls. It then adds any free-standing absorber area you specify and an allowance for occupants, sums everything into a total absorption figure, and divides volume by absorption with the Sabine constant in front.
The hero gives the Sabine RT60. The subline gives the Eyring result alongside it, because the two diverge in a predictable direction and the size of the gap is itself diagnostic. The grid shows room volume, total surface area, total absorption in sabins, and the mean absorption coefficient, which is total absorption divided by total surface area and is the single number that decides whether Sabine is safe to use here.
Absorption coefficients are frequency-dependent, so an RT60 is only meaningful when you say at which frequency. The defaults in the fields are mid-frequency figures, roughly the 500 Hz to 1 kHz region where most published single-number ratings sit. To get a full picture, run the calculator several times with the coefficients for each octave band and note how differently the room behaves at 125 Hz and at 4 kHz.
How to Use It
- Set the unit system first. The Sabine constant changes with it, and mixing metres into a feet calculation inflates the answer by more than a factor of three.
- Enter the room dimensions. Check the reported volume and surface area against your own figures before trusting anything downstream.
- Set a coefficient for each surface group. Use manufacturer data measured to a recognised reverberation-room method where you have it, and treat the hints as order-of-magnitude starting points only.
- Add occupants and free-standing absorbers. People are among the most effective absorbers in any room, and an audience often dominates the total.
- Set a target and read the gap. The note reports how many sabins you are short or over, which converts directly into a panel area once you pick a product.
The Formula: How RT60 Is Calculated
Sabine's equation is RT60 = 0.161 V ÷ A in metric units, where V is room volume in cubic metres and A is total absorption in metric sabins. In feet the constant becomes 0.049 and the sabins are in square feet. Total absorption is the sum of every surface area multiplied by its absorption coefficient, plus any air absorption term 4mV.
The constant is not arbitrary. It comes from the speed of sound and the geometry of a diffuse sound field: 24 ln(10) divided by the speed of sound, which at the 343 m/s value given in section 17.2 of OpenStax University Physics Volume 1, Speed of Sound, works out at 0.161. Because the speed of sound rises with temperature, the constant is very slightly temperature-dependent, though the effect is far smaller than the uncertainty in any absorption coefficient. The sixty-decibel decay itself is a factor of one million in intensity, which connects to the decibel definition set out in section 17.3, Sound Intensity.
Work the defaults. A room 8 by 6 by 3 metres has a volume of 144 m³. Floor and ceiling are 48 m² each, and the walls total 2 × (8 + 6) × 3 = 84 m², giving 180 m² of surface. Absorption is 48 × 0.30 + 48 × 0.60 + 84 × 0.05 = 14.4 + 28.8 + 4.2 = 47.4 sabins. Sabine gives 0.161 × 144 ÷ 47.4 = 0.489 s. Mean absorption is 47.4 ÷ 180 = 0.263, and the Eyring version, which uses −S ln(1 − 0.263) = 55.0 in place of 47.4, gives 0.421 s.
Why Sabine and Eyring Disagree, and Which to Believe
Sabine's derivation assumes sound energy decays continuously, which is a good approximation when each reflection removes only a small fraction of the energy. Eyring's version replaces the mean absorption coefficient with −ln(1 − α), which accounts properly for the fact that absorption happens in discrete reflection events. The two agree closely when mean absorption is small and diverge sharply when it is large.
The practical rule is that below a mean absorption of about 0.2 the difference is a few per cent and either formula is fine. Between 0.2 and 0.4 the Eyring figure is generally closer to measurement. Above roughly 0.5, Sabine becomes badly optimistic — it predicts a finite reverberation time even for a room with a mean coefficient of 1.0, a perfectly absorbing room that in reality has no reverberant field at all. Eyring correctly sends RT60 to zero in that limit, and this calculator reports the Eyring value as undefined rather than negative when the mean coefficient reaches one.
Neither formula is a substitute for measurement. Both assume a diffuse field in which sound energy is uniformly distributed and arrives from all directions equally, and both assume absorption is spread evenly over the boundaries. A room with a heavily absorbing ceiling and hard walls, which is the commonest office configuration and close to this page's defaults, violates the even-distribution assumption and will usually measure longer than either prediction.
Where the Diffuse-Field Assumption Fails
Three room types break these formulas badly enough that the number should be treated as indicative only. The first is any room with one dimension much smaller than the other two — a low, wide open-plan floor, or a long corridor. Sound in these spaces decays along a preferred axis rather than filling the volume, and measured decay curves often show two distinct slopes rather than one straight line.
The second is a small room at low frequency. Below the Schroeder frequency, where individual room modes are separated rather than overlapping, the sound field is not statistical at all and the concept of a single reverberation time loses meaning. For a domestic-sized room that transition sits somewhere in the low hundreds of hertz, which is exactly the region where control-room and studio problems live. Modal spacing is a wavelength problem, and our wavelength calculator converts a room dimension into the frequencies whose half-wavelengths fit it.
The third is a room that is very dead in one part and very live in another, such as a hall with an upholstered seating block and bare stone walls. The average coefficient tells you almost nothing about what a listener in either zone experiences. Serious work in these cases uses ray-tracing or an in-situ measurement following an established impulse-response method; university acoustics groups such as the Penn State Graduate Program in Acoustics publish extensively on where statistical models break down and what replaces them.
What RT60 Values Mean in Practice
Longer is not worse and shorter is not better; the right value depends entirely on what the room is for. Speech intelligibility wants a short decay, because each syllable's tail masks the next one. Music generally wants a longer decay, because the overlap is what gives an ensemble body and blend. Recording control rooms want short and, more importantly, smooth decay so that the loudspeakers rather than the room define what the engineer hears.
Two rooms with identical RT60 can still sound completely different if their decay is uneven across frequency. A room that measures 0.6 s at 1 kHz but 1.4 s at 125 Hz sounds boomy and unclear even though its single-number rating looks respectable, and that pattern is extremely common because most cheap absorbers are thin and work poorly at low frequency. Running this calculator once per octave band, using the coefficient for that band, is the single most useful thing you can do with it.
Reverberation and loudness are separate problems. A long decay raises the steady-state level in a room because energy accumulates, but reducing reverberation does not solve a noise-exposure problem at source. For that side of the question, the noise exposure calculator deals with time-weighted dose, and the sound level converter moves between pressure, intensity and decibel scales.
How This Differs From the Site's Delay and Reverb Tool
The boundary in one sentence: this page predicts the physical decay time of a real room from its geometry and materials, while the delay and reverb time calculator converts a musical tempo into the pre-delay, decay and tempo-synced delay settings you dial into a plugin. One is architectural acoustics; the other is production timing, and they share only a word.
Related physics tools cover neighbouring ground. The inverse square law calculator handles the direct-sound part of the level at a listener, which is the component reverberation does not affect. The frequency and period calculator converts between the two quantities you need when working per octave band, and the rectangular prism calculator is useful for checking volume and surface area on a room that is not a simple box.
Arb Digital builds free tools like this one because useful pages earn attention. If you want tools, calculators or content built for your own audience, we can help.
Browse All Free Tools Talk to Arb DigitalCommon Mistakes to Avoid
- Mixing metres and feet — the Sabine constant differs by a factor of about 3.3 between systems, so a units error swamps every other source of error in the calculation.
- Quoting an RT60 without a frequency — absorption coefficients vary enormously across the spectrum, and a single number hides the low-frequency problem that usually causes the complaint.
- Trusting Sabine in a very dead room — above a mean coefficient of about 0.5 it predicts far too long a decay. Read the Eyring figure instead.
- Forgetting the occupants — an audience can easily double the absorption of a hall, so an empty-room measurement badly overstates what a full room will do.
- Assuming coefficients above one are errors — measured values slightly over 1.0 are routine because of edge diffraction in the test method, and clamping them to 1.0 understates the absorber.
Related Free Tools From Arb Digital
For production timing rather than room physics, use the delay and reverb time calculator. The sound level converter moves between sound pressure, intensity and decibels, the noise exposure calculator handles dose over a working day, and the inverse square law calculator covers direct-field level with distance. The wavelength calculator and the frequency and period calculator support per-band work, and the rectangular prism calculator checks volume and surface area. The full free online tools hub lists everything.
Frequently Asked Questions
It is the time a sound takes to fall by sixty decibels after the source stops, which is a factor of one million in intensity. In practice it is usually extrapolated from the first twenty or thirty decibels of decay, because background noise buries the rest.
Use Sabine when the mean absorption coefficient is below about 0.2, where they agree closely anyway. Above that, and especially above 0.5, the Eyring figure is the more realistic one because Sabine wrongly predicts a finite decay even in a fully absorbing room.
Because absorption coefficients do. Thin porous absorbers work well at high frequency and poorly at low frequency, so the same room can have a short treble decay and a long bass decay. Run the tool once per octave band to see the shape.
Twice as much, because RT60 is inversely proportional to total absorption. Halving a decay time means doubling the sabins, which is why treating a very live room is a much bigger job than people expect.
Yes, and substantially. A seated adult is worth roughly 0.45 metric sabins, so a hundred-person audience adds about 45 sabins, which is often more than all the fixed treatment combined.
Only in large volumes at high frequency, above about 2 kHz, where the 4mV term becomes comparable with surface absorption. In a domestic or office-sized room it is negligible and can be left at zero.
It is a useful first estimate, not a specification. Both formulas assume a diffuse sound field and evenly spread absorption, and real rooms with absorbing ceilings and hard walls routinely measure longer than predicted.
This tool is provided for educational and study use. Statistical reverberation formulas are approximations that assume a diffuse sound field, and a room intended for critical listening, speech intelligibility or regulatory compliance should be assessed by a qualified acoustic consultant using measured data.
