Room Reverb Time Calculator

Why some rooms sing and others shout

Walk into an empty tiled bathroom and clap: the sound rings on, bright and obvious. Walk into a carpeted bedroom full of books and curtains and clap: the sound dies almost instantly, dull and short. Both rooms received the same clap, but their reverberation — the way sound reflects, scatters and decays — is completely different. Reverberation time, written RT60, measures that difference with a single number: the seconds it takes for sound to fall by 60 decibels after the source stops. It is the most useful first measurement in room acoustics, because it predicts whether speech will be intelligible, music will sound warm or muddy, and a recording will sound professional or echoey.

This calculator estimates RT60 from room dimensions and an average absorption coefficient, using the classic Sabine formula. You do not need acoustic training to use it: measure the room, judge how absorptive its surfaces are on average, and read the verdict. A furnished living room, a bare concrete garage and a treated home studio sit at very different points on the scale, and the presets on the tool let you feel those differences instantly. The result will not replace a professional acoustic survey, but it will tell you whether your room is broadly in the right range or badly off, and what kind of treatment would move it.

The Sabine formula explained

Wallace Sabine, working at Harvard around 1900, discovered that reverberation time depends on just two things: room volume and total absorption. The formula is RT60 = 0.161 × V ÷ A, where V is volume in cubic metres and A is total absorption in sabins. Total absorption is the sum over every surface of its area times its absorption coefficient, a number from 0 (perfectly reflective) to 1 (perfectly absorptive). This calculator simplifies that sum by using one average coefficient for the whole room: A = total surface area × average coefficient. The total surface area of a rectangular room is 2 × (length × width + length × height + width × height). That simplification is honest for a first estimate and slightly optimistic for rooms with one very hard wall and one very soft wall, where the average hides the imbalance.

Notice the scale of the effect. Doubling a room's volume doubles RT60 if absorption stays the same, which is why cathedrals ring for seconds while cupboards do not ring at all. Doubling absorption halves RT60, which is why adding a rug and curtains to a bare room transforms it so dramatically. The formula also explains why small rooms are hard: their volume is small, which should shorten RT60, but their surface area is also small, so a few square metres of treatment can swing the result a long way. Small-room acoustics are therefore sensitive to exactly where treatment goes, not just how much there is.

A worked example

Take a room 5 metres long, 4 metres wide and 3 metres high, furnished like an ordinary living room with an average coefficient of 0.2. Volume is 5 × 4 × 3 = 60 cubic metres. Surface area is 2 × (20 + 15 + 12) = 94 square metres. Total absorption is 94 × 0.2 = 18.8 sabins. RT60 = 0.161 × 60 ÷ 18.8 = 0.51 seconds. That is a comfortable result for conversation, television and casual music: clear enough for speech, live enough not to sound dead. Strip the room bare to a coefficient of 0.05 and absorption falls to 4.7 sabins, pushing RT60 to 2.06 seconds — a shouting, echoey space where speech blurs. Treat it heavily to 0.5 and RT60 falls to 0.21 seconds, very dry and ideal for voice recording but possibly dull for music.

Now a larger example: a 10 by 8 by 4 metre hall with a coefficient of 0.3. Volume is 320 cubic metres, surface area is 2 × (80 + 40 + 32) = 304 square metres, absorption is 91.2 sabins, and RT60 = 0.161 × 320 ÷ 91.2 = 0.56 seconds. Despite being more than five times the volume of the living room, the hall's RT60 is similar because its absorption grew with its size. This is the key insight of the Sabine formula: reverberation is a ratio, not a size. Large rooms are not automatically echoey; they are echoey when their surfaces are too reflective for their volume.

What RT60 targets to aim for

• Voice recording / podcasting: 0.2–0.4 seconds. Dry, intimate, easy to mix.
• Classrooms and meeting rooms: 0.4–0.6 seconds. Speech must stay intelligible at the back.
• Home listening / practice rooms: 0.4–0.8 seconds. A little life without muddiness.
• Small concert / rehearsal spaces: 0.8–1.2 seconds. Warmth and blend for ensembles.
• Churches and concert halls: 1.5–2.2 seconds. Reverberation is part of the music itself.

Practical ways to fix a room

• If RT60 is too high, add absorption at reflection points: thick curtains over windows, a rug on a hard floor, panels on bare walls.
• Break up parallel hard walls with bookshelves or diffusers to kill flutter echo, the rapid ringing between two flat surfaces.
• Do not over-treat: a room below 0.2 seconds feels oppressive and makes musicians play louder to hear themselves.
• Treat the ceiling too in tall rooms; a hard ceiling over a treated floor leaves a long vertical reverberation path.
• Remember bass: thin foam absorbs treble but not bass. Thick panels, air gaps and bass traps are needed for low-frequency control.

Common mistakes and limits

The biggest mistake is entering a single surface's coefficient as the room average — for example, using 0.9 for acoustic panels when most of the room is plaster. Estimate the average honestly: mostly hard surfaces mean 0.05–0.15, a normally furnished room means 0.15–0.30, and a heavily treated room means 0.4 or more. The second mistake is expecting one number to describe all frequencies: real RT60 is longer at low frequencies and shorter at high ones, and this calculator gives a mid-frequency estimate. The third limit is shape: Sabine's formula assumes a diffuse sound field, and long corridors, L-shaped rooms and domes violate that assumption. Use the result as a guide, listen to the room, and adjust treatment by ear as well as by number.

Reverberation, echo and flutter: not the same thing

People often use reverberation, echo and flutter interchangeably, but they describe different experiences. Reverberation is the dense, blended wash of many reflections arriving so quickly that they merge into a smooth decay — the sound of a hall. Echo is a single distinct reflection arriving late enough to be heard separately, typically after more than about 50 milliseconds, as in a canyon or a long empty corridor. Flutter echo is a rapid, metallic ringing between two parallel hard surfaces, common in stairwells and empty rectangular rooms, where sound bounces back and forth many times. RT60 describes reverberation well, but it does not fully capture a strong echo or a flutter, which is why a room can measure acceptably yet still sound unpleasant. Treating flutter usually means breaking up one of the parallel surfaces with absorption or diffusion, rather than simply adding more absorption everywhere.

Listening to your result

After calculating, clap in the room and compare what you hear with the number. A 0.5 second room should sound clear with a short, pleasant tail; a 2 second room should sound obviously ringy. If the number and your ears disagree wildly, check your absorption estimate first — it is the most subjective input. Reverberation is not good or bad in itself: a cathedral without it would sound wrong, and a podcast studio with it sounds amateur. The goal is the right reverberation for the job, and this calculator is the fastest way to find out where your room stands.