Room Reverb Time Calculator
Sabine RT60 from room size and absorption, in metric or imperial.
What it does
Reverberation time, called RT60, is the seconds a sound takes to fade by 60 decibels after the source stops. It sets whether a room feels crisp or echoey. Teachers, podcasters, and musicians check it before treating a space.
The tool above applies the Sabine formula to a box room. You enter length, width, height, and one average absorption number. It returns volume, total absorption in sabins, and RT60. Switch between metric and imperial at the top; the physics stays the same.
Sabine treats sound as spread evenly through the room. That fits mid size rooms with mixed surfaces: classrooms, offices, living rooms, and small halls. A single average coefficient keeps the math fast while staying close enough for planning treatment.
The result guides real choices. If RT60 lands near 0.4 s, speech stays clear. Near 1.0 s, a classroom starts to blur consonants. Above 2 s, a gym or pool hall needs large soft areas or panels before it works for talks.
How to use
- Pick metric (meters) or imperial (feet).
- Enter room length, width, and height as inside faces.
- Enter the average absorption coefficient from 0.02 (bare concrete) to 1.0 (open window).
- Read volume, total absorption, and RT60 in seconds.
- Compare with the table below: classrooms aim near 0.4 to 0.6 s, living rooms near 0.3 to 0.5 s.
- If the time runs high, add soft area or panels and run the numbers again.
Formula
- V
- Room volume: cubic meters (metric) or cubic feet (imperial).
- A
- Total absorption in sabins: surface area times average coefficient.
- 0.161
- Metric constant from the speed of sound. Imperial uses 0.049.
V is room volume and A is total absorption in sabins (surface area times average coefficient). The constants 0.161 and 0.049 carry the speed of sound in metric and imperial units, so both forms agree. Keep absorption below 1.0 per surface and use average values for mixed finishes.
Worked example
Example: 6 m by 4 m by 3 m room
A rectangular room measures 6 m by 4 m by 3 m with average absorption 0.20. Here is the full path to the answer.
- Volume: 6 x 4 x 3 = 72.0 m3.
- Surface: 2 x (24 + 18 + 12) = 108.0 m2.
- Absorption: 108.0 x 0.20 = 21.6 m2 sabins.
- RT60: 0.161 x 72.0 / 21.6 = 0.54 s.
- Read: speech stays clear under about 0.6 s, so this room suits talks and classes.
Reference table
| Room | Target RT60 | Note |
|---|---|---|
| Bedroom or office | 0.3 to 0.5 s | Dry enough for calls |
| Classroom | 0.4 to 0.6 s | Speech stays clear |
| Living room | 0.3 to 0.5 s | Comfortable talk and TV |
| Music practice room | 0.6 to 0.9 s | Some bloom helps tone |
| Gym or pool hall | 1.5 to 2.5 s untreated | Treat before speeches |
Limits
- Box rooms only. L shapes, vaults, and coupled spaces need deeper methods.
- Single average coefficient. Real rooms mix glass, wood, and fabric.
- Sabine drifts high when average absorption passes about 0.40.
- Low bass modes and flutter echoes sit outside this one number.
Common mistakes
- Guessing absorption at 0.90 for a whole bare room, which forces an unreal low time.
- Measuring outside wall faces instead of inside faces.
- Mixing meters with the 0.049 imperial constant.
- Ignoring the audience: people add real absorption.
- Chasing 0.2 s in a living room and making it sound dead.
Try it yourself
Quick checks. Answers sit tucked under each question.
RT60 for 5 m by 3.5 m by 2.8 m at 0.25?
Volume is 49.0 m3, absorption is 20.7 m2 sabins, RT60 is 0.38 s. Quick check: 0.161 x 49.0 / 20.7 = 0.38 s.
RT60 for 16 ft by 12 ft by 9 ft at 0.25?
Volume is 1,728.0 ft3, absorption is 222.0 ft2 sabins, RT60 is 0.38 s.
Questions
How do I get the volume for an odd shaped room?
For a box room, multiply length by width by height. For L shaped rooms, split into boxes, add the parts, and use the total. Small errors in volume shift RT60 in direct proportion.
What absorption coefficient should I pick?
Painted drywall sits near 0.05, wood near 0.10, carpet near 0.30, and thick curtains or acoustic panels from 0.50 to 0.90. Area weight the parts: multiply each surface by its own coefficient, then add.
When does Sabine lose accuracy?
Sabine assumes a diffuse field and modest absorption. Above about 0.40 average absorption, or in tiny booths with heavy treatment, Eyring or measured data fits better.
Does a bigger room always ring longer?
Yes. RT60 scales with volume over absorption, so doubling the room while keeping surfaces the same roughly doubles the time. That is why gyms ring and small treated studios sound dry.
How does furniture change the result?
Furniture adds absorption and breaks up flutter. A bare room calculation is the upper bound. Add rugs, shelves with books, and soft seats, then recheck with a higher average coefficient.
How can I check the number by ear?
Clap and listen for a rough check, or record a balloon pop and read the decay in an audio editor. For disputes or paid work, hire an acoustician with a calibrated meter.
Sources
- Acoustical Society of America
- NIST: SI units and acoustics resources
- OpenStax University Physics: sound and hearing