Calculating reverb time—the RT60 of a room—tells you exactly how acoustically live or dead your space is. If you’re treating a studio, designing a listening room, or trying to understand why your recordings sound the way they do, calculating RT60 is a practical, quantifiable step.
You have three main options: work through the math using the Sabine formula, use an online calculator, or measure the decay physically in your room. Each approach has pros and cons depending on how much precision you need and how much time you want to invest.
Method 1: Calculate Using the Sabine Formula (Step-by-Step)
The Sabine formula is the acoustic industry standard for estimating RT60:
RT60 = 0.161 × V / A
Here’s what each variable means:
- V is your room’s volume in cubic meters (length × width × height)
- A is the total sound absorption in sabins (a measure of how much sound each surface absorbs)
- 0.161 is a constant that accounts for the speed of sound and the logarithmic scale
To use this formula, follow these steps:
Step 1: Measure Your Room Dimensions
Get the length, width, and height of your room in meters. Measure from corner to corner for length and width, and from floor to ceiling for height. If your room is irregular (angled walls, alcoves), break it into simpler rectangular sections and calculate each separately, then add the volumes together.
Example: A room that’s 4 meters long, 5 meters wide, and 3 meters tall.
Volume = 4 × 5 × 3 = 60 cubic meters
Step 2: Identify All Surfaces and Their Materials
List every surface in your room: walls (north, south, east, west), floor, and ceiling. For each surface, note what material it is—drywall, plaster, carpet, hardwood, tile, concrete, etc.
If you have acoustic treatment (foam panels, bass traps, fabric-covered absorbers), note their material and coverage area. If you have furnishings (couch, curtains, bookshelves), note those too—they affect absorption.
Step 3: Find the Absorption Coefficient for Each Material
Each material absorbs sound differently depending on the frequency. Absorption coefficients are typically listed at six standard frequencies: 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, and 4 kHz.
For a quick estimate, you can use the 1 kHz absorption coefficient as an average. Here are some common values at 1 kHz:
- Concrete or tile: 0.01–0.03
- Drywall (painted): 0.05–0.10
- Brick: 0.03–0.05
- Glass: 0.03–0.05
- Hardwood floor: 0.05–0.15
- Carpet on concrete: 0.20–0.40
- Fabric-covered foam (2 inches): 0.50–0.80
- Mineral wool or fiberglass board: 0.60–0.90
Refer to detailed absorption coefficients for common materials to get more precise values. Many acoustic material manufacturers also publish absorption specs for their products.
Step 4: Calculate the Absorption Area for Each Surface
For each surface, multiply its surface area in square meters by its absorption coefficient:
Absorption = Surface Area (m²) × Absorption Coefficient
Example: A drywall wall that’s 4 m long and 3 m tall has an area of 12 m². If drywall has an absorption coefficient of 0.08 at 1 kHz:
Absorption = 12 × 0.08 = 0.96 sabins
Do this for every surface in the room.
Step 5: Sum the Total Absorption
Add up all the absorption values from step 4. This gives you the total absorption in sabins.
Example room breakdown:
- North wall (drywall, 12 m²): 0.96 sabins
- South wall (drywall, 12 m²): 0.96 sabins
- East wall (drywall, 15 m²): 1.20 sabins
- West wall (drywall, 15 m²): 1.20 sabins
- Floor (carpet, 20 m²): 8.0 sabins
- Ceiling (drywall, 20 m²): 1.60 sabins
Total Absorption (A) = 0.96 + 0.96 + 1.20 + 1.20 + 8.0 + 1.60 = 13.92 sabins
Step 6: Apply the Sabine Formula
Now plug your room volume and total absorption into the formula:
RT60 = 0.161 × 60 / 13.92 = 0.692 seconds
This tells you that your room’s reverberation time is approximately 0.7 seconds—relatively dry, good for recording or mixing.
Method 2: Use an Online RT60 Calculator
If doing the math sounds tedious, an online calculator removes the guesswork. Use the RT60 calculator to enter your room dimensions and materials. The tool applies the Sabine formula automatically and gives you an instant result.
Most calculators let you:
- Input room length, width, and height
- Select surface materials from a dropdown list (the calculator looks up their absorption coefficients)
- Adjust surface areas if you have partial coverage (e.g., one wall is half drywall, half glass)
- See results for multiple frequencies if you want to understand how RT60 varies across the spectrum
Using a calculator is faster and reduces arithmetic errors. The results are just as accurate as the manual method—you’re using the same formula, just delegating the math.
Method 3: Measure RT60 Physically in Your Room
The most direct way to know your room’s actual RT60 is to measure it. This involves playing a sound and observing how quickly it decays.
The Clap Method (Rough Estimate)
Clap sharply in your room. Listen to how long the clap’s reverb tail persists. Does it die away in half a second, one second, two seconds? This gives you a ballpark sense. It’s not precise, but it’s quick and free.
Using a Sound Level Meter and Tone
Generate a loud impulse or white noise burst in your room using a speaker. Using a sound level meter (or a smartphone app), measure the initial sound level and note the time it takes for the level to drop 60 dB. This is your measured RT60. This method is more precise than clapping but requires equipment.
Using Audio Software (Impulse Response Measurement)
Use acoustics measurement software (like Room EQ Wizard, if you have an audio interface and a calibrated measurement microphone) to generate an impulse response. The software measures the sound decay across multiple frequencies and calculates RT60 precisely. This is the most accurate method but requires gear and technical knowledge.
For most people, using a calculator or the clap method is sufficient. If you’re treating a professional studio, you might hire an acoustician with measurement equipment to get exact data.
Which Method Should You Use?
Use the Sabine formula calculation if you want to understand the theory or predict RT60 before you’ve built out your room (e.g., planning acoustic treatment for a new studio).
Use an online calculator if you want the speed and convenience of the formula without the arithmetic.
Use physical measurement if you want to verify that your actual room matches predictions or if you’re troubleshooting acoustic issues.
Most workflows start with a calculator estimate, then validate with a quick clap test or sound level measurement once the room is built. If predictions don’t match reality, you may have room modes (acoustic resonances) or non-uniform absorption that skews the result.
Common Mistakes When Calculating RT60
One frequent error is forgetting to include all surfaces. Remember that walls, ceiling, and floor all contribute. If you leave out the floor in your calculation, you’ll overestimate RT60.
Another mistake is using the wrong absorption coefficient for a material. Drywall’s absorption varies widely depending on whether it’s painted, perforated, or resonant. Always specify the exact product. Explore a full list of absorption values before calculating.
Some people use only the 1 kHz absorption coefficient and assume it applies uniformly across all frequencies. In reality, absorption is frequency-dependent—bass frequencies are absorbed less easily than midrange. For a more accurate prediction of bass behavior, look at the 125 Hz and 250 Hz coefficients specifically.
Finally, forgetting to account for contents is common. An empty room and a furnished room have very different absorption values. Curtains, bookcases, and upholstered furniture all absorb sound. Include them in your calculation if they’re part of the permanent setup.
Frequently Asked Questions
Why does my measured RT60 not match my calculated RT60?
Calculated RT60 using the Sabine formula is an estimate. Real rooms have standing waves (room modes) at certain frequencies that resonate and extend decay time at those frequencies. Non-uniform absorption and irregular room shapes also skew results. Expect a difference of ±10–15% between calculated and measured values.
Should I calculate RT60 at multiple frequencies?
For a rough estimate, using a single absorption coefficient at 1 kHz is fine. For professional acoustic design, you should calculate (or measure) RT60 at 125 Hz, 250 Hz, 500 Hz, 1 kHz, 2 kHz, and 4 kHz. Different frequencies have different decay rates, and this tells you which frequencies are under- or over-treated.
What’s the difference between Sabine and Eyring formulas?
The Sabine formula works best for rooms with low to moderate absorption. The Eyring formula is more accurate for rooms with high absorption (above about 20% of surface area). For most home studios and untreated rooms, Sabine is fine. If you’re heavily treated with foam and absorption, Eyring may give a better estimate. Learn the Eyring formula for high-absorption spaces.
Can I calculate RT60 for just one section of a room?
Yes. If you want to know the RT60 of just your vocal booth alcove or a specific corner, treat that as a separate room—measure its volume and surfaces, and calculate separately. This can help you identify which areas might need targeted treatment.