How to Calculate RT60 Step by Step — From Room Dimensions to Compliance Check

42% of acoustic design errors originate in the RT60 calculation itself — not in material selection, not in construction, but in the fundamental prediction of how long sound will persist in a room. The most common error is using the wrong formula: applying Sabine's equation to a room with significant acoustic treatment, where it overestimates RT60 by 15–40%. The second most common error is calculating at a single frequency instead of across all six octave bands, missing the bass reverberation problem that afflicts 60% of under-treated rooms.

This tutorial walks through the complete RT60 calculation process in five steps, using a real conference room as the worked example. Every number is shown. Every intermediate value is calculated. By the end, you will know exactly how to go from room dimensions to a compliance verdict — and where the common traps are.

The Five Steps

1. List all room surfaces with their areas
2. Assign absorption coefficients (α) by frequency for each surface
3. Calculate total absorption A = Σ(αᵢ × Sᵢ) at each octave band
4. Apply the Sabine or Eyring formula
5. Check the result against the applicable standard

Step 1: List All Room Surfaces with Areas

Start with the room dimensions. For rectangular rooms, you need length, width, and height. For non-rectangular rooms, measure or calculate the area of each surface individually.

Worked Example: 20-Person Conference Room

Room dimensions: 10.0 m × 6.0 m × 3.0 m

• Volume (V): 10.0 × 6.0 × 3.0 = 180.0 m³
• Ceiling area: 10.0 × 6.0 = 60.0 m²
• Floor area: 10.0 × 6.0 = 60.0 m²
• Long walls (2): 2 × (10.0 × 3.0) = 60.0 m²
• Short walls (2): 2 × (6.0 × 3.0) = 36.0 m²
• Total surface area (S): 60.0 + 60.0 + 60.0 + 36.0 = 216.0 m²

Note that walls are broken into sub-surfaces where materials differ. The 8.0 m² of glazing on long wall 2 has very different absorption from the plasterboard around it. Treating the entire wall as one material introduces error.

Step 2: Assign Absorption Coefficients by Frequency

Look up the absorption coefficient (α) for each material at each of the six standard octave bands: 125, 250, 500, 1000, 2000, and 4000 Hz. Use values measured per ISO 354:2003 or ASTM C423, sourced from manufacturer data sheets or published reference tables.

Here are the values for our conference room surfaces:

Sources for Absorption Coefficients

• Manufacturer data sheets: The most accurate source. Always check that the values are for the specific product, mounting type, and cavity depth you are specifying.
• Published reference tables: Textbooks such as Vorlaender's "Auralization" (Springer, 2008) or the Acoustic Surfaces database provide generic values. These are suitable for early-stage estimates but should be replaced with manufacturer-specific data for final calculations.
• AcousPlan materials database: Contains absorption coefficients for over 5,600 products from 115 brands, all sourced from manufacturer-published ISO 354 test reports.

Common Error: Using NRC Instead of Octave-Band Values

The NRC (Noise Reduction Coefficient) is the average of α at 250, 500, 1000, and 2000 Hz. It is a single number that tells you nothing about low-frequency performance. A material with NRC 0.85 could have α₁₂₅ = 0.10 (terrible bass absorption) or α₁₂₅ = 0.70 (excellent bass absorption). Using NRC for RT60 calculation means you cannot predict frequency-dependent behaviour, which is the entire point of the exercise.

Always use octave-band values. NRC is for quick comparisons between products, not for room acoustic calculations.

Step 3: Calculate Total Absorption at Each Octave Band

Total absorption A at each frequency is the sum of each surface's area multiplied by its absorption coefficient at that frequency:

A(f) = Σ(αᵢ(f) × Sᵢ)

Calculate this for all six octave bands:

125 Hz

250 Hz

500 Hz

1000 Hz

2000 Hz

4000 Hz

Summary Table

The mean absorption coefficient ranges from 0.156 at 125 Hz to 0.398 at 4000 Hz. Since all values exceed 0.15, we should use the Eyring formula at every frequency for this room. The room has substantial treatment (acoustic ceiling + carpet), placing it firmly in the regime where Sabine overestimates.

Step 4: Apply the Formula

Sabine Formula (for reference)

T60 = 0.161V / A

Per ISO 3382-2:2008 §A.1, this is valid when ᾱ < 0.15 and absorption is distributed reasonably uniformly.

Eyring Formula (recommended for this room)

T60 = 0.161V / (-S × ln(1 - ᾱ))

Per ISO 3382-2:2008 §A.2, this accounts for the compounding effect of absorption at each reflection. The term -ln(1 - ᾱ) is always greater than ᾱ, producing a larger effective denominator and a shorter (more realistic) RT60.

Calculation at Each Octave Band

The Sabine-Eyring difference ranges from 8% at 125 Hz (where average absorption is low) to 22% at 2000–4000 Hz (where absorption is high). At 1000 Hz, Sabine predicts 0.364 s while Eyring predicts 0.291 s — a 20% overestimate that could mislead a designer into specifying additional treatment that is not needed.

Air Absorption Correction

For rooms larger than approximately 200 m³, air absorption becomes significant at frequencies above 2000 Hz. The correction adds 4mV to the denominator of the Sabine formula (or equivalently adjusts the Eyring formula), where m is the energy attenuation constant of air in Np/m. At 20°C and 50% relative humidity:

• m at 2000 Hz ≈ 0.006 Np/m → 4mV = 4 × 0.006 × 180 = 4.32 m² Sabine
• m at 4000 Hz ≈ 0.020 Np/m → 4mV = 4 × 0.020 × 180 = 14.40 m² Sabine

Step 5: Check Against the Applicable Standard

The final step is comparing the calculated RT60 values against the targets specified by the applicable standard. For a full clause-level breakdown of how each standard maps to AcousPlan's calculation engine, see our standards conformance matrix. For our 20-person conference room (V = 180 m³), the relevant standards might include:

WELL v2 Feature 74 Compliance Check

The WELL RT60 target for this room is ≤ 0.7 s averaged across 500, 1000, and 2000 Hz.

Eyring RT60 average (500–2000 Hz) = (0.391 + 0.291 + 0.265) / 3 = 0.316 s

This is well below the 0.7 s threshold. The room passes Part 1 with considerable margin. If the project budget is tight, you could potentially use a ceiling tile with lower absorption and still comply — though there is no acoustic reason to reduce ceiling performance below what is specified.

The 125 Hz Warning

Although the room passes every mid-frequency criterion, the RT60 at 125 Hz is 0.791 s (Eyring). This is more than twice the 500 Hz value of 0.391 s. The room will have noticeable bass reverberation — a phenomenon that sounds like a low "boom" or "hum" after speech, particularly noticeable during video calls. Some standards (DIN 18041, BB93) include frequency-dependent limits that catch this. Others (WELL v2) do not.

If bass control is required, add a bass-absorbing element: a membrane absorber on one wall, a ceiling tile with enhanced low-frequency performance (larger cavity depth), or wall-mounted mineral wool panels with a 100 mm air gap behind them.

Summary of the Five-Step Process

The process is deterministic. Given accurate inputs, the output is a reliable prediction of how the room will perform. The skill — and the source of most errors — is in Step 2: choosing the right absorption coefficients. Always use manufacturer data from ISO 354 tests, always check the mounting condition matches your design, and always calculate at all six octave bands.

Related Reading

• RT60 Calculation: When Sabine Gets It Wrong — why the formula choice matters more than most designers realise
• Reverberation Time Formula Derivation — the physics behind Sabine and Eyring
• What Is RT60? — the fundamental concept explained from first principles