Free RT60 Calculator Online — No Signup, Instant ISO 3382 Results

78% of architects who specify acoustic treatment for commercial buildings have never run an RT60 calculation themselves — they rely on acoustic consultants or skip the prediction entirely and hope the ceiling tile manufacturer's generic recommendation is sufficient. The result is either over-specification (wasting $15,000–$50,000 per floor in unnecessary treatment) or under-specification (discovering the problem after occupancy when remediation costs 4–10 times the original budget). A free RT60 calculator that implements the same ISO 3382-2 methodology used by professional consultants eliminates this information asymmetry entirely.

What the Free Calculator Does

The AcousPlan RT60 calculator accepts three categories of input and produces six categories of output, all without requiring registration, payment, or software installation.

Inputs

Room geometry. Length, width, and height in metres (or feet — the calculator converts automatically). The tool computes volume (V) and total surface area (S) from these three dimensions. For rooms with irregular geometry, use the equivalent rectangular room dimensions — the room with the same volume and total surface area as the actual space.

Surface materials. For each of the six surfaces (floor, ceiling, four walls), select a material from the database of 5,600+ acoustic materials spanning 115 manufacturer brands. Each material carries frequency-dependent absorption coefficients at six octave bands (125, 250, 500, 1000, 2000, and 4000 Hz), sourced from ISO 354:2003 test reports. You can also enter custom absorption coefficients if you have manufacturer data that is not in the database.

Room type. Select the intended use — classroom, meeting room, open plan office, concert hall, recording studio, hospital ward, restaurant, or one of 15 other room types. This determines which compliance standard the calculator checks against: ANSI S12.60 for classrooms, WELL v2 Feature 74 for offices, BB93 for UK schools, DIN 18041 for German educational spaces, and so on.

Outputs

RT60 per octave band. The calculator returns reverberation time at each of the six standard octave band frequencies: 125, 250, 500, 1000, 2000, and 4000 Hz. This frequency-dependent output is critical because absorption coefficients vary dramatically with frequency — a mineral fibre ceiling tile might absorb 0.15 at 125 Hz and 0.80 at 2000 Hz, producing vastly different reverberation times at low and high frequencies.

Broadband RT60. The arithmetic mean of the 500 Hz and 1000 Hz octave band values, which is the standard single-number descriptor per ISO 3382-2:2008 §4.2. Most compliance standards reference this broadband value.

Sabine and Eyring comparison. Both predictions are shown side by side so you can see the divergence. When the mean absorption coefficient exceeds 0.20, the Eyring result (which is more accurate for treated rooms per ISO 3382-2:2008 §A.2) is highlighted as the primary prediction.

Compliance status. A pass/fail indicator against the selected standard. If the room type is a classroom, the calculator checks against the ANSI S12.60-2010 §5 limit of 0.6 seconds (for rooms under 283 m³). If it is an office meeting room, the WELL v2 Feature S07 limit of 0.6 seconds for rooms under 500 m³ is applied.

Treatment recommendations. If the room fails compliance, the calculator suggests how much additional absorption is needed — in square metres of NRC 0.85 panel — to bring the RT60 below the target. This is the auto-solve feature that iteratively adds absorption using the Sabine equation until convergence (maximum 50 iterations).

Calculation transparency. Every intermediate step is shown: surface areas, absorption per surface per frequency, total absorption (A), mean absorption coefficient (alpha_bar), formula selection logic, and the final T60 computation. This is the "show your work" panel that lets you verify every number.

How the Calculation Works — Step by Step

The calculator implements the methodology described in ISO 3382-2:2008 Annex A. Here is the exact sequence of operations.

Step 1: Compute Room Geometry

From the three input dimensions (L, W, H), the calculator derives:

• Volume: V = L x W x H (in m³)
• Floor area: S_floor = L x W
• Ceiling area: S_ceiling = L x W
• Wall areas: S_wall1 = L x H, S_wall2 = L x H, S_wall3 = W x H, S_wall4 = W x H
• Total surface area: S = 2(LW + LH + WH)

Step 2: Look Up Absorption Coefficients

For each surface material, the calculator retrieves the absorption coefficient at each octave band frequency from the materials database. These coefficients are sourced from ISO 354:2003 test reports published by manufacturers.

Step 3: Calculate Total Absorption per Frequency

At each octave band, the total absorption A(f) is the sum of each surface's contribution:

A(f) = sum of [alpha_i(f) x S_i] for all surfaces i

Step 4: Calculate Mean Absorption Coefficient

alpha_bar(f) = A(f) / S

Step 5: Select Formula

If alpha_bar(f) > 0.20 at a given frequency, the calculator uses Eyring's formula (per ISO 3382-2:2008 §A.2). Otherwise, it uses Sabine's formula (per ISO 3382-2:2008 §A.1). This selection is made independently at each frequency.

Step 6: Compute RT60

Sabine: T60(f) = 0.161 V / A(f)

Eyring: T60(f) = 0.161 V / [-S x ln(1 - alpha_bar(f))]

Step 7: Check Compliance

The broadband RT60 (average of 500 Hz and 1000 Hz values) is compared against the target for the selected room type and standard.

Worked Example: 6m x 5m x 3m Classroom

Let us walk through a complete calculation for a primary school classroom — the room type most frequently assessed against acoustic standards worldwide.

Room Geometry

• Length: 6 m, Width: 5 m, Height: 3 m
• Volume: V = 6 x 5 x 3 = 90 m³
• Floor: 6 x 5 = 30 m²
• Ceiling: 6 x 5 = 30 m²
• Long walls: 2 x (6 x 3) = 36 m²
• Short walls: 2 x (5 x 3) = 30 m²
• Total surface area: S = 30 + 30 + 36 + 30 = 126 m²

Surface Materials

Absorption Coefficients

Total Absorption Calculation (at 1000 Hz)

Mean Absorption Coefficient at 1000 Hz

alpha_bar = 26.28 / 126 = 0.209

Since alpha_bar > 0.20, the calculator selects Eyring's formula.

Sabine Prediction (1000 Hz)

T60 = 0.161 x 90 / 26.28 = 14.49 / 26.28 = 0.551 s

Eyring Prediction (1000 Hz)

-ln(1 - 0.209) = -ln(0.791) = 0.234

Eyring denominator = 126 x 0.234 = 29.52

T60 = 0.161 x 90 / 29.52 = 14.49 / 29.52 = 0.491 s

Sabine vs Eyring Comparison

The Sabine prediction is 12.2% higher than Eyring at 1000 Hz. In this classroom, the difference is 0.060 seconds — which matters when the ANSI S12.60 target is 0.6 seconds. Both formulas predict compliance at 1000 Hz, but at other frequencies (particularly 125 Hz and 250 Hz where the ceiling absorbs much less), the Sabine overestimate could push the result above the threshold.

Full Octave Band Results

Compliance Check

The broadband RT60 (average of 500 Hz and 1000 Hz) = (0.80 + 0.49) / 2 = 0.645 s

Against ANSI S12.60-2010 §5 (limit: 0.6 s for rooms under 283 m³): FAIL (0.645 s > 0.600 s)

The classroom fails because of the high RT60 at 500 Hz. The mineral fibre ceiling tile absorbs only 55% of sound at 500 Hz, and the hard floor and walls contribute minimal absorption at that frequency. The calculator would recommend adding approximately 3.5 m² of wall-mounted acoustic panel (NRC 0.85) to bring the 500 Hz absorption up and reduce the broadband RT60 below 0.6 seconds.

How It Compares to Paid Acoustic Software

The free calculator implements the same fundamental methodology as commercial tools, but there are important differences in scope and capability.

What the Free Calculator Matches

What Paid Software Adds

Commercial tools like ODEON, EASE, and Treble Technologies offer capabilities that go beyond statistical reverberation formulas:

Ray tracing and image source methods. These tools model individual sound reflections geometrically, accounting for room shape, source and receiver positions, and directional scattering. This is essential for performance spaces where early reflection patterns, clarity (C80), and lateral fraction (LF) matter as much as RT60.

3D room modelling. Import CAD/BIM models and simulate sound propagation in geometrically complex spaces — auditoriums with balconies, churches with vaulted ceilings, factories with equipment obstructions.

Auralization. Render what the room will sound like by convolving the simulated impulse response with a dry audio signal. This lets architects and clients "hear" the room before it is built.

Spatial analysis. Map RT60, STI, and sound pressure levels across a grid of receiver positions, not just at a single point.

For the majority of projects — offices, classrooms, meeting rooms, healthcare facilities, restaurants — the statistical method (Sabine/Eyring) provides predictions within 10–15% of measured values, which is sufficient for compliance verification. Ray tracing becomes necessary for large, geometrically complex, or performance-critical spaces.

When to Use the Free Calculator vs. Hiring a Consultant

The free calculator is appropriate when:

• You need a quick compliance check during the design phase
• The room is roughly rectangular (aspect ratios within 3:1)
• The room volume is between 30 m³ and 2,000 m³
• You need to compare material options and see how they affect RT60
• You are preparing a preliminary acoustic specification for tender

• The room has complex geometry (coupled volumes, curved surfaces, balconies)
• The room is a performance space where C80, D50, LF, and EDT matter
• Sound insulation between rooms is a concern (STC/Rw calculations)
• Environmental noise impact assessment is required
• The project requires ISO 3382-1 measurement and formal certification

Tips for Getting the Most Accurate Results

Use octave-band absorption coefficients, not NRC. The calculator uses frequency-dependent coefficients from its materials database. If you enter a custom material, provide the full octave band data from the manufacturer's ISO 354 test certificate. Never estimate RT60 from a single NRC value — the frequency-dependent variation is too large for a single-number average to be meaningful.

Account for furnishings. Empty rooms have longer reverberation times than furnished rooms. A typical office desk with chair adds approximately 0.8–1.2 m² Sabine of absorption. Twenty desks in a classroom add 16–24 m² Sabine — a significant contribution that can reduce RT60 by 0.2–0.4 seconds. The calculator's material database includes "occupied" categories (e.g., "audience seated in upholstered chairs") that let you model this.

Check all six octave bands. A room can pass the broadband RT60 target (average of 500 and 1000 Hz) while failing at individual frequencies. Low-frequency reverberation (125–250 Hz) is the most common problem in rooms with lightweight ceiling tiles and hard floors. The calculator shows all six bands so you can catch this.

Consider air absorption in large rooms. For rooms larger than 500 m³, air absorption at 2000 Hz and 4000 Hz becomes significant — the calculator includes the 4mV correction term per ISO 9613-1:1993 automatically.

Who Uses This Calculator

The free calculator serves three primary user groups:

Architects. Check RT60 compliance during schematic design before the acoustic consultant is engaged. Compare ceiling tile options and see how each affects the broadband RT60 and compliance status. Generate preliminary acoustic specifications for tender documents.

Facility managers. Diagnose acoustic complaints in existing buildings. Measure the room dimensions, identify the surface materials, and run the calculator to see if the RT60 exceeds the recommended target for the room's use. This provides evidence-based justification for acoustic remediation budgets.

Students and researchers. Learn how the Sabine and Eyring equations work by entering different room configurations and observing how each parameter affects the result. The calculation transparency panel shows every step, making it an effective teaching tool for architectural acoustics courses.

Try the Free Calculator

Enter your room dimensions and surface materials — the calculator returns RT60 per octave band, compliance status, and treatment recommendations in under two seconds. No signup. No installation. No cost.

Open the free RT60 calculator

Further Reading

• Your RT60 Calculation Is Probably Wrong — And Sabine's Formula Is Why — why Eyring gives more accurate results for treated rooms
• WELL v2 Feature 74 Decoded — every acoustic requirement for WELL certification
• What Is RT60 — And Why It Determines Whether Your Room Sounds Good or Terrible — the fundamentals of reverberation time

References

• ISO 3382-2:2008 — Acoustics — Measurement of room acoustic parameters — Part 2: Reverberation time in ordinary rooms
• ISO 354:2003 — Acoustics — Measurement of sound absorption in a reverberation room
• ISO 9613-1:1993 — Acoustics — Attenuation of sound during propagation outdoors — Part 1: Calculation of the absorption of sound by the atmosphere
• ANSI S12.60-2010 — Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools
• IEC 60268-16:2020 — Sound system equipment — Part 16: Objective rating of speech intelligibility by speech transmission index
• ASTM C423 — Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method