How to Calculate RT60: Step-by-Step Guide for Non-Specialist Architects

TL;DR — RT60 in 90 Seconds

Reverberation Time (RT60) is the number of seconds it takes for sound to decay by 60 dB after a source stops. It is the single most important acoustic parameter an architect controls through design decisions: room volume, surface finishes, and geometry. The Sabine equation — RT60 = 0.161 × V / A — relates room volume (V, in cubic metres) to total absorption (A, in metric sabins). To calculate RT60, you list every room surface, multiply each area by its absorption coefficient at each octave band, sum the results, and divide into the volume term. This guide walks through that process using a real 200 m³ primary school classroom, with every number shown, so you can replicate it on your next project without needing an acoustic consultant. By the end, you will know whether your room passes or fails BB93, WELL Feature 74, or ANSI S12.60 — and what to change if it fails.

The Story: A Classroom That Failed Its Acoustic Test

In 2024, a London borough primary school completed a two-storey extension with six new classrooms. The architect specified a suspended mineral fibre ceiling and vinyl flooring — a standard approach. At handover, the acoustic consultant measured RT60 at 1.1 seconds in the 500 Hz band. BB93:2015 requires a maximum of 0.6 seconds for primary school classrooms under 200 m³. The school failed. Remediation cost £14,000 per classroom — exposed soffit areas needed additional wall-mounted absorbers, and one room required a complete ceiling replacement because the original tiles had an NRC of 0.55 rather than the specified 0.85. The architect had never run the numbers. A five-minute calculation would have caught the problem at RIBA Stage 3.

Step 1: Measure the Room Dimensions

Start with the internal finished dimensions of the room. For a rectangular room, you need length, width, and height. Our example classroom:

The surface area breakdown:

For non-rectangular rooms, break the geometry into planar surfaces and sum. Do not forget reveals, soffits, or column faces — they contribute absorption area.

Step 2: Assign Absorption Coefficients

Every surface material has an absorption coefficient (α) at each octave band frequency, ranging from 0.00 (perfectly reflective) to 1.00 (perfectly absorptive). Values come from manufacturer data sheets tested to ISO 354:2003, or from published reference tables.

For our classroom, the material assignments are:

Critical note: Always use octave-band coefficients, not the single-number NRC. NRC is an average of 250–2000 Hz and hides low-frequency performance completely.

Step 3: Calculate Absorption Per Surface Per Band

Multiply each surface area by its absorption coefficient at each frequency. This gives absorption in metric sabins (m²) — the unit representing equivalent open-window area.

At 500 Hz (the most commonly cited band):

Repeat this calculation for all six octave bands (125, 250, 500, 1000, 2000, 4000 Hz).

Step 4: Apply the Sabine Equation

The Sabine equation, per ISO 3382-2:2008 Annex A.1:

RT60 = 0.161 × V / A

For our classroom at 500 Hz:

RT60₅₀₀ = 0.161 × 200 / 66.25 = 32.2 / 66.25 = 0.49 seconds

Now calculate across all bands:

The mid-frequency average (500–2000 Hz) is: (0.49 + 0.43 + 0.46) / 3 = 0.46 seconds.

Step 5: Check Against the Applicable Standard

For a UK primary school classroom under 200 m³, BB93:2015 Table 1.2 requires:

The room passes the primary BB93 criterion but has severe bass reverberation at 125 Hz. This is exactly the problem that single-frequency calculations miss. The room will sound "boomy" during lessons despite meeting the mid-frequency target. Students will struggle to hear consonants clearly when low-frequency HVAC noise is present.

Calculate Now: Use AcousPlan's free RT60 calculator to run this same analysis instantly — enter your room dimensions and materials, and get octave-band RT60 results with automatic standard compliance checking.

Step 6: Iterate to Fix Problems

The 125 Hz problem is caused by insufficient low-frequency absorption. The mineral fibre ceiling only provides α = 0.15 at 125 Hz. Solutions:

1. Add a 200 mm air gap behind the ceiling tiles: This increases low-frequency absorption to approximately α = 0.45 at 125 Hz, bringing RT60 down to about 0.90 seconds.
2. Install fabric-wrapped absorber panels on the rear wall: A 50 mm panel with 50 mm air gap achieves α ≈ 0.50 at 125 Hz. Covering 15 m² of the rear wall adds 7.5 sabins, reducing 125 Hz RT60 to approximately 1.22 seconds.
3. Combine both approaches: Ceiling air gap plus 10 m² of wall panels brings 125 Hz RT60 below 0.8 seconds.

Common Mistakes in RT60 Calculations

Mistake 1: Using NRC instead of octave-band data. NRC averages four frequencies and completely masks low-frequency performance. A panel with NRC 0.85 might have α = 0.10 at 125 Hz.

Mistake 2: Forgetting furniture and people. An occupied classroom with 30 students and furniture adds approximately 15–20 sabins at mid-frequencies. The unoccupied room will have longer RT60 than the occupied room. Design for the occupied condition but check the unoccupied condition too — teachers work in empty rooms after hours.

Mistake 3: Applying Sabine to heavily treated rooms. When the average absorption coefficient exceeds about 0.3, switch to the Eyring equation. In our example, the average at 1000 Hz is 74.6/250 = 0.30, right at the threshold. A recording studio with ᾱ = 0.6 would see Sabine overestimate RT60 by roughly 35%.

Mistake 4: Ignoring air absorption. Above 2000 Hz in rooms larger than about 500 m³, air itself absorbs sound. Per ISO 3382-2 Annex A, add 4mV to the denominator, where m is the energy attenuation constant (typically 0.01–0.02 per metre at 4000 Hz for 50% RH). In our small classroom this is negligible, but in a sports hall it can reduce 4000 Hz RT60 by 20%.

Mistake 5: Not checking all six octave bands. Standards increasingly require broadband compliance. WELL Feature 74 specifies limits at 500, 1000, and 2000 Hz individually. BB93 advises checking 125 Hz. A room that passes at 500 Hz can easily fail at 125 Hz, as our example demonstrates.

When to Use Software Instead of a Spreadsheet

Hand calculation works for simple rectangular rooms with uniform surface assignments. Use software (like AcousPlan) when:

• The room is non-rectangular (L-shaped, raked seating, curved walls)
• You need to compare multiple material options quickly
• The project requires compliance reporting against multiple standards simultaneously
• You need to account for furniture, people, and air absorption automatically
• The client needs a visual output showing RT60 across all frequency bands

Summary

Calculating RT60 is six steps: measure dimensions, assign absorption coefficients, multiply area by alpha at each frequency, sum the absorption, divide into 0.161V, and check against the relevant standard. The entire process takes 10–15 minutes by hand for a rectangular room and under 30 seconds with software. The investment is trivial compared to the cost of remediation — which averages £8,000–£15,000 per room for schools that fail BB93 at handover.

The key lesson from our 200 m³ classroom: always check all six octave bands, particularly 125 Hz. Bass reverberation is the most common acoustic failure mode in rooms that meet mid-frequency targets, and it is entirely preventable with proper low-frequency absorption specification at design stage.

Ready to calculate? Open AcousPlan's RT60 calculator — enter your room, pick your materials, and get instant octave-band compliance results with automatic standard checking. No spreadsheet required.