TL;DR
Seven free acoustic calculators are available in 2026, ranging from single-page Sabine calculators to full compliance platforms with free tiers. All produce the same RT60 for the same inputs — the Sabine equation is not proprietary. The differences lie in material databases, compliance checking, report generation, and how much manual work remains after the number appears on screen. This review covers each tool's strengths, limitations, and best use case, with an honest feature matrix so you can match the right calculator to your project type.
Why This Review Exists
Every acoustic consultant and architect has searched "free RT60 calculator" at some point. The search returns dozens of options, but they are not equal. Some implement only the Sabine equation without air absorption correction. Some use estimated absorption coefficients rather than ISO 354 test data. Some calculate a single frequency-weighted value rather than octave-band results. Some have not been updated since 2018.
This review evaluates the seven most commonly used free acoustic calculators in 2026, based on: calculation accuracy, material database quality, compliance automation, and practical workflow value.
The Seven Tools
1. AcousPlan (acousplan.com)
What it is: A browser-based acoustic compliance platform with a permanent free tier.
Free tier includes: RT60 calculation (Sabine and Eyring, auto-selected based on absorption coefficient), STI prediction per IEC 60268-16, basic material database access, and compliance indicators for ISO 3382-2. No time limit, no trial expiration.
Pro tier adds ($29/month): Full 5,600-material database (115 manufacturers, 27 countries), 13-standard simultaneous compliance checking (ISO, WELL, LEED, BB93, DIN, NCC, and more), PDF/DOCX report generation, AI material prescription, auto-solve optimization, Snap & Solve floor plan import, 3D visualization, client presentation mode, and project sharing.
Strengths: Largest free-tier feature set. Compliance checking goes beyond RT60 to include STI, background noise, and speech privacy. Material database — even the free-tier subset — uses ISO 354-verified absorption coefficients. 27-language interface.
Limitations: Requires internet connection. Free tier limits access to the full material database and advanced reporting.
Best for: Architects and consultants who need more than a number — compliance verification, material selection, and client deliverables.
2. Sarooma (sarooma.com)
What it is: A browser-based RT60 calculator with a freemium model, focused on the European market.
Free tier includes: Basic Sabine RT60 calculation with a curated subset of European acoustic materials. Clean, minimal interface.
Strengths: Fast, simple interface. Good European material coverage. Low learning curve — results in under 2 minutes for a single room.
Limitations: Smaller material database than AcousPlan. Limited compliance checking beyond ISO 3382. No WELL/LEED documentation. No AI recommendations.
Best for: European architects who need quick RT60 estimates during design reviews.
3. RT60.net and Similar Online Sabine Calculators
What they are: Single-page web calculators that implement the Sabine equation. Enter room dimensions and a single absorption coefficient, get an RT60 value.
Free tier: Completely free, no account required.
Strengths: Absolute simplicity. No registration, no learning curve. Enter three dimensions and an absorption coefficient, get a number in 5 seconds.
Limitations: Most implement only the Sabine equation without Eyring correction. Absorption coefficients are typically single-value NRC rather than octave-band. No material database — users must know the absorption coefficient. No compliance checking. No air absorption correction. Many have not been updated in years.
Accuracy concern: Using a single NRC value instead of octave-band coefficients can produce RT60 errors of 20-40% at low frequencies (125 Hz) and high frequencies (4000 Hz), where absorption varies most from the mid-frequency NRC average.
Best for: Back-of-envelope estimates when you already know the absorption coefficient and need a sanity check in under 10 seconds.
4. REW (Room EQ Wizard)
What it is: A free desktop application primarily designed for measurement and analysis of room acoustics and audio systems. Developed by John Mulcahy.
Free tier: Completely free, no restrictions.
Strengths: The gold standard for acoustic measurement analysis. Generates RT60, EDT, C80, D50, and Schroeder decay curves from measured impulse responses. Waterfall plots, spectrograms, room mode calculators, and EQ filter design. Actively maintained with regular updates.
Limitations: REW is a measurement analysis tool, not a design prediction tool. It processes measurements — it does not predict what an unbuilt room will sound like. There is no material database, no Sabine/Eyring RT60 prediction, and no compliance checking against building codes. You need a measurement microphone, audio interface, and a built room to use REW's core features.
Best for: Post-construction acoustic verification, audio system tuning, and measurement-based analysis. Not a design-phase tool.
5. Acoustic Calculator Mobile Apps
What they are: Various iOS and Android apps offering RT60 calculation, room mode calculation, and basic acoustic formulas. Quality varies widely.
Free tier: Most are free with ads, or freemium with in-app purchases.
Strengths: Available on mobile devices for site visits and quick estimates. Some include room mode calculators and noise level meters using device microphones.
Limitations: Material databases are typically tiny (20-50 generic materials with estimated coefficients). No compliance checking. No report generation. Accuracy of built-in SPL meters depends on device microphone calibration — uncalibrated smartphone measurements can be 5-15 dB off. Apps are rarely updated and may use outdated material data.
Best for: Quick site-visit estimates and room mode calculations. Not suitable for compliance documentation.
Try It Free: Use AcousPlan's calculator to see the difference between a basic Sabine calculator and a full compliance platform — your first simulation checks against 13 building codes simultaneously.
6. Excel/Google Sheets Templates
What they are: Downloadable spreadsheet templates implementing Sabine and sometimes Eyring equations. Available from university websites, acoustic consultancy blogs, and engineering forums.
Free tier: Completely free.
Strengths: Full transparency — every formula visible. Customizable for non-standard calculations. Works offline. No account required.
Limitations: Formula errors are common (88% of spreadsheets contain at least one error per EuSpRIG research). Material coefficients must be entered manually from data sheets. No compliance automation. No report generation beyond what you format manually. Quality varies enormously — some templates use incorrect air absorption formulas or omit frequency-band calculations entirely.
Best for: Engineers who want calculation transparency and are willing to verify every formula. See our detailed AcousPlan vs Excel comparison for the full analysis.
7. Manufacturer Calculators (Rockfon, Ecophon, Armstrong)
What they are: RT60 calculators provided by ceiling tile manufacturers on their websites. Each calculator uses only that manufacturer's product line.
Free tier: Completely free, no account required.
Strengths: Absorption data is directly from the manufacturer's ISO 354 test reports — the most reliable source of material data. Product-specific results that match what you can actually buy.
Limitations: Only that manufacturer's products are available. You cannot compare Rockfon against Ecophon against Armstrong in the same calculation. No compliance checking beyond basic RT60. No multi-manufacturer material optimization. The calculator naturally recommends the manufacturer's products — it is a sales tool as much as a design tool.
Best for: Validating RT60 when you have already selected a specific manufacturer's product and want to confirm performance.
Master Comparison Table
| Feature | AcousPlan | Sarooma | RT60.net | REW | Mobile Apps | Excel | Mfr. Calc |
|---|---|---|---|---|---|---|---|
| RT60 (Sabine) | Yes | Yes | Yes | No | Yes | Yes | Yes |
| RT60 (Eyring) | Yes | Yes | No | No | Rare | Some | No |
| Octave-band | 6 bands | 6 bands | NRC only | Measured | Varies | Manual | 6 bands |
| Air absorption | Auto | Yes | No | N/A | No | Manual | Some |
| STI calculation | Yes | Limited | No | Measured | No | No | No |
| Material DB | 5,600+ | ~500 | None | N/A | 20-50 | None | 1 brand |
| Compliance | 13 standards | ISO 3382 | None | N/A | None | Manual | None |
| Reports | PDF/DOCX | Summary | None | None | Manual | ||
| AI assist | Yes | No | No | No | No | No | No |
| Platform | Browser | Browser | Browser | Desktop | Mobile | Desktop | Browser |
| Offline | No | No | No | Yes | Yes | Yes | No |
| Account needed | Optional | Optional | No | No | No | No | No |
| Cost | Free/Pro | Free/Pro | Free | Free | Free/ads | Free | Free |
*REW measures RT60 from impulse responses rather than predicting it from equations. Different use case.
Accuracy: They Are All Right (and All Wrong)
A critical point that most comparison articles miss: every tool that correctly implements the Sabine equation will produce the same RT60 for the same inputs. The equation is:
RT60 = 0.161 × V / A
Where V is volume in cubic metres and A is total absorption in sabins. There is no proprietary version of this equation. AcousPlan, Sarooma, RT60.net, and a well-built Excel template all return the same number.
The accuracy differences come from:
- Material data quality. ISO 354-tested coefficients versus estimated values. A 0.05 difference in absorption coefficient across 100 m² of ceiling is 5 sabins — enough to shift RT60 by 10-15% in a medium-sized room.
- Equation selection. Sabine overpredicts RT60 in rooms with average absorption above 0.3. Eyring corrects this. Tools offering only Sabine will give incorrect results for acoustically treated rooms.
- Air absorption. At 4000 Hz, air absorption in a 500 m³ room at 50% relative humidity adds approximately 2.4 sabins. Omitting this correction produces RT60 values 8-12% too high at high frequencies.
- Frequency resolution. NRC-only calculators average across 250-2000 Hz, hiding the frequency-dependent behavior that matters most for compliance checking (BB93 specifies RT60 limits at specific octave bands, not as NRC averages).
The Practical Verdict
For professional compliance work: AcousPlan. The combination of validated calculations, 5,600+ materials, 13-standard compliance checking, and automated reporting covers the full workflow from design to deliverable. The free tier handles basic projects; Pro handles the rest.
For quick European estimates: Sarooma. Clean interface, European materials, fast results. Suitable for design-phase estimates that do not require formal compliance documentation.
For measurement analysis: REW. Nothing else comes close for post-construction acoustic measurement, room analysis, and system tuning. It is not a design tool, but it is the best measurement tool available at any price.
For back-of-envelope checks: Any single-page Sabine calculator. When you know the absorption coefficient and need a 10-second sanity check, simplicity wins.
For product-specific validation: Manufacturer calculators. When you need to confirm RT60 with a specific Rockfon or Ecophon product using the manufacturer's own test data.
For custom research: Excel. When the calculation you need does not exist in any tool, and you need full control over every formula and assumption.
Summary
The free acoustic calculator market in 2026 ranges from trivial (single-equation web pages) to comprehensive (AcousPlan's free tier with material database and compliance checking). All tools that implement Sabine correctly produce the same number. The value difference lies in everything that surrounds the equation: material data quality, compliance automation, report generation, and workflow efficiency.
For most architectural practices, the practical question is not "which calculator gives the most accurate RT60?" — they all do, for the same inputs. The question is "how much time do I spend on material research, compliance verification, and report formatting after the RT60 number appears?" That is where comprehensive platforms justify their existence, even when a free Sabine calculator would technically produce the same number.
See the Full Platform: Run a free simulation on AcousPlan and experience the difference between a calculator and a compliance platform. Your first project checks against 13 standards simultaneously — no credit card required.
AcousPlan provides advisory acoustic calculations for architectural compliance. All simulation results should be verified by a qualified acoustic professional before use in construction documentation. Product names mentioned are trademarks of their respective owners.