STC & IIC sound rating estimator

What an STC and IIC calculator does

Sound Transmission Class (STC) rates how well a wall or floor blocks airborne sound such as speech and television, and Impact Insulation Class (IIC) rates how well a floor-ceiling blocks footsteps and dropped objects. This estimator gives you a first answer two ways: it looks up indicative laboratory STC and IIC ranges for common wood-stud, steel-stud, masonry, concrete, wood-joist and CLT assemblies, or it estimates STC for a solid single-leaf wall or slab from its weight.

Both modes compare the result with the target you enter. The International Building Code (IBC Section 1206) asks for STC 50 and IIC 50 in the lab, or 45 measured in the field, between dwelling units, so 50 is the default. The chart shows where the typical range sits against that line, and the margin at the low end tells you how many points a design may need to find.

The mass estimate uses the TMS 302 equation for concrete masonry, STC = 20.5 × W^0.234 with W in pounds per square foot. It suits concrete, CMU and other dense single leaves. Stud walls, cavity walls and floating floors are double-leaf systems that perform very differently from their weight, which is why the assembly lookup exists. Neither mode replaces a tested design with a report number, and neither accounts for flanking, penetrations or workmanship.

How to use the STC & IIC sound rating estimator

1. Choose Imperial or Metric. Thickness, density and surface mass switch between inches, lb/ft³ and lb/ft² and millimetres, kg/m³ and kg/m².
2. Set Estimate from to Typical assembly to look up a construction, or to Single-leaf mass (TMS 302) to estimate a solid wall or slab from its layers.
3. For Typical assembly, pick the closest Assembly. Floors also report IIC.
4. Enter the Target STC, and for floors the Target IIC, from your code or brief. 50 matches the IBC lab requirement between dwelling units.
5. For the mass estimate, add one row per layer: choose a Layer material and enter its Thickness. CMU presets carry a whole-wall weight and ignore thickness; choose Custom density to enter your own Custom density (if chosen).
6. Read the typical range or estimated STC, the margin to your target and the chart, then download the PDF or CSV or copy the result.

Worked example

Imperial: A 2×4 wood-stud wall with batts and one layer of ⅝ in Type X each side typically tests STC 35–39, 15 points short of STC 50 at the low end. In mass mode, 6 in of 150 lb/ft³ concrete weighs about 74.9 lb/ft² and estimates STC 56.

Metric: A 2×4 wood-stud wall with batts and one layer of ⅝ in Type X each side typically tests STC 35–39, 15 points short of STC 50 at the low end. In mass mode, 150 mm of 2,400 kg/m³ concrete weighs 360 kg/m² and estimates STC 56.

STC and IIC requirements between dwelling units

In the US, the IBC sets minimum sound ratings for walls, partitions and floor-ceiling assemblies that separate dwelling units and sleeping units from each other and from public or service areas. A laboratory rating comes from a test on a full-size specimen; a field rating is measured in the finished building, where sound also travels through flanking paths, so the code accepts a lower number.

The IRC has an optional appendix for two-family dwellings with lower targets, and many condominium associations, lenders and green-building programs ask for more than the code minimum. Enter whichever value governs your project.

Typical STC ratings of common wall and floor assemblies

The ranges below are the presets in the tool. They summarise published laboratory tests, such as NRC Canada's IR-761 series on gypsum board walls and NRC tests of CLT floors, and they spread because stud spacing, board type, insulation, the brand and spacing of resilient channel and the laboratory all change the result. Treat them as a way to rank options, then select a specific tested design.

The pattern is consistent: a single row of wood studs with one layer of gypsum each side lands in the mid-to-high 30s whatever you put in the cavity, because the studs couple the two faces. Decoupling the faces with resilient channel, staggered studs or a double stud wall is what moves a wall past STC 50.

Estimating STC from wall weight (TMS 302)

For concrete masonry, The Masonry Society's TMS 302 relates STC to wall weight with a best-fit equation: STC = 20.5 × W^0.234, where W is the weight of the wall in lb/ft². It applies to concrete masonry at least 3 in thick with sealed or fine-textured faces and full mortar joints. The calculator rounds the result down to a whole number and also reports the field-incidence mass-law transmission loss at 500 Hz, TL = 20 log10(m × f) − 47 dB with m in kg/m², as a cross-check.

Heavier is better, but slowly: doubling the weight raises the TMS 302 estimate by about 18 %, roughly 8 to 10 points in the usual range. Light single leaves such as gypsum board, glass and plywood fall below the masonry data and also have a coincidence dip, so their real STC can differ from any weight-based estimate.

Worked example: concrete wall against an STC 50 target

A 150 mm normal-weight concrete wall at 2,400 kg/m³ weighs 0.150 × 2,400 = 360 kg/m², or 73.7 lb/ft². TMS 302 gives 20.5 × 73.7^0.234 = 56.1, so the estimate is STC 56, six points above an STC 50 target, and the mass-law transmission loss at 500 Hz is 58 dB. In imperial units, 6 in of the same concrete weighs about 74.9 lb/ft² and also estimates STC 56.

Switch to Typical assembly and choose a 2×4 wood-stud wall with batts and one layer of ⅝ in Type X each side, and the range is STC 35 to 39: 11 to 15 points short. The cheapest fixes are decoupling, not mass: resilient channel on one side typically brings the same wall to STC 45 to 52, and a double stud wall to 54 to 60.

Who it is for and when to use it

Method, formulas & assumptions

Typical assembly: indicative laboratory STC (ASTM E90/E413) and, for floors, IIC (ASTM E492/E989) ranges for common wood-stud, steel-stud, masonry, concrete, wood-joist and CLT assemblies; the margin to your target is the low end minus the target. Single-leaf mass: surface mass = sum of density × thickness (or a fixed CMU wall weight per layer); STC = 20.5 × W^0.234 with W in lb/ft² (TMS 302-12, concrete masonry ≥ 3 in), rounded down; field-incidence mass-law TL at 500 Hz = 20 log10(m × 500) − 47 dB, m in kg/m². A bar chart compares the result with the target.

Imperial inputs are converted with exact factors (1 ft = 0.3048 m, 1 lb = 0.45359237 kg, 1 US gal = 3.785411784 L) before the calculation runs in SI, then results are converted back.

Common mistakes to avoid

• Treating a lab STC as what you will hear on site. Flanking through floors, ceilings and junctions usually takes several points off the field result.
• Adding insulation to a single wood-stud wall and expecting STC 50. Without decoupling, the studs carry sound straight through.
• Shorting out resilient channel with a long screw into a stud or a cabinet fixed through the board. One bridge can cost several points.
• Using the mass estimate for a stud or cavity wall. Double-leaf walls do not follow a weight relationship.
• Ignoring penetrations. Back-to-back outlets, recessed lights and unsealed perimeter gaps leak sound through an otherwise good wall.
• Comparing STC and IIC as if they were the same. STC is airborne sound; IIC is footfall and impact, and finishes change IIC far more than STC.

Scope & limits

Assembly ranges are indicative lab results summarised from published tests (NRC Canada IR-761, NRC CLT floor tests, TMS 302), not listings; a specific tested design governs. Field ratings are usually lower because of flanking, penetrations and workmanship. The mass equation is for dense single leaves and does not apply to stud, cavity or floating constructions. The target check compares with a value you enter, not a code lookup.

Frequently asked questions

Further reading

• Wall build-ups template
• Typical exterior wall assemblies
• All building performance tools
• All free architecture & construction tools
• Harth resource library

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