What STC Rating Does My Wall Have?
A 2×4 wall with resilient channel on one side, one layer of 5/8 in type X drywall on each side and 3½ in batts tests at STC 45 to 50, depending on the stud spacing. Without the batts the same wall tests at 40. STC is a laboratory result, not a sum of materials, so the calculator reports the published tests for the wall you pick.
Tested wood stud walls, lowest and highest result for each:
| Framing | Drywall | Insulation | STC |
|---|---|---|---|
| 2×4 studs | 1/2 in type X, one layer each side | No insulation | 32–35 |
| 2×4 studs | 1/2 in type X, one layer each side | 3½ in batts | 34–39 |
| 2×4 studs | 5/8 in type X, one layer each side | No insulation | 32–34 |
| 2×4 studs | 5/8 in type X, one layer each side | 3½ in batts | 36 |
| 2×4 studs | 1/2 in, two layers one side | No insulation | 38 |
| 2×4 studs | 1/2 in, two layers one side | 3½ in batts | 40 |
| 2×4 studs | 1/2 in type X, two layers each side | 3½ in batts | 38–45 |
| 2×4 studs, resilient channel | 5/8 in type X, one layer each side | No insulation | 40 |
| 2×4 studs, resilient channel | 5/8 in type X, one layer each side | 3½ in batts | 45–50 |
| 2×4 studs, resilient channel | 1/2 in type X, one layer each side | 3½ in batts | 43–46 |
| Double 2×4 studs | 5/8 in type X, one layer each side | No insulation | 45 |
| Double 2×4 studs | 5/8 in type X, one layer each side | 3½ in batts | 54–56 |
| Double 2×4 studs | 5/8 in type X, one layer each side | Batts in both rows | 56–60 |
The same walls side by side, lowest and highest test:
Two patterns stand out. In a single row of studs, batts add only 2 to 4 points and a second layer of drywall a few more. Once the two sides are decoupled, by resilient channel or by a second row of studs, the same batts add 5 to 15 points.
How the Rating Is Worked Out
A lab builds the wall between two rooms, plays sound in one and measures how much is lost through the wall in sixteen one-third-octave bands from 125 to 4,000 Hz (ASTM E90). Then the rating rule of ASTM E413 takes over:
- 1A reference curve of a fixed shape is laid over the sixteen measured losses.
- 2The curve is moved up until the measured points below it fall short by no more than 32 dB in total, and by no more than 8 dB at any one frequency.
- 3The STC is the value of the curve at 500 Hz.
Because of the 8 dB rule, one weak frequency can hold back the whole rating. In USG's worked example, an STC 50 wall is held at 50 by dips at 2,500 and 3,150 Hz, each exactly 8 dB under the curve, with 29 dB short in all.
Building an STC 50 Wall
STC 50 is the code line between apartments. A wood stud wall gets there by keeping the two faces apart, so sound can't cross through the studs:
- Resilient channel on one side holds the drywall off the studs. The tests reach STC 50 with studs 24 in on center and 45 to 46 at 16 in. Drywall screws long enough to reach a stud through the channel short it out, which is why installers use short screws.
- A double stud wall has two rows of 2×4 studs on separate plates 1 in apart. With batts in one row it tests at 54 to 56, in both rows 56 to 60. It costs floor space: about 9 1/4 in finished against 4 1/2 in for a plain wall.
- Batts fill the cavity in either case; count the batt packages and the studs for a second row.
Doors, Outlets and Gaps
A wall rated STC 50 in the lab rarely performs that well in a building, which is why the IBC accepts NNIC 45 in the field. Sound leaks through gaps at the floor and ceiling, through back-to-back outlet boxes and around doors. The IBC exempts entrance doors from the rating but requires them to fit tight to the frame and sill. Heavier solid-core doors block more sound than hollow ones; compare door weights by core.
STC vs. NRC, IIC and OITC
| Rating | What it measures | Where it applies |
|---|---|---|
| STC | Airborne sound blocked by a wall or floor | IBC 1206.2: 50 between dwelling units |
| IIC | Impact sound, such as footsteps, through a floor-ceiling | IBC 1206.3: 50 between dwelling units |
| NRC | Sound absorbed by a surface inside a room | Ceiling tiles and panels |
| OITC | Outdoor noise, weighted to low traffic sounds | Windows and exterior walls |
STC and NRC are often confused. Soft panels with a high NRC make a room less echoey but do almost nothing to stop sound reaching the next room; that takes mass, decoupling and airtight walls.
The IBC's sound rules cover apartments, condos, hotels and dorms: walls and floors between dwelling or sleeping units, or between them and public or service areas. A single-family house under the IRC has no STC requirement in the code's main chapters. In metric, the calculator gives the wall thickness in millimeters: 133 mm for the default wall.
Frequently Asked Questions
What is STC?
Sound Transmission Class, a single number for how well a wall or floor blocks airborne sound such as voices. A lab measures the sound lost through the wall at sixteen frequencies from 125 to 4,000 Hz (ASTM E90) and fits a reference curve to them (ASTM E413); the higher the number, the quieter the other side.
What STC is required between apartments?
The IBC (Section 1206.2) requires STC 50 for walls and floor-ceilings between dwelling units when lab tested, or NNIC 45 when tested in the field. Entrance doors are exempt but must fit tight.
What is the STC of a standard interior wall?
About 32 to 35 for 2×4 studs with one layer of ½ in drywall each side and an empty cavity; adding batts raises it only to about 34 to 39.
How do I get an STC 50 wall?
Decouple the two sides. 2×4 studs with resilient channel on one side, 5/8 in type X drywall each side and batts test at 45 to 50; a double row of studs with batts tests at 54 to 60.
Does insulation improve STC?
In a single row of studs, only a few points, about 2 to 4. In a decoupled wall it does much more: resilient channel walls gain 5 to 10 points and double-stud walls 9 to 15.
Does resilient channel raise STC?
Yes, by separating one layer of drywall from the studs. With 5/8 in type X each side and batts, a plain 2×4 wall tests at STC 36 and the same wall with resilient channel on one side at 45 to 50.
Does a second layer of drywall help?
Some. On 2×4 studs with batts, one layer of 1/2 in drywall each side tests at STC 34 to 39 and two layers each side at 38 to 45: extra mass helps, but less than decoupling.
What is the difference between STC, NRC and IIC?
STC rates how much airborne sound a wall or floor blocks; IIC rates impact sound such as footsteps through a floor-ceiling (the IBC also asks IIC 50); NRC rates how much sound a surface absorbs inside a room.
Does the STC calculator cover metal stud walls?
No. It covers wood stud walls from published tests. Steel-stud walls behave differently and have their own tested assemblies in the gypsum and insulation makers’ guides.
Sources
- International Code Council. (2020). 2021 International building code (Section 1206).
- Owens Corning. (2024). Wall & floor assembly guide: Insulation for sound & fire rated assemblies (Pub. No. 300746E).
- PABCO Gypsum. (2026). Assembly selector (PGD-WD-468-16).
- USG Corporation. (n.d.). Sound transmission class [Rating procedure under ASTM E413].
Updated .
Cite this page as:
Daniel, P. STC Calculator. Available at: https://constructioncalculators.com/stc-calculator/. Accessed: October 28, 2026.