R-Value Converter: Compare Insulation & U-Values Instantly (Free Tool)
You see “R-19” on a roll of insulation. The building code calls for “R-15” in the walls. But you are looking at a European window with a U‑value of 1.5 W/m²·K. How do you compare them?
R‑value and U‑value are the two main ways to measure insulation performance. R‑value measures resistance to heat flow (bigger is better). U‑value measures heat transfer rate (smaller is better). In the United States, R‑values are quoted in ft²·°F·h/BTU. In most other countries, they use metric RSI (m²·K/W) or U‑values (W/m²·K).
Today, I give you a free R-Value Converter.
You enter an R-value in US units (or metric RSI). The calculator shows the equivalent in the other system and the corresponding U‑value.
Let me explain how R-value works and how to convert between different units.
What is R-Value? (Simple Explanation)
R‑value is a measure of a material’s resistance to heat flow . The higher the R‑value, the better the insulation . It is defined as the temperature difference required to drive one unit of heat through one square foot of material .
The formula:
R = thickness ÷ thermal conductivity
For a single material, the R‑value depends on thickness and the material’s thermal conductivity . For an assembly of multiple layers (like a wall), total R‑value is the sum of each layer’s R‑value .
US vs. Metric units:
- US R‑value: ft²·°F·h/BTU – used in the United States and Canada
- Metric RSI: m²·K/W – used in Europe, Australia, and other countries
The key conversion: 1 RSI = 5.678 US R‑value. Or 1 US R‑value = 0.176 RSI .
Why This R-Value Converter Matters
Here is why you need to convert R-values.
Reason 1: Compare products from different countries
Insulation manufacturers in the US use US R‑values. European products use metric RSI or U‑values. You cannot compare them directly .
Reason 2: Understand building code requirements
Some codes specify US R‑values. Others use metric RSI. In Canada, both are often listed .
Reason 3: Convert U-values to R-values
Windows are often rated by U‑value (W/m²·K). U‑value is the reciprocal of RSI . To find RSI: RSI = 1 ÷ U‑value . To find US R‑value: R = 1 ÷ (U‑value × 0.176).
Reason 4: Verify insulation performance
When you know the thermal conductivity (k‑value, λ) and thickness of a material, you can calculate its R‑value: R = thickness ÷ k .
Reason 5: International projects
If you work on building projects in different countries, you need to convert between unit systems .
LIVE R-Value Converter
Enter an R-value or RSI value. The calculator shows the equivalent and U‑value instantly.
📐 R-Value Converter
📊 Input Value
📐 Input Unit
📐 R-Value Converter
Convert between US R-value, Metric RSI, and U-value
📊 Input Value
📐 Input Unit
US R-value (ft²·°F·h/BTU) Metric RSI (m²·K/W) U-value (W/m²·K)
📊 US R-value (ft²·°F·h/BTU):—
📊 Metric RSI (m²·K/W):—
📊 U-value (W/m²·K):—
0%
💡 1 US R-value = 0.176 RSI • 1 RSI = 5.678 US R-value • U-value = 1 ÷ RSI
📐 For U-value, lower is better. For R-value, higher is better.
How to Use This R-Value Converter
Follow these 3 simple steps.
Step 1: Enter the value you want to convert
Step 2: Select the input unit (US R-value, Metric RSI, or U‑value)
Step 3: Read the equivalent values in all three units
The calculator shows:
- US R‑value (ft²·°F·h/BTU)
- Metric RSI (m²·K/W)
- U‑value (W/m²·K)
Common R-Values
| Material | R-Value per Inch | R-Value for 3.5" | R-Value for 6" |
|---|---|---|---|
| Fiberglass batt | 3.0-3.7 | R-11 to R-13 | R-19 to R-22 |
| Cellulose | 3.7 | R-13 | R-22 |
| Spray foam | 4.0-6.5 | R-14 to R-23 | R-24 to R-39 |
| Polystyrene foam | 4.0-5.0 | R-14 to R-18 | R-24 to R-30 |
R-Value to U-Value Conversion
US R-value to US U-value:
U = 1 ÷ R
Metric RSI to metric U-value:
U = 1 ÷ RSI
US R-value to metric U-value:
RSI = R ÷ 5.678, then U = 1 ÷ RSI
Example: R-19 US = 19 ÷ 5.678 = 3.347 RSI → U = 1 ÷ 3.347 = 0.299 W/m²·K.
Understanding the Units
US R-value (ft²·°F·h/BTU):
This is the standard in the United States and Canada . It measures how many square feet of material are needed to resist one BTU of heat flow for one hour with a one-degree temperature difference .
Metric RSI (m²·K/W):
This is the SI unit for thermal resistance . It measures how many square meters of material are needed to resist one watt of heat flow with a one-degree Kelvin temperature difference .
U-value (W/m²·K):
U-value is the reciprocal of RSI. It measures how much heat flows through one square meter of material for each degree of temperature difference . Lower U‑value = better insulation.
Real Examples
Example 1: Fiberglass batt
- Material: 3.5" fiberglass batt (R-3.7 per inch)
- US R-value: 13
- RSI: 13 ÷ 5.678 = 2.29 m²·K/W
- U-value: 1 ÷ 2.29 = 0.437 W/m²·K
Example 2: European window
- U‑value: 1.5 W/m²·K
- RSI: 1 ÷ 1.5 = 0.667 m²·K/W
- US R-value: 0.667 × 5.678 = 3.79
Example 3: R-19 fiberglass
- US R-value: 19
- RSI: 19 ÷ 5.678 = 3.35 m²·K/W
- U-value: 1 ÷ 3.35 = 0.299 W/m²·K
Frequently Asked Questions (FAQs)
1. What is the difference between R-value and U-value?
R‑value measures resistance to heat flow (higher is better). U‑value measures heat flow rate (lower is better). They are reciprocals .
2. How do I convert US R-value to metric RSI?
Divide US R-value by 5.678 .
3. How do I convert metric RSI to US R-value?
Multiply RSI by 5.678 .
4. How do I convert U-value to R-value?
RSI = 1 ÷ U‑value. US R‑value = RSI × 5.678 .
5. What is a good R-value for a wall?
R-13 to R-15 for exterior walls in most US climates.
6. What is a good R-value for an attic?
R-38 to R-60 in cold climates, R-30 to R-38 in moderate climates.
7. What are RSI units?
RSI is the metric unit for R‑value: m²·K/W . It is used in Europe, Australia, and many other countries .
8. Why do US R-values seem higher than metric RSI?
Because 1 US R‑value = 0.176 RSI . The US unit is about 5.68 times larger.
9. Can I add R-values together?
Yes. For a wall assembly, total R‑value = sum of all layer R‑values .
10. What is the R-value of air?
Air has an R‑value of about 0.68 per inch of still air (air film resistance).
How to Calculate R-Value from k-Value
If you know the thermal conductivity (k‑value, λ) and thickness of a material:
R = thickness ÷ k
Example: 100mm (0.1 m) of foam with k=0.035 W/m·K
R = 0.1 ÷ 0.035 = 2.86 m²·K/W = 2.86 × 5.678 = 16.2 US R-value .
Common Mistakes
Mistake #1: Confusing US and metric R-values
US R‑values are about 5.68× larger than metric RSI. Always check units .
Mistake #2: Thinking higher U‑value is better
U‑value measures heat loss. Lower U‑value is better .
Mistake #3: Adding R-values without accounting for layers
For an assembly, total R‑value is the sum of all layers . For parallel paths (e.g., studs in a wall), use area-weighted averaging .
Mistake #4: Ignoring air films
Interior and exterior air films add to the total R‑value .
Mistake #5: Using the wrong conversion factor
Always use 5.678 for US to RSI conversion. Using a rounded value like 5.7 is acceptable for rough estimates, but 5.678 is more accurate .
Final Thoughts
Understanding R‑value and converting between units is essential for international building projects and comparing insulation products.
My R-Value Converter gives you:
- US R‑value, metric RSI, and U‑value
- Instant conversion between all three
- Understanding of what each number means
Bookmark this page. Use it when buying insulation, designing buildings, or working on international projects.
The next time you see an insulation rating in an unfamiliar unit, you will know exactly how it compares.
Disclaimer: This is an educational tool based on standard thermal resistance definitions. For actual building design, consult a qualified engineer.
External Links (Authority Backlinks):
- Wikipedia – R-value (insulation){:target="_blank" rel="noopener noreferrer"}
- EIA – Glossary (R-value definition){:target="_blank" rel="noopener noreferrer"}
- Kingspan – How to Calculate a U-value{:target="_blank" rel="noopener noreferrer"}
- ASHRAE – Fundamentals Handbook{:target="_blank" rel="noopener noreferrer"}
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