Sources and scope for this page: ASHRAE Handbook — Fundamentals (2021, SI, Chapter 4, Table 1), covering five broad SI coefficient ranges; no geometry-specific or product recommendations; and DOE-HDBK-1012/2-92 (June 1992, Volume 2), covering heat-transfer balance, resistance method and boiling-regime limitations (one more source is tabulated below). Independent checks are on record (2026-10-25): Compared the five SI row ranges with the publicly accessible ASHRAE Table 1. This checks the table transcription, not suitability for a specific application; Checked the BtuIT conversion against Appendix B.9, printed p. 60. Calculated examples use explicitly assumed inputs. Nothing outside those scopes has been checked. Full sources, values and limits on this page.
Page last changed 2026-10-25 · based on 2021, SI, Chapter 4, Table 1
Quick Answer
Broad coefficient ranges — ASHRAE 2021, Chapter 4, Table 1
| Convection type[3] | Fluid class[3] | h min W/m²·K[3] | h max W/m²·K[3] | h min BtuIT/(h·ft²·°F)[1] | h max BtuIT/(h·ft²·°F)[1] |
|---|---|---|---|---|---|
| Natural convection # | Gases | 2 | 25 | 0.3522 | 4.403 |
| Natural convection # | Liquids | 10 | 1000 | 1.761 | 176.1 |
| Forced convection # | Gases | 25 | 250 | 4.403 | 44.03 |
| Forced convection # | Liquids | 50 | 20000 | 8.806 | 3522 |
| Boiling / condensation # | Phase change | 2500 | 100000 | 440.3 | 1.761e+04 |
These are broad published ranges, not guaranteed minima or maxima for a particular plate, pipe, fluid or machine. Source: ASHRAE 2021 Fundamentals, Chapter 4, Table 1. Boiling and condensation require phase-change methods; this table does not resolve their operating regimes.
Choose a coefficient for the actual flow
Record the fluid, temperature, pressure, surface geometry, characteristic length, orientation and flow conditions. Specify whether the coefficient is local or an average over the stated area. A gas-range entry does not establish an air coefficient for a particular fan speed.
A calculation using a correlation must meet that correlation’s geometry, flow-regime and property-evaluation conditions. Do not infer a universal multiplier between air and water, or assume that doubling velocity doubles h. See DOE-HDBK-1012/2-92, convection heat transfer, printed pages 18–25.
Calculate heat rate from an assumed h
Use Q̇ = hA(Ts − T∞), with Q̇ in W, h in W/(m²·K), A in m² and the temperature difference in K. Positive Q̇ means heat leaves the surface. The simple form assumes representative uniform temperatures and a coefficient appropriate to the area.
Illustrative inputs, not a recommended operating condition: h = 50 W/(m²·K), A = 0.20 m², Ts = 60°C and T∞ = 20°C. Then ΔT = 40 K and Q̇ = 50 × 0.20 × 40 = 400 W. If h is instead assumed to be 25, the result is 200 W. This shows sensitivity to h; it does not verify either coefficient. Radiation is excluded from this example.
Compare resistances on the same area basis
Total convection resistance is R = 1/(hA) in K/W. Area-normalized resistance is R″ = RA = 1/h in m²·K/W. For one-dimensional conduction through a plane wall, R″ = L/k. Do not add total resistance (K/W) to area-normalized resistance (m²·K/W).
Assumed example: L = 0.005 m and k = 200 W/(m·K) give R″wall = 0.000025 m²·K/W. At h = 50, R″conv = 0.020; at h = 5000, it is 0.000200. The convection term falls by a factor of 100. For this wall plus one convection boundary, the sum changes from 0.020025 to 0.000225, a factor of 89.
This is a same-area, one-dimensional resistance comparison. It does not establish a heat-sink size reduction: real equipment also depends on fin efficiency, spreading, contacts, additional boundaries, radiation and changes in flow. A surface coefficient h is not the overall coefficient U of an entire heat exchanger.
Convert SI and Btu units
NIST SP 811 (2008), Appendix B.9, printed page 60, lists 1 BtuIT/(h·ft²·°F) ≈ 5.678263 W/(m²·K). Divide the SI value by 5.678263 for the BtuIT column. For example, 50 W/(m²·K) ≈ 8.806 BtuIT/(h·ft²·°F). Values here are rounded to four significant figures; the rounded conversion is not described as exact.
Use temperature differences: 1 K difference equals 1°C difference. The BtuIT convention is specified because thermochemical Btu uses a different factor. Read the NIST conversion table.
When a broad range is not enough
Before choosing a value, document its source, geometry and applicable range. Check whether a surface changes temperature substantially along the flow, whether the coefficient varies over the surface, and whether a phase change occurs. These cases can require an average, integration or a different heat-transfer model.
Do not choose the largest number in the table as a performance promise. In boiling, transition to film boiling can reduce heat transfer; increasing heat input does not imply an ever-increasing usable coefficient. DOE discusses nucleate boiling, film boiling and critical heat flux on printed pages 40–43.
Frequently Asked Questions
- Is h a constant material property?
- No. A coefficient must be tied to the surface and flow conditions. Fluid properties matter, but a material name alone does not identify a usable value.
- Does a larger h make the complete cooler proportionally better?
- Not necessarily. The same-area resistance example changes the convection term by 100 times but the wall-plus-convection sum by 89 times. Other resistances and changing flow can further alter the result.
- Can I use a table endpoint as a design value?
- The endpoints only describe a broad published range. Use a correlation within its stated conditions or an appropriate measured coefficient for the actual geometry, fluid and flow.
Related
Value Sources
References and calculation methods used for this page are listed below. Numbered markers connect table columns where present. A reference listing does not itself establish that every value has been independently checked.
| # | Source | Type | Revision / method |
|---|---|---|---|
| [1] | BtuIT column calculated from SI values | derived | SI value divided by 5.678263, following NIST SP 811 (2008), Appendix B.9, printed page 60; rounded to four significant figures. |
| [2] | DOE Fundamentals Handbook, Thermodynamics, Heat Transfer, and Fluid Flow, Volume 2 | reference | DOE-HDBK-1012/2-92; printed pp. 18–25 and 40–43 — source |
| [3] | ASHRAE Handbook — Fundamentals, Chapter 4, Table 1 | reference | 2021 SI edition; broad convection-type ranges — source |
| [4] | NIST SP 811 — coefficient of heat transfer conversion | reference | 2008 edition, Appendix B.9, printed p. 60 — source |
Data Sources
| Standard | Revision | What it covers on this page |
|---|---|---|
| ASHRAE Handbook — Fundamentals | 2021, SI, Chapter 4, Table 1 | Five broad SI coefficient ranges; no geometry-specific or product recommendations. |
| DOE-HDBK-1012/2-92 | June 1992, Volume 2 | Heat-transfer balance, resistance method and boiling-regime limitations. |
| NIST SP 811 | 2008, Appendix B.9, p. 60 | BtuIT heat-transfer coefficient unit conversion. |
Verification scope
- 2026-10-25 — Compared the five SI row ranges with the publicly accessible ASHRAE Table 1. This checks the table transcription, not suitability for a specific application. Evidence
- 2026-10-25 — Checked the BtuIT conversion against Appendix B.9, printed p. 60. Calculated examples use explicitly assumed inputs. Evidence
Derived values — the following values on this page are calculated, not taken directly from the standard:
| Value | How it is derived |
|---|---|
| BtuIT table columns | SI / 5.678263; four significant figures. |
| Worked heat rate | Assumed 50 × 0.20 × (60 − 20) = 400 W; not measured equipment performance. |
| Worked area-normalized resistance | 0.005 / 200 = 0.000025; convection terms 1/50 and 1/5000; summed-resistance ratio 0.020025 / 0.000225 = 89. |
Source review on 2026-10-25 replaces the earlier geometry-specific compilation. No independently verified coefficients for particular fans, plates, pipes or products are claimed.
Cite This Page
Read the source notes, calculation methods and verification scope above before reusing a value. To cite this page:
Each row in the tables above also has a permanent link — hover a row and use the # link to cite a single value rather than the whole page.