The Nusselt number tells how much convection enhances heat transfer over pure conduction through a still fluid. Engineers get h from Nusselt correlations for pipes, plates and cylinders.
How to use the Nusselt Number Calculator
- Find
- Heat transfer coefficient h
- Nusselt number
- Characteristic length L
- Fluid thermal conductivity k
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose what to find and enter the values.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Find | Choose one: Nusselt number, Heat transfer coefficient h |
| Heat transfer coefficient h | Units: W/(m²·K), BTU/(h·ft²·°F) |
| Nusselt number | A number |
| Characteristic length L | Units: m, cm, mm, km, in, ft … |
| Fluid thermal conductivity k | Units: W/(m·K), BTU/(h·ft·°F) |
What each result means
- Nusselt number = [Heat transfer coefficient h]×[Characteristic length L]÷[Fluid thermal conductivity k]
- Heat transfer coefficient h = [Nusselt number]×[Fluid thermal conductivity k]÷[Characteristic length L]
Example values
How the main result changes when Heat transfer coefficient h changes and everything else stays at the example values above:
| Heat transfer coefficient h (W/(m²·K)) | Nusselt number |
|---|---|
| 25 | 48.0769 |
| 37.5 | 72.1154 |
| 50 | 96.1538 |
| 62.5 | 120.1923 |
| 75 | 144.2308 |
| 100 | 192.3077 |
Formula
Nu = h·L / kWorked example
Inputs — Find: Nusselt number; Heat transfer coefficient h: 50 W/(m²·K); Characteristic length L: 0.05 m; Fluid thermal conductivity k: 0.026 W/(m·K).
Results — Nusselt number = 96.15385.
Key terms
- Convection
- Heat transfer by fluid motion.
Frequently asked questions
What does Nu = 1 mean?
Heat transfer is by conduction alone.
What is the characteristic length?
Diameter for pipes, length for plates.
How is Nu found?
From correlations such as Dittus–Boelter: Nu = 0.023 Re^0.8 Pr^0.4.
How is it different from the Biot number?
Nu uses the fluid's k; Bi uses the solid's.
What formula does the Nusselt Number Calculator use?
Nu = h·L / k.