# Nusselt Number Calculator

URL: https://speedycalculator.com/calculators/physics/nusselt-number/
Last updated: 2026-10-30

> The Nusselt number calculator finds the ratio of convective to conductive heat transfer at a surface from the heat transfer coefficient, the characteristic length and the fluid's thermal conductivity – or the heat transfer coefficient from Nu.

## Inputs

- Find
- Heat transfer coefficient h (W/(m²·K), BTU/(h·ft²·°F))
- Nusselt number
- Characteristic length L (m, cm, mm, km, in, ft, yd, mi)
- Fluid thermal conductivity k (W/(m·K), BTU/(h·ft·°F))

## Results

- Nusselt number
- Heat transfer coefficient h

## Formula

```
Nu = h·L / k
```

## Explanation

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

FieldWhat to enterFindChoose one: Nusselt number, Heat transfer coefficient hHeat transfer coefficient hUnits: W/(m²·K), BTU/(h·ft²·°F)Nusselt numberA numberCharacteristic length LUnits: m, cm, mm, km, in, ft …Fluid thermal conductivity kUnits: 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 number2548.076937.572.11545096.153862.5120.192375144.2308100192.3077

## Worked 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.

## FAQ

**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.


## Sources

- [Incropera – Fundamentals of Heat and Mass Transfer (reference text)](https://www.wiley.com/en-us/Fundamentals+of+Heat+and+Mass+Transfer%2C+8th+Edition-p-9781119320425)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)