# Refrigerant Capillary Tube Calculator

URL: https://speedycalculator.com/calculators/physics/refrigerant-capillary-tube/
Last updated: 2026-10-30

> The refrigerant capillary tube calculator finds the length of a replacement capillary tube with a different inner diameter that gives the same flow restriction as the original.

## Inputs

- Original inner diameter (in, mm)
- Original length (ft, m, in)
- Replacement inner diameter (in, mm)

## Results

- Replacement length
- Length ratio

## Formula

```
L₂ = L₁·(d₂/d₁)^{4.6}
```

## Explanation

Small refrigerators and air conditioners use a thin copper capillary tube as the expansion device. If the original diameter isn't available, a different diameter can be used with an adjusted length to keep the same restriction.

### How to use the Refrigerant Capillary Tube Calculator

- **Original inner diameter**
- **Original length**
- **Replacement inner diameter**

Results update as you type, every field has its own unit menu, and everything is calculated in your browser.

### How it works

Enter the original inner diameter and length, and the inner diameter of the replacement tube.

### Inputs explained

FieldWhat to enterOriginal inner diameterUnits: in, mmOriginal lengthUnits: ft, m, inReplacement inner diameterUnits: in, mm

### What each result means

- **Replacement length** = [Original length]×([Replacement inner diameter]÷[Original inner diameter])^4.6
- **Length ratio** = ([Replacement inner diameter]÷[Original inner diameter])^4.6

### Example values

How the main result changes when **Original inner diameter** changes and everything else stays at the example values above:

Original inner diameter (in)Replacement length (ft)0.0155482.46490.023374.72160.03119.89430.03887.12760.04653.08110.0620.8203

## Worked example

Inputs — Original inner diameter: 0.031 in; Original length: 10 ft; Replacement inner diameter: 0.036 in.

Results — **Replacement length = 19.8943 ft**; Length ratio = 1.9894 ×.

## Key terms

- **Capillary tube**: Small-bore tube used as a fixed expansion device.

## FAQ

**Why the exponent 4.6?**
Empirically, capillary flow restriction scales with about d^4.6.

**Should I use a smaller or larger tube?**
Either works with the adjusted length; a larger one becomes very long.

**Is this exact?**
It is a rule of thumb – verify the system charge and pressures.

**What else affects capillary flow?**
Refrigerant type, subcooling and pressures.


## Sources

- [OpenStax – University Physics Volume 1](https://openstax.org/details/books/university-physics-volume-1)
- [The Engineering ToolBox](https://www.engineeringtoolbox.com/)