# How to read a dip rod: cm, mm and bars

> Explains how to read dip rods marked in mm, or in cm with four 2 mm bars between each centimetre, and why writing '58.4' for 58 cm and 4 bars as 58.4 cm costs tens of litres.

Canonical: https://dipdensity.watsinfo.com/guides/dip-rod-cm-mm-and-bars/
Last updated: 2026-11-02

**Short answer:** Some dip rods mark every millimetre; others mark each centimetre with four bars between, each bar 2 mm. On a bar rod, 58 cm and 4 bars is 58.8 cm. If it is written 58.4 and read as a decimal, the dip is 4 mm low: about 50 litres in a 20 KL tank near half full.

## Two kinds of marking

**Millimetre rods** mark every millimetre between the centimetre numbers. A wet line three marks past 58 is **58.3 cm** or **583 mm**. Simple.

**Bar rods** mark each centimetre with **four bars**, splitting it into five steps of **2 mm**. The fifth step reaches the next centimetre. A wet line at the fourth bar past 58 is:

```
58 cm + 4 bars × 0.2 cm = 58.8 cm
```

## The decimal trap

On a bar rod people often write "58.4" meaning 58 cm and 4 bars. It looks like a decimal, and anyone who types it into a chart as 58.4 cm is **0.4 cm low**. In a 20 KL tank near half full, 4 mm is around 50 litres. On a tanker reconciliation with two dips, the error can double.

| Written | Means | Correct dip |
|---|---|---|
| 58.1 | 58 cm + 1 bar | 58.2 cm |
| 58.2 | 58 cm + 2 bars | 58.4 cm |
| 58.3 | 58 cm + 3 bars | 58.6 cm |
| 58.4 | 58 cm + 4 bars | 58.8 cm |

A figure after the point of 5 or more cannot come from a bar rod, so if you see 58.7 in a bar-rod register, someone has mixed the two.

## Make it one way

Agree with everyone who dips: write **cm and mm** always, or write bars as **58 + 4** so nobody mistakes it for a decimal. The [dip to litres calculator](https://dipdensity.watsinfo.com/tools/dip-to-litres-calculator/) has a cm.bars setting that reads 58.4 correctly as 58.8 cm.

## Questions

### How do I know which kind of rod I have?

Count the marks between two centimetre numbers. Nine small marks make ten 1 mm spaces. Four marks make five 2 mm spaces: a bar rod.

### Does the app take bars?

The app takes cm or mm. Convert bars first: centimetres plus bars × 0.2. The calculators on this site have a cm.bars option that does it for you.