---
title: Calculate a sum or difference of water meters in Chart’nX
description: With the Chart’nX Operations protocol, add or subtract the meters of a water group to get a calculated volume, for example the difference of two meters.
canonical: https://wiki.can-nx.com/en/guides/chart-water-operations/
language: en
product: Chart’nX
audience:
  - administrateur
  - installateur
context: "Chart’nX: water dashboard generator, meter set to Operations within a group"
---

# Calculate a sum or difference of water meters in Chart’nX

Context: Chart’nX: water dashboard generator, meter set to Operations within a group

In a Chart’nX water dashboard, the **Operations** protocol calculates a meter’s value from other meters of the same group: a sum or a difference of known measurements. Fictitious example of a need: a measured primary volume minus a measured secondary volume. To add up all the secondaries in the primary, the **Virtual (∑ secondaries)** mode described in [Add a water group](https://wiki.can-nx.com/en/guides/chart-water-groups/) is enough.

Intended for administrators and installers of a Chart’nX water dashboard.

## Before you start

- The meters to combine already exist in the same group.
- Prepare the composition from the measurements of the installation: the measurements must cover the same periods and must not be counted several times.
- If the composition requires a chain of calculations that depend on one another, or a virtual primary that indirectly reuses its own result, have it validated by support before saving it.

## Choose the calculated meter

1. Open the water dashboard with **Configure**, then find the group in **Sub-meters**.
2. In a secondary, or in the primary set to **Real**, give the result an explicit name and choose **Operations** in **Protocol**.

## Compose the calculation

1. Click **+ Add operations**. The **Compose the calculation** window lists, in its **Meters** section, the other meters of this group. The meter being composed cannot select itself.
2. Click each row to include. For each one, choose **Sum** to add it or **Subtract** to deduct it.
3. If the list is long, use **Search for a meter…**. **Select all** or **Deselect all** applies to the visible rows of the section. A selection hidden by the search stays kept: clear the search and reread everything before confirming.
4. Click **Save** in the window. The components and their signs appear in the card. **Cancel** in the window keeps the previous composition.
5. Reread the unit of the calculated meter and the selected components, then [save the dashboard](https://wiki.can-nx.com/en/guides/chart-save/).

The **Energy balance** and **Sub-meters** shortcuts of the energy total window are not part of this flow: the water window only shows the **Meters** section.

## Check the result

Open the dashboard over a known period and compare the result of the calculation with the expected measurements.

## Troubleshooting

| Message | Action |
| --- | --- |
| **No meter is available for this operation.** | Check that other meters have been created in the same group. |
| **No meter matches this search.** | Clear the search. |
| **Unavailable meter** | Reopen **Add operations** and identify the expected component. Remove a reference that has become unnecessary only if the calculation remains correct. |

After deleting a meter, reread all the calculations concerned, their signs and their unit. To create another composition, add a new secondary and compose it explicitly.

If saving does not produce the expected components or if the values remain inconsistent after checking, see [the water troubleshooting](https://wiki.can-nx.com/en/guides/chart-water-help/). If you cannot establish the correct composition, contact support@can-nx.com.