---
title: Calculating the 'system.mem.used' metric
description: Datadog, the leading service for cloud-scale monitoring.
breadcrumbs: Docs > Metrics > Metrics Guides > Calculating the 'system.mem.used' metric
---

# Calculating the 'system.mem.used' metric

The manner in which Datadog calculates the `system.mem.used` metric produces a value that may sometimes be different from what might be displayed by common system resource reporting tools.

For example, running 'free -m' on an Ubuntu machine may produce the following memory breakdown (values represent megabytes):

|  |
|  |
| total  | used | free  | shared | cached | available |
| 128831 | 1203 | 71975 | 4089   | 55653  | 122380    |

A Datadog Agent running on this same machine reports a `system.mem.used` metric with a value of 56856 MB—clearly different from the 'free -m' used memory value of 1203 MB.

The reason for this discrepancy is that Datadog includes cached memory in its formula for used memory, where 'free -m' does not.

Datadog calculates used memory as follows:

- system.mem.used(56856) = system.mem.total(128831) - system.mem.free(71975)

Again, Datadog's `system.mem.used` metric includes cached memory, so subtracting this cached memory from used memory results in the following value:

- system.mem.used(56856) - system.mem.cached(55653) = 1203

1203 MB—identical to the used memory value reported by 'free -m' in the example above.

**The `system.mem.usable` metric represents free memory plus cached memory plus buffers** (on Linux, it reflects "MemAvailable" attribute from /proc/meminfo whenever possible).

- [Learn more about Metrics](https://docs.datadoghq.com/metrics.md)