---
title: "Use bounded labels | Grafana Labs"
description: "The most important labeling rule and the values to never use as metric labels"
---

> For a curated documentation index, see [llms.txt](/llms.txt). For the complete documentation index, see [llms-full.txt](/llms-full.txt).

## Use bounded labels

A metric label should be a dimension you aggregate or filter by, with a bounded set of values.

The single most important rule is that **label values must be bounded**. A label like `env` has a handful of values forever. A label like `user_id` is an **unbounded label**: it grows without limit, and every new value creates another active series that’s billed and indexed.

Never use these as metric label values:

- User, customer, or session IDs
- Request or trace IDs
- IP addresses
- Email addresses
- Timestamps

Each of these creates a new series per unique value, and the combinations multiply against every other label on the metric.

[A chart comparing active series over time: a bounded label like env stays flat while an unbounded label like user_id grows without limit](unbounded-label-growth.svg "A chart comparing active series over time: a bounded label like env stays flat while an unbounded label like user_id grows without limit")