Meter

A graphical display of a numeric value within a defined range.

Demo

Native HTML

Use Native HTML First

Before using this custom component, consider using native <meter> elements. They provide built-in semantics, work without JavaScript, and require no ARIA attributes.

<label for="battery">Battery Level</label>
<meter id="battery" value="75" min="0" max="100">75%</meter>

Use custom implementations only when you need custom styling that native elements cannot provide, or when you need programmatic control over the visual appearance.

Use Case	Native HTML	Custom Implementation
Basic value display	Recommended	Not needed
JavaScript disabled support	Works natively	Requires fallback
low/high/optimum thresholds	Built-in support	Manual implementation
Custom styling	Limited (browser-dependent)	Full control
Consistent cross-browser appearance	Varies by browser	Consistent
Dynamic value updates	Works natively	Full control

The native <meter> element supports low, high, and optimum attributes for automatic color changes based on value thresholds. This functionality requires manual implementation in custom components.

Accessibility Features

WAI-ARIA Role

Role	Element	Description
`meter`	Container element	Identifies the element as a meter displaying a scalar value within a known range.

The meter role is used for graphical displays of numeric values within a defined range. It is not interactive and should not receive focus by default.

WAI-ARIA Properties

`aria-valuenow`

Indicates the current numeric value of the meter. Required for all meter implementations.

Type	Number
Required	Yes
Range	Must be between `aria-valuemin` and `aria-valuemax`

`aria-valuemin`

Specifies the minimum allowed value for the meter.

Type	Number
Required	Yes
Default	0

`aria-valuemax`

Specifies the maximum allowed value for the meter.

Type	Number
Required	Yes
Default	100

`aria-valuetext`

Provides a human-readable text alternative for the current value. Use when the numeric value alone doesn't convey sufficient meaning.

Type	String
Required	No (recommended when value needs context)
Example	"75% complete", "3 out of 4 GB used"

Keyboard Support

Not applicable. The meter pattern is a display-only element and is not interactive. It should not receive keyboard focus unless explicitly made focusable for specific use cases.

Accessible Naming

Meters must have an accessible name. This can be provided through:

Visible label - Using the label prop to display a visible label
aria-label - Provides an invisible label for the meter
aria-labelledby - References an external element as the label

Visual Design

This implementation follows WCAG guidelines for accessible visual design:

Visual fill bar - Proportionally represents the current value
Numeric display - Optional text showing the current value
Visible label - Optional label identifying the meter's purpose
Forced colors mode - Uses system colors for accessibility in Windows High Contrast Mode

References

Source Code

Meter.svelte

<script lang="ts">
  interface MeterProps {
    value: number;
    min?: number;
    max?: number;
    clamp?: boolean;
    showValue?: boolean;
    label?: string;
    valueText?: string;
    /** Format pattern for dynamic value display (e.g., "{value}%", "{value} of {max}") */
    format?: string;
    [key: string]: unknown;
  }

  let {
    value,
    min = 0,
    max = 100,
    clamp = true,
    showValue = true,
    label,
    valueText,
    format,
    ...restProps
  }: MeterProps = $props();

  function clampNumber(val: number, minVal: number, maxVal: number, shouldClamp: boolean): number {
    if (!Number.isFinite(val) || !Number.isFinite(minVal) || !Number.isFinite(maxVal)) {
      return val;
    }
    return shouldClamp ? Math.min(maxVal, Math.max(minVal, val)) : val;
  }

  // Format value helper
  function formatValueText(
    val: number,
    formatStr: string | undefined,
    minVal: number,
    maxVal: number
  ): string {
    if (!formatStr) return String(val);
    return formatStr
      .replace('{value}', String(val))
      .replace('{min}', String(minVal))
      .replace('{max}', String(maxVal));
  }

  const normalizedValue = $derived(clampNumber(value, min, max, clamp));

  const percentage = $derived(() => {
    if (max === min) return 0;
    const pct = ((normalizedValue - min) / (max - min)) * 100;
    return Math.max(0, Math.min(100, pct));
  });

  const ariaValueText = $derived(
    valueText ?? (format ? formatValueText(normalizedValue, format, min, max) : undefined)
  );

  const displayText = $derived(
    valueText ? valueText : formatValueText(normalizedValue, format, min, max)
  );
</script>

<div
  role="meter"
  aria-valuenow={normalizedValue}
  aria-valuemin={min}
  aria-valuemax={max}
  aria-valuetext={ariaValueText}
  aria-label={label || restProps['aria-label']}
  aria-labelledby={restProps['aria-labelledby']}
  aria-describedby={restProps['aria-describedby']}
  class="apg-meter {restProps.class || ''}"
  id={restProps.id}
  tabindex={restProps.tabindex}
  data-testid={restProps['data-testid']}
>
  {#if label}
    <span class="apg-meter-label" aria-hidden="true">
      {label}
    </span>
  {/if}
  <div class="apg-meter-track" aria-hidden="true">
    <div class="apg-meter-fill" style="width: {percentage()}%"></div>
  </div>
  {#if showValue}
    <span class="apg-meter-value" aria-hidden="true">
      {displayText}
    </span>
  {/if}
</div>

Usage

Example

<script>
  import Meter from './Meter.svelte';
</script>

<!-- Basic usage with aria-label -->
<Meter value={75} aria-label="CPU Usage" />

<!-- With visible label -->
<Meter value={75} label="CPU Usage" />

<!-- With format pattern -->
<Meter
  value={75}
  label="Progress"
  format="{value}%"
/>

<!-- Custom range -->
<Meter
  value={3.5}
  min={0}
  max={5}
  label="Rating"
  format="{value} / {max}"
/>

<!-- With valueText for screen readers -->
<Meter
  value={75}
  label="Download Progress"
  valueText="75 percent complete"
/>

API

Prop	Type	Default	Description
`value`	number	required	Current value of the meter
`min`	number	0	Minimum value
`max`	number	100	Maximum value
`clamp`	boolean	true	Whether to clamp value to min/max range
`showValue`	boolean	true	Whether to display the value text
`label`	string	-	Visible label (also used as aria-label)
`valueText`	string	-	Human-readable value for aria-valuetext
`format`	string	-	Format pattern for display and aria-valuetext (e.g., "{value}%", "{value} of {max}")

One of label, aria-label, or aria-labelledby is required for accessibility.

Testing

Tests verify APG compliance for ARIA attributes, value handling, and accessibility requirements. Since Meter is a display-only element, keyboard interaction tests are not applicable.

Test Categories

High Priority: ARIA Attributes

Test	Description
`role="meter"`	Element has the meter role
`aria-valuenow`	Current value is correctly set
`aria-valuemin`	Minimum value is set (default: 0)
`aria-valuemax`	Maximum value is set (default: 100)
`aria-valuetext`	Human-readable text is set when provided

High Priority: Accessible Name

Test	Description
`aria-label`	Accessible name via aria-label attribute
`aria-labelledby`	Accessible name via external element reference
`visible label`	Visible label provides accessible name

High Priority: Value Clamping

Test	Description
`clamp above max`	Values above max are clamped to max
`clamp below min`	Values below min are clamped to min
`no clamp`	Clamping can be disabled with clamp=false

Medium Priority: Accessibility

Test	Description
`axe violations`	No accessibility violations detected by axe-core
`focus behavior`	Not focusable by default

Medium Priority: Edge Cases

Test	Description
`decimal values`	Handles decimal values correctly
`negative range`	Handles negative min/max ranges
`large values`	Handles large numeric values

Low Priority: HTML Attribute Inheritance

Test	Description
`className`	Custom class is applied to container
`id`	ID attribute is set correctly
`data-*`	Data attributes are passed through

Testing Tools

Vitest (opens in new tab) - Test runner for unit tests
Testing Library (opens in new tab) - Framework-specific testing utilities (React, Vue, Svelte)
Playwright (opens in new tab) - Browser automation for E2E tests
axe-core/playwright (opens in new tab) - Automated accessibility testing in E2E

Meter.test.svelte.ts

import { render, screen } from '@testing-library/svelte';
import { axe } from 'jest-axe';
import { describe, expect, it } from 'vitest';
import Meter from './Meter.svelte';
import MeterWithLabel from './MeterWithLabel.test.svelte';

describe('Meter (Svelte)', () => {
  // 🔴 High Priority: ARIA Attributes
  describe('ARIA Attributes', () => {
    it('has role="meter"', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress' },
      });
      expect(screen.getByRole('meter')).toBeInTheDocument();
    });

    it('has aria-valuenow set to current value', () => {
      render(Meter, {
        props: { value: 75, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuenow', '75');
    });

    it('has aria-valuemin set (default: 0)', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuemin', '0');
    });

    it('has aria-valuemax set (default: 100)', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuemax', '100');
    });

    it('has custom aria-valuemin when provided', () => {
      render(Meter, {
        props: { value: 50, min: 10, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuemin', '10');
    });

    it('has custom aria-valuemax when provided', () => {
      render(Meter, {
        props: { value: 50, max: 200, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuemax', '200');
    });

    it('has aria-valuetext when valueText provided', () => {
      render(Meter, {
        props: { value: 75, valueText: '75 percent complete', 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuetext', '75 percent complete');
    });

    it('does not have aria-valuetext when not provided', () => {
      render(Meter, {
        props: { value: 75, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).not.toHaveAttribute('aria-valuetext');
    });

    it('uses format for aria-valuetext', () => {
      render(Meter, {
        props: {
          value: 75,
          min: 0,
          max: 100,
          format: '{value}%',
          'aria-label': 'Progress',
        },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuetext', '75%');
    });
  });

  // 🔴 High Priority: Accessible Name
  describe('Accessible Name', () => {
    it('has accessible name via aria-label', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'CPU Usage' },
      });
      expect(screen.getByRole('meter', { name: 'CPU Usage' })).toBeInTheDocument();
    });

    it('has accessible name via aria-labelledby', () => {
      render(MeterWithLabel);
      expect(screen.getByRole('meter', { name: 'Battery Level' })).toBeInTheDocument();
    });

    it('has accessible name via visible label', () => {
      render(Meter, {
        props: { value: 50, label: 'Storage Used' },
      });
      expect(screen.getByRole('meter', { name: 'Storage Used' })).toBeInTheDocument();
    });
  });

  // 🔴 High Priority: Value Clamping
  describe('Value Clamping', () => {
    it('clamps value above max to max', () => {
      render(Meter, {
        props: { value: 150, min: 0, max: 100, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuenow', '100');
    });

    it('clamps value below min to min', () => {
      render(Meter, {
        props: { value: -50, min: 0, max: 100, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuenow', '0');
    });

    it('does not clamp when clamp=false', () => {
      render(Meter, {
        props: { value: 150, min: 0, max: 100, clamp: false, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuenow', '150');
    });
  });

  // 🔴 High Priority: Focus Behavior
  describe('Focus Behavior', () => {
    it('is not focusable by default', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).not.toHaveAttribute('tabindex');
    });

    it('is focusable when tabindex is provided', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress', tabindex: 0 },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('tabindex', '0');
    });
  });

  // 🟡 Medium Priority: Accessibility
  describe('Accessibility', () => {
    it('has no axe violations', async () => {
      const { container } = render(Meter, {
        props: { value: 50, 'aria-label': 'Progress' },
      });
      const results = await axe(container);
      expect(results).toHaveNoViolations();
    });

    it('has no axe violations with visible label', async () => {
      const { container } = render(Meter, {
        props: { value: 50, label: 'CPU Usage' },
      });
      const results = await axe(container);
      expect(results).toHaveNoViolations();
    });

    it('has no axe violations with valueText', async () => {
      const { container } = render(Meter, {
        props: { value: 75, valueText: '75% complete', 'aria-label': 'Progress' },
      });
      const results = await axe(container);
      expect(results).toHaveNoViolations();
    });
  });

  // 🟡 Medium Priority: Edge Cases
  describe('Edge Cases', () => {
    it('handles decimal values correctly', () => {
      render(Meter, {
        props: { value: 33.33, 'aria-label': 'Progress' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuenow', '33.33');
    });

    it('handles negative min/max range', () => {
      render(Meter, {
        props: { value: 0, min: -50, max: 50, 'aria-label': 'Temperature' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('aria-valuenow', '0');
      expect(meter).toHaveAttribute('aria-valuemin', '-50');
      expect(meter).toHaveAttribute('aria-valuemax', '50');
    });
  });

  // 🟡 Medium Priority: Visual Display
  describe('Visual Display', () => {
    it('shows value when showValue is true (default)', () => {
      render(Meter, {
        props: { value: 75, 'aria-label': 'Progress' },
      });
      expect(screen.getByText('75')).toBeInTheDocument();
    });

    it('hides value when showValue is false', () => {
      render(Meter, {
        props: { value: 75, showValue: false, 'aria-label': 'Progress' },
      });
      expect(screen.queryByText('75')).not.toBeInTheDocument();
    });

    it('displays formatted value when format provided', () => {
      render(Meter, {
        props: {
          value: 75,
          format: '{value}%',
          'aria-label': 'Progress',
        },
      });
      expect(screen.getByText('75%')).toBeInTheDocument();
    });

    it('displays visible label when label provided', () => {
      render(Meter, {
        props: { value: 50, label: 'CPU Usage' },
      });
      expect(screen.getByText('CPU Usage')).toBeInTheDocument();
    });
  });

  // 🟢 Low Priority: HTML Attribute Inheritance
  describe('HTML Attribute Inheritance', () => {
    it('applies class to container', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress', class: 'custom-meter' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveClass('custom-meter');
    });

    it('sets id attribute', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress', id: 'my-meter' },
      });
      const meter = screen.getByRole('meter');
      expect(meter).toHaveAttribute('id', 'my-meter');
    });

    it('passes through data-* attributes', () => {
      render(Meter, {
        props: { value: 50, 'aria-label': 'Progress', 'data-testid': 'custom-meter' },
      });
      expect(screen.getByTestId('custom-meter')).toBeInTheDocument();
    });
  });
});

Resources

WAI-ARIA APG: Meter Pattern (opens in new tab)
MDN: <meter> element (opens in new tab)
AI Implementation Guide (llm.md) (opens in new tab) - ARIA specs, keyboard support, test checklist