Is #ffffff on #498500 accessible?

Yes for WCAG 2.2 level AA: the contrast ratio is 4.52:1, above the 4.5:1 needed for normal text. It does not reach the 7:1 needed for AAA.

What is the contrast ratio of #ffffff and #498500?

4.52:1, calculated with the WCAG 2.2 relative luminance formula. The ratio is truncated, not rounded, so it never shows a failing pair as passing.

4.52:1

White text (#ffffff) on a dark green background (#498500) meets WCAG 2.2 level AA for normal text. It passes 4 of 5 checks below.

Large text sample

Normal body text in #ffffff on #498500. The quick brown fox jumps over the lazy dog.

Simulated with the Machado et al. (2009) model at full severity.

Hex, rgb() or hsl(). Alpha is composited.

Text colour

Background colour

Large heading text

Normal body text at 14 px. The quick brown fox jumps over the lazy dog.

Outlined buttonA text link

Simulated with Machado et al. (2009), full severity.

4.52:1

WCAG 2.2 contrast verdicts
Check	Criterion	Needs	Result
Normal text	1.4.3 AA	4.5:1	Pass
Large text	1.4.3 AA	3:1	Pass
UI components & graphics	1.4.11 AA	3:1	Pass
Normal text	1.4.6 AAA	7:1	Fail
Large text	1.4.6 AAA	4.5:1	Pass

Same hue, lightness adjusted in OKLCH.

AA — 4.5:1

Already passes.

AAA — 7:1

Change text: no colour with this hue reaches the target.

Change background

#2b6500 · 7.08:1

Large text means at least 24 CSS px, or 18.66 CSS px bold. Ratios are truncated, never rounded, so 4.49:1 is shown as failing 4.5:1.

Is #ffffff on #498500 accessible?: Yes for WCAG 2.2 level AA: the contrast ratio is 4.52:1, above the 4.5:1 needed for normal text. It does not reach the 7:1 needed for AAA.
What is the contrast ratio of #ffffff and #498500?: 4.52:1, calculated with the WCAG 2.2 relative luminance formula. The ratio is truncated, not rounded, so it never shows a failing pair as passing.