# Color contrast checker (WCAG 2.2)

> Check the WCAG 2.2 contrast ratio between a text color and its background, with AA and AAA pass or fail for normal text, large text and UI parts.

Phiên bản tương tác: https://www.calcopenly.com/vi/programming/wcag-contrast-checker
Chủ đề: Máy tính lập trình và công nghệ

WCAG measures contrast as (L1 + 0.05) ÷ (L2 + 0.05), where L1 and L2 are the relative luminances of the lighter and darker color. Each luminance comes from the linearized sRGB channels, weighted 0.2126 for red, 0.7152 for green and 0.0722 for blue, so the ratio runs from 1:1 for identical colors to 21:1 for black on white.

Designers and developers check it before shipping text, buttons and icons. The default, #1F2937 slate text on a white background, measures 14.67:1 and passes all five checks, while #777777 on white measures 4.48:1, just under the 4.5:1 that AA needs for normal text.

The ratio is truncated to two decimals and compared unrounded, as WCAG requires, so 4.499:1 fails the 4.5:1 threshold. Alpha is ignored; blend a semi-transparent color onto its background first.

## Dữ liệu đầu vào

- **Text color**: #RRGGBB, a CSS name, rgb(), hsl() or oklch().
- **Background color**

## Kết quả

- Contrast ratio (:1) — kết quả chính
- AA normal text (≥ 4.5:1)
- AA large text (≥ 3:1)
- AAA normal text (≥ 7:1)
- AAA large text (≥ 4.5:1)
- UI components (≥ 3:1)
- Text luminance
- Background luminance

## Công thức

$$
\begin{aligned} \text{ratio} &= \frac{L_1 + 0.05}{L_2 + 0.05} \\[4pt] L &= 0.2126 R_{lin} + 0.7152 G_{lin} \\ &\quad + 0.0722 B_{lin} \end{aligned}
$$

## Ví dụ có lời giải

### Black on white is the maximum

- Text color: #000000
- Background color: #FFFFFF
- **Contrast ratio: 21.00 :1**
- **AA normal text (≥ 4.5:1): Đạt**
- **AAA normal text (≥ 7:1): Đạt**
- Nguồn đối chiếu: WCAG 2.2 definition: (1 + 0.05) / (0 + 0.05) = 21

### #777777 on white just misses AA

- Text color: #777777
- Background color: #FFFFFF
- **Contrast ratio: 4.48 :1**
- **AA normal text (≥ 4.5:1): Không đạt**
- **AA large text (≥ 3:1): Đạt**
- **UI components (≥ 3:1): Đạt**
- Nguồn đối chiếu: Python 3.8 decimal: luminance of 0x77 = 0.18447…, (1.05) / (0.23447…) = 4.4781…

### #767676 on white passes AA

- Text color: #767676
- Background color: #FFFFFF
- **Contrast ratio: 4.54 :1**
- **AA normal text (≥ 4.5:1): Đạt**
- **AAA normal text (≥ 7:1): Không đạt**
- **AAA large text (≥ 4.5:1): Đạt**
- Nguồn đối chiếu: Python 3.8 decimal: 1.05 / (0.18116… + 0.05) = 4.5422…

### Same color on itself (edge)

- Text color: teal
- Background color: #008080
- **Contrast ratio: 1.00 :1**
- **AA large text (≥ 3:1): Không đạt**
- **UI components (≥ 3:1): Không đạt**
- Nguồn đối chiếu: WCAG 2.2 definition: (L + 0.05) / (L + 0.05) = 1

### Order doesn't matter

- Text color: #FFFFFF
- Background color: #1F2937
- **Contrast ratio: 14.68 :1**
- **AAA normal text (≥ 7:1): Đạt**
- Nguồn đối chiếu: Python 3.8 decimal: luminances 1 and 0.021530…, 1.05 / 0.071530… = 14.679…; WCAG puts the lighter color on top

## Câu hỏi

### What contrast ratio does WCAG require?

Level AA needs at least 4.5:1 for normal text and 3:1 for large text (success criterion 1.4.3), and 3:1 for user interface components and meaningful graphics (1.4.11). Level AAA raises text to 7:1 and large text to 4.5:1 (1.4.6). Logos, disabled controls and purely decorative text have no requirement. Most laws and procurement rules point to level AA.

### What counts as large text in WCAG?

Large text is at least 18 point, or 14 point when bold. In CSS, where 1 pt is 1.333 px, that is 24 px for regular weight and about 18.66 px for bold. W3C's Understanding document puts these at roughly 1.5 em and 1.2 em of default body text, and warns that very thin fonts may need more. Large text only needs 3:1 at AA instead of 4.5:1.

### What is the lightest gray text that passes AA on white?

#767676, which gives 4.54:1 on #FFFFFF. One step lighter, #777777, gives 4.48:1 and fails, because WCAG compares the exact ratio with 4.5 and allows no rounding up. For large text you can go much lighter: any gray that reaches 3:1, such as #949494 at 3.03:1, passes AA.

### Did WCAG 2.2 change the contrast requirements?

No. WCAG 2.2 became a W3C Recommendation on 5 October 2023 and added 9 success criteria, mainly on focus visibility, target size and authentication, but the contrast criteria 1.4.3, 1.4.6 and 1.4.11 are the same as in WCAG 2.1. A color pair that passed under 2.1 still passes under 2.2, at the same 4.5:1 and 3:1 thresholds.

### Does the order of text and background colors matter?

No. The formula always puts the lighter color's luminance on top, so white text on #1F2937 and #1F2937 text on white both measure 14.67:1. What changes is how the pair reads: light text on a dark background can look thinner or bolder depending on the font and screen, so check the actual rendering too, especially at small sizes.

### “Color contrast checker (WCAG 2.2)” chính xác đến mức nào?

Độ chính xác phụ thuộc vào dữ liệu nhập và giả định của phương pháp. Phép tính thập phân dùng 50 chữ số có nghĩa, nhưng ước lượng, phương pháp số và dữ liệu nguồn có thể kém chính xác hơn; làm tròn khi hiển thị không loại bỏ các giới hạn đó. Ví dụ có lời giải đã đối chiếu với nguồn độc lập: 5. Ví dụ, “Black on white is the maximum” được kiểm tra bằng WCAG 2.2 definition: (1 + 0.05) / (0 + 0.05) = 21.

### Phương pháp này lấy từ đâu?

W3C WCAG 2.2 — Success Criteria 1.4.3, 1.4.6 and 1.4.11; definitions of contrast ratio and relative luminance; W3C Understanding SC 1.4.3 — large text is at least 18 pt, or 14 pt bold.

## Nguồn

- [W3C WCAG 2.2 — Success Criteria 1.4.3, 1.4.6 and 1.4.11; definitions of contrast ratio and relative luminance](https://www.w3.org/TR/WCAG22/#dfn-contrast-ratio)
- [W3C Understanding SC 1.4.3 — large text is at least 18 pt, or 14 pt bold](https://www.w3.org/WAI/WCAG22/Understanding/contrast-minimum.html)