# Aspect ratio calculator (16:9, 4:3, 21:9)

> Find the aspect ratio of any resolution, such as 1920 × 1080 = 16:9, resize it to a new width or height, or fit it in a screen with letterbox bars.

Phiên bản tương tác: https://www.calcopenly.com/vi/programming/aspect-ratio-resolution-calculator
Chủ đề: Máy tính lập trình và công nghệ

The aspect ratio is the width divided by the height, written in lowest terms by dividing both sides by their greatest common divisor: 1920 ÷ 120 = 16 and 1080 ÷ 120 = 9, so 1920 × 1080 is 16:9. Resizing keeps that quotient fixed, so the new height is the new width × H ÷ W. Fitting a picture inside another frame uses the largest scale that fits both ways, s = min(frame width ÷ W, frame height ÷ H), and the leftover space becomes bars.

Scaling 1920 × 1080 to 1280 wide gives 1280 × 720. Showing that 16:9 picture on a 1024 × 768 screen gives a 1024 × 576 picture with 96 px letterbox bars above and below, so the picture uses 75% of the screen.

Sizes stay exact fractions until the end. Scaled sizes are then rounded to whole pixels, which can shift the ratio slightly.

## Dữ liệu đầu vào

- **I want to** (lựa chọn: Find the ratio, Resize keeping the ratio, Fit inside another frame)
- **Source ratio** (lựa chọn: Use the width and height below, 16:9 (HD video, most monitors), 4:3 (older TVs, iPad), 21:9 (ultrawide, as marketed), 9:16 (vertical phone video), 1:1 (square), 3:2 (35 mm photo), 16:10 (laptops), 4:5 (portrait social post), 1.85:1 (flat cinema), 2.39:1 (scope cinema), 32:9 (super ultrawide))
- **Chiều rộng**
- **Chiều cao**
- **New size is given as** (lựa chọn: Chiều rộng, Chiều cao)
- **New width or height**
- **Frame width**
- **Frame height**

## Kết quả

- Kết quả
- Simplified ratio — kết quả chính
- Width ÷ height
- Closest common ratio
- New width (px)
- New height (px)
- Picture width in frame (px)
- Picture height in frame (px)
- Bars
- Each bar (px)
- Frame area used by the picture

## Công thức

$$
\begin{aligned} W{:}H &= \frac{W}{\gcd(W,H)} : \frac{H}{\gcd(W,H)} \\[4pt] s &= \min\!\left(\frac{W_f}{W}, \frac{H_f}{H}\right)\end{aligned}
$$

## Ví dụ có lời giải

### Full HD

- I want to: Find the ratio
- Chiều rộng: 1920 px
- Chiều cao: 1080 px
- **Simplified ratio: 16:9**
- **Width ÷ height: 1.7778**
- **Closest common ratio: 16:9**
- Nguồn đối chiếu: Python 3.8: fractions.Fraction(1920, 1080) = 16/9

### Ultrawide 2560 × 1080

- I want to: Find the ratio
- Chiều rộng: 2560 px
- Chiều cao: 1080 px
- **Simplified ratio: 64:27**
- **Width ÷ height: 2.3704**
- Nguồn đối chiếu: Python 3.8: fractions.Fraction(2560, 1080) = 64/27 = 2.37037…

### 1366 × 768 laptop is not exactly 16:9 (edge)

- I want to: Find the ratio
- Chiều rộng: 1366 px
- Chiều cao: 768 px
- **Simplified ratio: 683:384**
- **Width ÷ height: 1.7786**
- **Closest common ratio: 16:9 (0.05% wider)**
- Nguồn đối chiếu: Python 3.8: Fraction(1366, 768) = 683/384; (683/384) / (16/9) − 1 = 0.0488%

### Decimal cinema ratio

- I want to: Find the ratio
- Chiều rộng: 2.39 px
- Chiều cao: 1 px
- **Simplified ratio: 239:100**
- Nguồn đối chiếu: Python 3.8: Fraction('2.39') / 1 = 239/100

### Scale 1920 × 1080 to 1280 wide

- I want to: Resize keeping the ratio
- Source ratio: Use the width and height below
- Chiều rộng: 1920 px
- Chiều cao: 1080 px
- New size is given as: Chiều rộng
- New width or height: 1280 px
- **New width: 1,280 px**
- **New height: 720 px**
- **Kết quả: 1280 × 720 px**
- Nguồn đối chiếu: Python 3.8: 1280 × Fraction(1080, 1920) = 720

### 4:3 at 1080 tall

- I want to: Resize keeping the ratio
- Source ratio: 4:3 (older TVs, iPad)
- New size is given as: Chiều cao
- New width or height: 1080 px
- **New width: 1,440 px**
- **New height: 1,080 px**
- Nguồn đối chiếu: Python 3.8: 1080 × Fraction(4, 3) = 1440

## Câu hỏi

### What is the aspect ratio of 1920 × 1080?

1920 × 1080 is 16:9: both sides divide by their greatest common divisor, 120, to give 16 and 9, and the width is 1.7778 times the height. ITU-R BT.709 defines 1920 × 1080 as the HDTV picture format, and the same 16:9 shape covers 1280 × 720, 2560 × 1440 and 3840 × 2160 (4K UHD).

### Is a 21:9 ultrawide monitor really 21:9?

No. Ultrawide monitors sold as 21:9 are 2560 × 1080 (64:27, 2.370), 3440 × 1440 (43:18, 2.389) or 3840 × 1600 (12:5, 2.4), while 21:9 itself is 2.333. The label is rounded marketing, and the gap shows up in practice: true 21:9 video on a 3440 × 1440 panel fills 3360 × 1440 and leaves a 40 px pillarbox bar on each side.

### How do I calculate the new height when resizing an image?

Multiply the new width by the original height over the original width: new height = new width × H ÷ W. Scaling 1920 × 1080 to 1280 wide gives 1280 × 1080 ÷ 1920 = 720. The other way round, new width = new height × W ÷ H, so a 4:3 picture 1080 tall is 1440 wide. Round to whole pixels at the end, and to even numbers for most video encoders.

### What is the difference between letterbox and pillarbox?

Letterboxing adds bars above and below when the picture is wider than the screen, such as 16:9 video on a 1024 × 768 display, which gets 96 px bars. Pillarboxing adds bars on the left and right when the picture is narrower, such as 4:3 content on a 1920 × 1080 screen, which leaves 240 px on each side. Zooming to fill removes the bars but crops the picture, by 25% in that 4:3 case.

### What aspect ratios do cinema films use?

Digital cinema uses two DCI container shapes: flat at 1.85:1 (1998 × 1080 in the 2K container) and scope at 2.39:1 (2048 × 858). On a 1920 × 1080 TV, a 2.39:1 film shows letterbox bars of about 138 px above and below, and a 1.85:1 film bars of about 21 px, because both are wider than the TV's 16:9 (1.78:1).

### “Aspect ratio calculator (16:9, 4:3, 21:9)” 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: 9. Ví dụ, “Full HD” được kiểm tra bằng Python 3.8: fractions.Fraction(1920, 1080) = 16/9.

### Phương pháp này lấy từ đâu?

ITU-R BT.709-6 — HDTV picture format 1920 × 1080 at 16:9; DCI Digital Cinema System Specification — flat 1998 × 1080 (1.85:1) and scope 2048 × 858 (2.39:1) containers.

## Nguồn

- [ITU-R BT.709-6 — HDTV picture format 1920 × 1080 at 16:9](https://www.itu.int/rec/R-REC-BT.709)
- [DCI Digital Cinema System Specification — flat 1998 × 1080 (1.85:1) and scope 2048 × 858 (2.39:1) containers](https://www.dcimovies.com/specification)