FLUX Resolution Guide: Choose Dimensions and Output Sizes

Set FLUX.2 pro output size with width and height; use aspect_ratio for FLUX1.1 pro Ultra. Both hosted Black Forest Labs models support outputs up to four megapixels. 2, 4, 5

What are the key facts about FLUX image resolution?

How do FLUX dimensions and megapixels work?

Ultra exposes an aspect_ratio control for choosing image shape. FLUX.2 text-to-image examples instead specify width and height, letting an application request concrete pixel dimensions. 2, 4

This distinction matters when preparing images for a layout. Decide whether the delivery needs a particular shape, an exact pixel size, or both, then choose a route that exposes the appropriate controls.

BFL's FLUX.2 prompting guide documents output up to four megapixels and requires output dimensions to be multiples of sixteen. Its examples include square, landscape, and portrait compositions. 5

Think of resolution as an area budget. Multiplying width by height gives the pixel count; doubling both dimensions produces four times as many pixels. These are arithmetic relationships, not model performance claims.

For example, 1024 × 1024 contains 1,048,576 pixels, while 2048 × 2048 contains 4,194,304 pixels. BFL uses 1024 × 1024 as its pricing definition of one megapixel. 6

The distinction between width and area prevents a common planning mistake. A square image with a side of 4096 pixels contains 16,777,216 pixels, calculated by multiplication; it is much larger than a four-megapixel output.

For a banner, sketch the placement of the subject and the empty area intended for text. Use that composition to choose the aspect ratio before asking the model for more detail.

What limits apply to FLUX output and reference images?

The FLUX.2 maximum does not apply to every FLUX endpoint. Ultra has its own request fields, and its documented output controls should be read independently from a FLUX.2 tutorial. 2, 4

Similarly, a resolution limit is not a guarantee of readable text or preserved detail. BFL publishes separate guidance for typography, including explicit wording and placement instructions. Those decisions still belong in the prompt. 7

For critical labels, inspect the actual output at its intended display size. Check letters, spacing, borders, and object edges. Reject a larger image if it fails requirements that a smaller candidate satisfies.

FLUX.2 pro's editing API has a combined nine-megapixel budget for input and output. A larger output leaves less capacity for reference images. 8

BFL documents up to eight references at one-megapixel output and seven at two-megapixel output for pro. Check that combined budget when changing a resolution preset. 8

Budgeting also requires the pricing definition of megapixels. BFL rounds output and reference-image sizes for billing; a small dimension change can move a request into a different billed size. 6

When evaluating a larger output, keep the creative requirements fixed. Otherwise, changes in composition, wording, or reference material can obscure whether the extra pixels actually helped your intended use.

How do you set FLUX image dimensions through the API?

1. Write down the intended shape and final placement. A wide page header and a tall mobile illustration call for different compositions; BFL's aspect-ratio guide maps common ratios to these uses. 9

2. Choose the model and check its dimension rules. For FLUX.2, use a size within its published output limit and compatible with its dimension increment. 5

3. Create BFL API credentials using the official quick start. Put the key in BFL_API_KEY before making a request. 10

4. Submit a modest, deliberate test. This FLUX.2 pro example uses 1536 × 864, a widescreen size shown in BFL's prompting guide. The particular scene is an illustrative prompt. 4, 5
   

curl --fail-with-body https://api.bfl.ai/v1/flux-2-pro \
  -H "x-key: ${BFL_API_KEY}" \
  -H 'Content-Type: application/json' \
  -d '{
    "prompt": "A ceramic teapot on the right of a wooden table, soft window light, empty wall on the left",
    "width": 1536,
    "height": 864
  }'

1. Poll the polling_url from the response. When the status becomes Ready, download the image from result.sample; the initial response only identifies the generation task. 4

2. For Ultra, follow its separate example: submit to /v1/flux-pro-1.1-ultra and pass an aspect_ratio, such as 16:9. Use the documented Ultra controls rather than assuming the FLUX.2 payload transfers unchanged. 2

Check the downloaded file's actual dimensions before placing it in your layout. Record requested and delivered dimensions alongside the endpoint and prompt so that a later comparison remains meaningful.

Try the image in the intended crop. If the subject is too close to an edge, revise the composition instruction before spending effort on a higher-resolution version of the same unsuitable framing.

For composition ideas, see the FLUX photography guide. The sibling FLUX.2 online guide explains hosted access and billing.

How do Ultra and FLUX.2 pro resolution controls compare?

Choose between these routes using an actual delivery requirement. If both can produce a suitable shape, compare completed images for framing, text accuracy, and the amount of revision you need.

Keep resolution and visual style as separate evaluation questions. Ultra's optional Raw mode is an aesthetic control documented by BFL; it should not be treated as a new pixel-size setting. 2

Does four megapixels mean 4096 × 4096?

FLUX.2's four-megapixel square example is 2048 × 2048. A 4096 × 4096 image contains 16,777,216 pixels, calculated by multiplying its dimensions. 5, 6

Can I request any width and height from FLUX.2?

FLUX.2 output dimensions must be multiples of sixteen and stay within the documented output limit. Validate width and height before submitting a request. 5

Does a larger image always solve small-text problems?

For FLUX.2 text rendering, specify the exact words and their placement as BFL's typography guide recommends. Inspect the resulting letters at the intended display size before selecting a larger output. 7

Should I set width and height for Ultra?

FLUX1.1 pro Ultra's documented request uses aspect_ratio. FLUX.2 pro examples use width and height; follow the schema for the endpoint you call. 2, 4

Sources

• Ultra release announcement
• Ultra API and resolution controls
• FLUX.2 launch announcement
• FLUX.2 text-to-image API
• FLUX.2 prompting and dimension rules
• BFL pricing and megapixel definition
• BFL typography guidance
• Multi-reference editing budgets
• Aspect-ratio guidance
• BFL API setup

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