300x1200 half-page ad image: it is exactly 1:4, so request 1:4

A 300x1200 ad slot has a width-to-height ratio of exactly 1:4, and Nano Banana 2.1 on Sume lists 1:4 in its aspect_ratio descriptor, so you can request the slot shape directly instead of cropping. One image costs $0.075 at 0.5K, $0.10 at 1K, $0.15 at 2K and $0.20 at 4K; the ratio does not change the price.

Google added 1:4, 4:1, 1:8 and 8:1 to Nano Banana 2.1 at 1K, 2K and 4K on Oct 6 2026 (Gemini API changelog, read 2026-10-09).

What a tall slot costs

The Image API bills a completed generation at the catalog price and bills nothing for a failed one. These are per-image prices; there is no per-second rate for stills.

Request the slot shape

Send aspect_ratio: "1:4" with a resolution tier. Pick the tier for how sharp the slot must be; a slot that shows at 300 CSS pixels wide rarely needs 4K.

import os, requests

body = {
    "model": "google/nano-banana-2.1",
    "prompt": "Tall half-page ad for a running shoe, product at the bottom, empty headline space at the top",
    "aspect_ratio": "1:4",
    "resolution": "1K",
}
r = requests.post(
    "https://api.sume.com/v1/images",
    headers={"Authorization": f"Bearer {os.environ['SUME_API_KEY']}"},
    json=body, timeout=60,
)
print(r.status_code)  # 200 = image body, 202 = job envelope
if r.status_code == 200:
    out = r.json()
    print(out["data"][0]["url"], out["usage"]["cost"])

If your slot is not exactly 1:4

A 160x600 slot is 4:15, which is 0.267 against 0.25 for 1:4. Generate 1:4 and trim height: for a width w the 1:4 image is 4w tall and the slot needs 3.75w, so you crop 6.25% of the height. A 300x600 slot is 1:2, which Nano Banana 2.1 does not list; generate 2:3 (0.667) and crop 25% of the width, or generate 1:4 and accept a different layout.

Keep the headline and logo away from the top and bottom edges so a trim never touches them.

Gotchas

• Read supported_parameters.aspect_ratio.values from GET /v1/images/models before pinning a ratio; a value the model does not list returns 400 unsupported_parameter.
• Nano Banana Pro does not list 1:4, so do not swap the model id and keep the ratio.
• A 4K call can exceed the 30-second wait and return 202; branch on the status code, not the body shape.