Your photo is measured, not uploaded
Drag an image into the drop zone or tap it to browse your device, and the file is read entirely on this page with the browser's own FileReader. A local preview appears, but the bytes never travel anywhere: there is no upload form, no server, no account and no analytics, which is why the page works safely with baby scans, private phone photos and anything you simply do not want to send away. As soon as the image finishes decoding, the tool reads its true naturalWidth and naturalHeight — the full sensor or export dimensions — rather than the shrunken size shown on screen, because enlargement is the thing that decides print sharpness.
Next you choose the print you intend to make. The quick bar covers the standard photo and poster sizes from 4×6 through 24×36 inches plus A4, A3, A2 and A1, and the four custom fields accept any rectangle in inches or centimetres with the two unit systems linked live. Effective DPI is then computed separately for width and height as natural pixels divided by physical inches, and the verdict always uses the weaker of the two axes so a borderline file cannot be rescued by averaging. Four bands keep the answer honest: 300 DPI and above is Excellent, 200 to 299 is Good for most framed work, 150 to 199 is Acceptable for large posters seen from across a room, and below 150 is Risky for anything viewed up close.
The largest crisp print, and a result card to keep
Underneath the verdict the tool inverts the same equation at a strict 300 DPI and lists the largest rectangle your pixels can fill while staying sharp, in both inches and centimetres — use it to pick a size you already have instead of ordering a print that softens. On the right, the readiness card gathers the file name, native pixel dimensions, chosen target and a large colour-coded DPI number with a matching badge over a thumbnail of your image, and Download repaints the whole card from scratch at 300 DPI in the print size selected in the menu. Everything runs offline in the tab once the page is open; close it and the photo leaves with you.
图片是被测量，而不是被上传
把图片拖入拖放区，或点击后从设备浏览选择，文件会完全由本页通过浏览器自带的 FileReader 在本地读取。页面会显示本地预览，但字节不会发往任何地方：没有上传表单、没有服务器、无需注册，也没有统计代码，因此宝宝扫描件、私人手机照片以及任何你不愿外传的图片都可以放心检查。图片解码完成后，工具读取的是它真实的 naturalWidth 与 naturalHeight——完整的传感器或导出像素——而不是屏幕上缩小后的显示尺寸，因为决定印刷锐度的恰恰是放大后的像素密度。
接着选择你打算打印的尺寸。快捷栏覆盖 4×6 到 24×36 英寸的标准照片与海报规格，以及 A4、A3、A2、A1；四个自定义框接受任意英寸或厘米矩形，两种单位实时联动。有效 DPI 按宽、高两个方向分别计算：自然像素除以物理英寸，判定永远取两个方向中较弱的一个，临界文件无法靠平均值蒙混过关。四档结论让结果诚实可信：300 DPI 及以上为 Excellent，200 至 299 对多数装框作品是 Good，150 至 199 对隔房间观看的大海报是 Acceptable，低于 150 对近看的任何印刷品都是 Risky。
最大清晰尺寸，以及可留存的结果卡
判定下方，工具用严格的 300 DPI 反解同一公式，列出你的像素在保持清晰前提下能填满的最大矩形，同时给出英寸与厘米——据此直接选择你现有像素撑得起的尺寸，而不是订一张发虚的印刷品。右侧的就绪卡把文件名、原始像素、目标尺寸与带颜色徽标的超大 DPI 数字连同你的图片缩略图汇总在一起；点击下载时，整张卡片会按菜单所选印刷尺寸以 300 DPI 从零重绘。页面打开后一切都在标签页内离线运行；关掉它，图片也随你离开，不留任何痕迹。
Why is my effective DPI lower than the 300 DPI tag in my file?
DPI metadata is only an instruction to the printer; it does not add pixels. A 1200×1500 image tagged 300 DPI is still only sharp at 4×5 inches. This checker ignores the tag and measures real density: native pixels divided by the target size on each axis, taking the weaker figure.
为什么有效 DPI 比我文件里标注的 300 DPI 低？
DPI 元数据只是给打印机的一条指令，并不能增加像素。一张 1200×1500 的图片即使标记为 300 DPI，也只在 4×5 英寸以内清晰。本检查器忽略该标记，测量真实密度：原始像素除以目标尺寸，两个方向分别计算并取较弱值。
What is the biggest size my photo will print sharply?
The result panel divides both native dimensions by 300 and shows the answer in inches and centimetres. That is the largest rectangle with no upscaling at all. If you accept viewing distance, Good and Acceptable bands show how much farther a large wall poster can be pushed while still looking clean.
我的照片最大能清晰打印到多大？
结果面板会把原始宽高都除以 300，同时给出英寸与厘米答案。那是完全不放大的最大矩形。若考虑观看距离，Good 与 Acceptable 两档会提示大型墙面海报还能放大到什么程度仍然干净。
Is my image uploaded or stored anywhere?
No. The file is read locally with FileReader and turned into an in-memory preview; nothing is submitted, and refreshing or closing the tab releases it. The result card you export is generated on your own device and contains only the thumbnail needed to draw the picture.
我的图片会被上传或保存到哪里吗？
不会。文件通过 FileReader 在本地读取并生成内存预览，没有任何提交；刷新或关闭标签页即释放。你导出的结果卡也在自己的设备上生成，其中仅包含绘图所需的缩略图。
