"300 DPI" appears on every print shop's upload page and almost nobody explains what it means — or that the phrase is technically wrong. This guide untangles pixels, inches and dots, gives you exact pixel dimensions for every common poster size, and shows why a file that looks flawless on a laptop can print disappointingly soft. For the arithmetic itself, the free print DPI calculator converts inches, centimeters and pixels in both directions; the poster printing guide covers what to do once your file passes this stage.

PPI and DPI are not the same thing

PPI — pixels per inch — describes a digital file: how many image pixels are packed into each inch of its intended printed width. It is a property of the file's dimensions, and it is what upload portals actually test. A 2400×3000 pixel image is 300 PPI only when printed at 8×10 inches.

DPI — dots per inch — describes the hardware: how many microscopic ink droplets a printer lays down per inch. Most photo printers run 300, 600 or even 1440 DPI regardless of your file, using several colored droplets to reproduce one pixel. The terms are used interchangeably in everyday language, which is harmless, but the distinction matters when diagnosing a bad print: a 72 PPI file sent to a 1440 DPI printer still contains only 72 pixels of information per inch, and the extra dots cannot invent missing detail.

Why 300 became the gold standard

At a normal reading distance of about 25–30 cm (10–12 inches), the human eye stops resolving separate marks once they are packed tighter than roughly 290–300 per inch. Above that threshold, more pixels buy no visible sharpness on a standard matte or photo print; below it, individual pixels or jagged edges start to show. Magazines settled on 300 DPI decades ago, photo labs inherited the convention, and kiosks now warn or reject files that fall near 100 DPI at print size. Stars on a dark poster make the rule slightly stricter, because bright points on a near-black background reveal soft edges faster than busy photographs do.

The pixel math, with full tables

The arithmetic is one multiplication: pixels = inches × 300 for each side. At sizes where the eye follows slightly farther — large framed prints, wall art above the sofa — viewing distance forgives slightly lower density, but 300 stays the safe target. Common US photo sizes at 300 DPI are:

Print sizeInches (cm approx.)Pixels at 300 DPI
4 × 610.2 × 15.2 cm1200 × 1800
5 × 712.7 × 17.8 cm1500 × 2100
8 × 1020.3 × 25.4 cm2400 × 3000
11 × 1427.9 × 35.6 cm3300 × 4200
12 × 12 (square)30.5 × 30.5 cm3600 × 3600
16 × 2040.6 × 50.8 cm4800 × 6000
18 × 2445.7 × 61.0 cm5400 × 7200
20 × 3050.8 × 76.2 cm6000 × 9000
24 × 3661.0 × 91.4 cm7200 × 10800

Notice that the largest file is not huge by modern standards — around 2.2 megapixels per square inch of math works out to 78 megapixels at 24×36, which compresses to a perfectly portable PNG of tens of megabytes. This is why our star map tools repaint at export rather than enlarging the preview.

A-series sizes from A4 to A1

Metric paper follows the ISO 216 A-series, where each step halves the long edge. The ratio is always √2, so every A-size is the same shape and scales without cropping — useful when a design must be offered in several sizes.

SizeCentimetersInchesPixels at 300 DPI
A421.0 × 29.78.27 × 11.692480 × 3508
A329.7 × 42.011.69 × 16.543508 × 4961
A242.0 × 59.416.54 × 23.394961 × 7016
A159.4 × 84.123.39 × 33.117016 × 9933

One warning for cross-system ordering: an A4 file does not perfectly fill an 8.5×11 letter sheet, and A-series aspect ratios do not match 2:3 photo paper, so mixing metric and imperial products forces a crop.

When 150 DPI is genuinely enough

Resolution requirements scale with viewing distance. A gallery-wrapped canvas or a framed 24×36 seen from across a living room is typically viewed at two meters or more, where 150 PPI — even 100 PPI for big exhibition graphics — reads as sharp because the eye cannot resolve the dots at that distance. Bold shapes with soft edges also survive scaling better than fine serif text. The practical rule: use 300 for anything viewed within arm's reach, including every keepsake gift; drop toward 150 only when the product is large, matte-textured or canvas, and never below the lab's stated minimum.

Crisp on screen, soft on paper

Monitors show about 72–144 pixels per inch, so the same 2400-pixel-wide file that measures a crisp 40 cm on a display stretches over 50 cm at 300 DPI... and a 1500-pixel web graphic looks sharp at full screen but covers barely five printed inches before the pixels become visible. Screens also emit light while paper reflects it, which hides small softnesses on the monitor; in print, every pixel edge is compared against a white page under bright light. Zoom your image to 100% in an image viewer and compare a 2.5 cm (1 inch) slice to a real ruler — that view is the honest preview of what the printer will do.

JPEG artifacts: what to watch for

JPEG shrinks files by discarding subtle color changes, and the damage clusters in exactly the places posters suffer: sharp contrasts like white text on navy, hard edges, and smooth dark skies. On a night-sky map, over-compression shows as mosquito noise around the brightest stars, faint 8×8 square blocking in the gradient, and smeared halos around captions. Prevention is simple: export from the original at high quality (95–100), never re-save a JPG repeatedly because each pass adds damage, and prefer lossless PNG for dark graphics with text. If a file only exists as a medium-quality JPG, do not upscale it — send it to the print readiness checker and choose a smaller product size rather than printing enlarged flaws.

What vectors and SVG actually are

A JPG or PNG is a bitmap: a fixed grid of pixels, so enlargement beyond its native density only spreads the same dots. A vector graphic stores instructions — "a line from here to there, a circle of this radius, text set in this typeface" — and the renderer rebuilds those instructions at whatever resolution it needs, from a phone screen to a billboard. SVG is the web's vector format, which is why this site draws all star points, constellation lines and labels as vector-like canvas paths, then repaints the entire poster at 300 DPI on export: the artwork is resolution-independent even though the delivered file is a high-resolution PNG for universal lab compatibility. Logos and type stay razor sharp; only embedded photographic textures need true pixel data.

Check the file before you print

Three numbers settle the resolution question: pixel width, pixel height, and intended inches. Divide pixels by inches, confirm 300 (or your lab's minimum), verify sRGB color and the right aspect ratio, and inspect a 100% crop around the smallest text. Our own downloads do this arithmetic automatically in your chosen size, and the DPI calculator reverses it for any outside image. Pixel counts are an honest, checkable contract — once the numbers match the product, the only remaining variables are paper and framing, covered in our paper and frames guide.

"300 DPI"写在每家印厂的上传页上，却几乎没人解释它的含义——而且严格说，这个词还并不准确。本文理清像素、英寸与墨点的关系，给出每种常见海报尺寸所需的精确像素，并解释为什么一个在笔记本上看起来完美无瑕的文件，打印出来却可能发虚。计算本身可以交给免费的打印 DPI 计算器，它能在英寸、厘米与像素之间双向换算；文件过了分辨率这一关之后该怎么做，则见海报印刷指南。

PPI 与 DPI 不是一回事

PPI（每英寸像素数）描述的是数字文件：在预定的印刷宽度里，每一英寸容纳多少个图像像素。它是文件尺寸的属性，也是上传门户真正检测的指标。一张 2400×3000 像素的图，只有印成 8×10 英寸时才是 300 PPI。

DPI（每英寸点数）描述的是硬件：打印机每英寸落下多少个微小墨滴。多数照片打印机无论你的文件如何，都工作在 300、600 甚至 1440 DPI，用数个彩色墨滴去还原一个像素。日常交流中两个词混用并无大碍，但排查印刷问题时区别很关键：一张 72 PPI 的文件送进 1440 DPI 的打印机，每英寸仍只包含 72 个像素的信息，多出来的墨点变不出缺失的细节。

300 为何成为金标准

在约 25–30 厘米的正常阅读距离下，当并列的标记密到每英寸约 290–300 个以上时，人眼就无法分辨独立的点。超过这个阈值，更多像素在普通哑光纸或相纸上换不来可见的锐度；低于它，单个像素与锯齿边缘就会显露。杂志界几十年前定下 300 DPI，照片印厂沿用至今，自助机如今会在成品尺寸低于约 100 DPI 时警告甚至拒收。深色海报上的星点让这条规则略微更严：近黑背景上的亮点，比内容繁杂的照片更容易暴露发虚的边缘。

像素计算与完整对照表

算法只有一次乘法：每边 像素 = 英寸 × 300。对于装框大画、沙发上方墙饰这类眼睛离得稍远的尺寸，观看距离会宽容略低的密度，但 300 仍是安全目标。常见美式照片尺寸在 300 DPI 下所需像素如下：

成品尺寸英寸（约厘米）300 DPI 所需像素
4 × 610.2 × 15.2 cm1200 × 1800
5 × 712.7 × 17.8 cm1500 × 2100
8 × 1020.3 × 25.4 cm2400 × 3000
11 × 1427.9 × 35.6 cm3300 × 4200
12 × 12（方形）30.5 × 30.5 cm3600 × 3600
16 × 2040.6 × 50.8 cm4800 × 6000
18 × 2445.7 × 61.0 cm5400 × 7200
20 × 3050.8 × 76.2 cm6000 × 9000
24 × 3661.0 × 91.4 cm7200 × 10800

注意最大的文件以今天的标准并不夸张——按每平方英寸约 220 万像素计算，24×36 约为 7800 万像素，压成 PNG 不过几十 MB，传输毫无压力。这正是本站星空工具在导出时重新绘制、而不是放大预览图的原因。

从 A4 到 A1 的 A 系列尺寸

公制纸张遵循 ISO 216 的 A 系列：每升一号，长边对折。所有 A 号纸长宽比都是 √2，形状完全一致，放大缩小无需裁切——一个设计要提供多个尺寸时尤其方便。

尺寸厘米英寸300 DPI 所需像素
A421.0 × 29.78.27 × 11.692480 × 3508
A329.7 × 42.011.69 × 16.543508 × 4961
A242.0 × 59.416.54 × 23.394961 × 7016
A159.4 × 84.123.39 × 33.117016 × 9933

跨体系下单要有一个提醒：A4 文件不能恰好铺满 8.5×11 的美式信纸，A 系列的比例也不等于 2:3 相纸；公制与英制产品混选时必然产生裁切。

150 DPI 什么时候真的够用

分辨率要求随观看距离而变。帆布包框或客厅里的 24×36 装框画，通常在两米或更远观看，在这个距离上 150 PPI——大型展览画面甚至 100 PPI——都显得锐利，因为人眼在该距离已无法分辨墨点。轮廓大胆、边缘柔和的形状也比细小衬线文字更耐放大。实用法则：一臂距离内观看的一切（包括所有纪念礼物）一律用 300；只有当成品尺寸大、表面带哑光纹理或为帆布时，才降到 150 左右，且绝不要低于印厂标明的下限。

屏幕锐利、纸面发虚的原因

显示器每英寸只显示约 72–144 个像素，所以同一张 2400 像素宽的文件，在屏幕上可覆盖约 40 厘米而依然清晰，在 300 DPI 下却铺满 50 厘米以上；一张 1500 像素的网络配图全屏看很锐利，印成五英寸以上像素就开始可见。屏幕发光而纸张反光，也使轻微虚焦在显示器上被掩盖；印刷时每个像素边缘都在强光下与白纸对比。在看图软件里把图像放大到 100%，取 2.5 厘米（1 英寸）宽的一块与实物尺子对照——那才是打印机效果的诚实预览。

JPG 压缩伪影如何识别

JPG 靠丢弃细微色彩变化来缩小文件，而损伤恰好集中在海报最怕的地方：深藏青底上的白字、硬边缘与平滑深色天空。在夜空星图上，过度压缩表现为最亮恒星周围的"蚊状噪声"、渐变里隐约的 8×8 方块，以及说明文字旁被拖脏的光晕。预防很简单：从原图以高质量（95–100）导出；绝不反复另存 JPG，每一次保存都会叠加损伤；深色带文字图形优先用无损 PNG。如果手头只有一张中等质量 JPG，不要放大它——交给印刷就绪检查器评估，并选择更小的成品尺寸，而不是把缺陷放大印出。

矢量与 SVG 到底是什么

JPG 与 PNG 是位图：一个固定的像素网格，超出原生密度放大只会摊大同样的点。矢量图形存的是指令——"从这里到那里一条线、这个半径的圆、这段文字用这种字体排"——渲染器在任何需要的分辨率上重建这些指令，从手机屏到户外巨幕皆可。SVG 就是网络世界的矢量格式，这也是本站把所有星点、星座连线与标签画成类矢量画布路径，并在导出时以 300 DPI 整体重绘的原因：交付文件虽为保证印厂通用性而采用高分辨率 PNG，图形本身却与分辨率无关。标志与文字始终锋利如刀，只有嵌入的照片纹理才真正需要像素数据。

印刷前先检查文件

三个数字就能定分辨率：像素宽、像素高与目标英寸。像素除以英寸，确认达到 300（或印厂下限），再核对 sRGB 色彩与正确长宽比，最后在 100% 放大下检查最小的文字。本站下载文件已按你选的尺寸自动完成这些计算，DPI 计算器则可对任意外部图片反推。像素数字是一份诚实、可核对的契约——当数字与产品匹配后，剩下的变量就只有纸张与装裱，详见纸张与装裱指南。