kdp-cover-math

Pure-function Amazon KDP paperback cover-wrap and interior-page geometry. Returns the inch dimensions you need to lay out a print-ready cover or interior — trim, bleed, spine width, full wrap dimensions, safe areas, barcode box.

Zero dependencies. ESM + CJS + types. Works in Node, the browser, edge runtimes, and any TypeScript project.

npm i kdp-cover-math

Why this exists

Every KDP paperback designer rebuilds the same math. The official numbers are spread across KDP docs, most third-party tools hide the math behind paid services, and the only open-source equivalents are buried inside coupled application code (or licensed under restrictive non-commercial terms).

This package extracts the math as plain functions you can import anywhere — no UI, no opinions on rendering, no telemetry.

Quick start

import {
  coverDimensions,
  interiorPageDimensions,
  spineWidthInches,
  spineSupportsText,
  TRIM_SIZES,
} from "kdp-cover-math";

// Full cover wrap for a 40-page, 8.5"x11", white-paper paperback.
const cover = coverDimensions("8.5x11", 40, "white");
//          ^ trim id          ^ pages    ^ paper
// → {
//     trimWidthIn:  8.5,
//     trimHeightIn: 11,
//     spineIn:      0.09008,
//     bleedIn:      0.125,
//     fullWidthIn:  17.34008,        // 2 × 8.5 + spine + 2 × 0.125
//     fullHeightIn: 11.25,           // 11 + 2 × 0.125
//     safeInsetIn:  0.25,
//     barcodeBoxIn: { widthIn: 2.0, heightIn: 1.2 },
//   }

// Interior page dimensions including bleed (or not).
const page = interiorPageDimensions("8.5x11", "bleed");
// → { trimWidthIn: 8.5, trimHeightIn: 11, pageWidthIn: 8.625, pageHeightIn: 11.25, bleedIn: 0.125, safeInsetIn: 0.25, gutterInsetIn: 0.375 }

// Spine math.
spineWidthInches(120, "cream");     // → 0.3 in
spineSupportsText(60, "white");     // → false  (KDP requires ≥ 79 interior pages for spine text)
spineSupportsText(120, "cream");    // → true

// The four trim sizes this package knows about.
TRIM_SIZES;
// → [{ id: "8.5x11",  widthIn: 8.5, heightIn: 11,  label: '8.5" × 11"',  description: "..." },
//    { id: "8x10",    widthIn: 8,   heightIn: 10,  ... },
//    { id: "9.5x9.5", widthIn: 9.5, heightIn: 9.5, ... },
//    { id: "8.5x8.5", widthIn: 8.5, heightIn: 8.5, ... }]

API

Types

• TrimSizeId — "8.5x11" | "8x10" | "9.5x9.5" | "8.5x8.5".
• PaperType — "white" | "cream".
• BleedSetting — "no-bleed" | "bleed".

Constants

| Constant | Value | What it represents | |---|---|---| | BLEED_INCHES | 0.125 | Outer-edge bleed for both interior pages and the cover wrap. | | SAFE_AREA_INSET_IN | 0.25 | Inset from trim where you should keep critical content. | | GUTTER_INSET_IN | 0.375 | Extra inset on the inside edge of interior pages. | | SPINE_WHITE_PER_PAGE | 0.002252 | Inches of spine thickness per page on white paper. | | SPINE_CREAM_PER_PAGE | 0.0025 | Inches of spine thickness per page on cream paper. | | SPINE_TEXT_MIN_PAGES | 79 | KDP's minimum interior page count for reliable printed spine text. | | SPINE_TEXT_MIN_INCHES | 0.06 | The thinnest spine KDP will print readable text on. |

Functions

• getTrim(id: TrimSizeId): TrimDimensions — look up one trim record.
• interiorPageDimensions(trimId, bleed): InteriorPageDimensions — page geometry for an interior layout.
• spineWidthInches(pageCount, paper): number — spine width in inches.
• spineSupportsText(pageCount, paper): boolean — whether KDP will print spine text on this book.
• coverDimensions(trimId, pageCount, paper): CoverDimensions — full cover wrap geometry.

TRIM_SIZES

Read-only catalogue of every trim this package supports. Each entry exposes id, label, widthIn, heightIn, and a short description suitable for UI.

Constraints and assumptions

• Paperback-only. No hardcover, no PDF cover proofing, no Kindle eBook geometry.
• Black-and-white interior assumption (the spine constants follow KDP's B&W price tier; color print uses different stocks and spine rates).
• USD-agnostic; this package does not do pricing math. See [PRICING] for the conventional separate package if you need that.
• Values are returned in inches. Convert to points or millimeters yourself if your renderer wants those.

Compatibility

Tested on Node 18+. Pure ECMAScript with no platform APIs — works in:

• Node.js (ESM and CJS)
• Browser bundlers (Webpack / Vite / esbuild / Turbopack / Bun)
• Edge runtimes (Vercel Functions, Cloudflare Workers, Deno)
• React / Vue / Svelte / SolidJS / vanilla

Sources

All formulas pulled from current KDP documentation:

• Print options for paperbacks
• Cover sizing & bleed
• Spine text requirements
• Barcode safe area

KDP can and does update these from time to time — file an issue if you spot drift.

License

MIT © alanherron — use freely in commercial work, no attribution required (but appreciated).

Related

This package was extracted from starofthepage — a coloring-book studio that turns photos into KDP-ready coloring books. The starofthepage app uses this package to compute every paperback wrap it lays out.