diff --git a/src/core/changes.ts b/src/core/changes.ts
index 51de119..0355b43 100644
--- a/src/core/changes.ts
+++ b/src/core/changes.ts
@@ -7,6 +7,7 @@ import type { PierInput } from './types';
 export const EDITABLE_FIELDS = [
   { key: 'copingWidthMm', label: '코핑 폭 B', unit: 'mm' },
   { key: 'copingLengthMm', label: '코핑 길이 L', unit: 'mm' },
+  { key: 'columnDiameterMm', label: '기둥 직경 D', unit: 'mm' },
   { key: 'copingH1Mm', label: '끝단 높이 h1', unit: 'mm' },
   { key: 'copingH2Mm', label: '테이퍼 높이 h2', unit: 'mm' },
   { key: 'levelWidthMm', label: 'Level 폭', unit: 'mm' },
@@ -21,6 +22,7 @@ export type EditableKey = (typeof TRACKED_KEYS)[number];
 export const FIELD_LABELS: Record<EditableKey, string> = {
   copingWidthMm: '코핑 폭 B',
   copingLengthMm: '코핑 길이 L',
+  columnDiameterMm: '기둥 직경 D',
   copingH1Mm: '끝단 높이 h1',
   copingH2Mm: '테이퍼 높이 h2',
   levelWidthMm: 'Level 폭',
diff --git a/src/core/column-check.ts b/src/core/column-check.ts
new file mode 100644
index 0000000..227952b
--- /dev/null
+++ b/src/core/column-check.ts
@@ -0,0 +1,30 @@
+/**
+ * 기둥 직경 D (승인 2026-10-23): free input 1200–3000 mm in 50 mm steps. Checks, all in mm:
+ *  - D < Level 폭 (the column meets the Level zone);
+ *  - D + 2·c ≤ 코핑 폭 B (c = 125, 기둥철근.md: 기둥 면 ~ P1 중심 피복);
+ *  - the arc spacing of the 주철근 (2π·r1 / n_div, n_div = 46 kept) ≥ max(1.5·d_b, 40), with d_b = 32 (S32).
+ * Pure: gives the messages for the 알림 list.
+ */
+import type { PierModel } from './geometry';
+
+export const COLUMN_D_RANGE = { min: 1200, max: 3000, step: 50 } as const;
+export const COLUMN_COVER_MM = 125;
+const N_DIV = 46;
+const DB_MAIN = 31.8;
+
+export function columnDiameterIssues(dMm: number, model: PierModel): string[] {
+  const out: string[] = [];
+  if (!Number.isFinite(dMm) || dMm < COLUMN_D_RANGE.min || dMm > COLUMN_D_RANGE.max || Math.round(dMm) % COLUMN_D_RANGE.step !== 0) {
+    out.push(`기둥 직경은 ${COLUMN_D_RANGE.min}~${COLUMN_D_RANGE.max} mm, ${COLUMN_D_RANGE.step} mm 단위여야 합니다.`);
+    return out;
+  }
+  const levelW = model.copingGeom.levelWidth;
+  if (dMm >= levelW) out.push(`기둥 직경 ${dMm} mm가 Level 폭 ${levelW} mm 이상입니다.`);
+  const copingW = model.copingB * 1000;
+  if (dMm + 2 * COLUMN_COVER_MM > copingW) out.push(`기둥 직경 ${dMm} mm와 피복이 코핑 폭 ${copingW} mm를 넘습니다.`);
+  const r1 = dMm / 2 - COLUMN_COVER_MM;
+  const arc = (2 * Math.PI * r1) / N_DIV;
+  const minArc = Math.max(1.5 * DB_MAIN, 40);
+  if (arc < minArc) out.push(`주철근 순간격 ${arc.toFixed(1)} mm가 최소 ${minArc.toFixed(1)} mm보다 작습니다.`);
+  return out;
+}
diff --git a/src/core/geometry.ts b/src/core/geometry.ts
index 50367f3..f99e5a8 100644
--- a/src/core/geometry.ts
+++ b/src/core/geometry.ts
@@ -166,6 +166,8 @@ export interface PierModel {
   endBottom: number;
   /** Coping heights and Level width in mm, as used by the coping rebar (1단계). */
   copingGeom: CopingGeom;
+  /** Column diameter D (mm) used by the model and the column rebar. */
+  columnDiameterMm: number;
   bearings: BearingLocal[];
   blocks: BlockModel[];
   columns: ColumnModel[];
@@ -225,6 +227,8 @@ export function buildPier(input: PierInput): PierModel {
   const copingH1 = (input.copingH1Mm ?? P1_COPING_GEOM.h1) / 1000;
   const copingH2 = (input.copingH2Mm ?? P1_COPING_GEOM.h2) / 1000;
   const levelW = (input.levelWidthMm ?? P1_COPING_GEOM.levelWidth) / 1000;
+  // Column diameter (m): the input, with 2.0 m when absent (승인 2026-10-23).
+  const colD = (input.columnDiameterMm ?? DIM.columnDiameter * 1000) / 1000;
   const planHalfB = B / 2;
   const ip1v: Vec2 = { e: ip1.x, n: ip1.y };
   const ip2v: Vec2 = { e: ip2.x, n: ip2.y };
@@ -367,7 +371,7 @@ export function buildPier(input: PierInput): PierModel {
     k: colK[i],
     top: levelBottom,
     bottom: footingTop,
-    diameter: DIM.columnDiameter,
+    diameter: colD,
   }));
 
   const footings: FootingModel[] = [];
@@ -378,7 +382,7 @@ export function buildPier(input: PierInput): PierModel {
 
   if (input.foundationType === '현장타설말뚝') {
     columns.forEach((c) => {
-      piles.push({ s: c.s, k: c.k, n: 0, top: input.footingBottomEl, bottom: pileTip, diameter: DIM.columnDiameter });
+      piles.push({ s: c.s, k: c.k, n: 0, top: input.footingBottomEl, bottom: pileTip, diameter: colD });
     });
   } else {
     for (const f of footingSets) {
@@ -448,6 +452,7 @@ export function buildPier(input: PierInput): PierModel {
     levelBottom,
     endBottom,
     copingGeom: { h1: copingH1 * 1000, h2: copingH2 * 1000, levelWidth: levelW * 1000 },
+    columnDiameterMm: colD * 1000,
     bearings: [...frontPlaced, ...backPlaced].map((b) => ({ no: b.no, row: b.row, k: b.k, s: b.s, g: b.g, n: b.n, z: b.z })),
     blocks,
     columns,
diff --git a/src/core/rebar/column.ts b/src/core/rebar/column.ts
index 07e597d..23a5f9f 100644
--- a/src/core/rebar/column.ts
+++ b/src/core/rebar/column.ts
@@ -54,7 +54,7 @@ export function columnInputFor(model: PierModel, base: ColumnInput = COLUMN_P1):
   const eP1 = Math.round((seg.top - (c3 - 0.1)) * 1000);
   // e_top: the coping middle level (Level 하면 + 계단 상면)/2 is the top of P3 and P6 (승인 2026-10-23).
   const eTop = Math.round((seg.top - (model.levelBottom + seg.top) / 2) * 1000);
-  return { ...base, Hc, hcop, eP1, eTop };
+  return { ...base, Hc, hcop, eP1, eTop, D: model.columnDiameterMm };
 }
 
 export const COLUMN_P1: ColumnInput = {
diff --git a/src/core/types.ts b/src/core/types.ts
index 88a3cd5..cc1a6e6 100644
--- a/src/core/types.ts
+++ b/src/core/types.ts
@@ -30,6 +30,8 @@ export interface PierInput {
   foundationType: FoundationType;
   pileLengthMm: number;
   copingWidthMm: number;
+  /** 기둥 직경 D (mm, 1200–3000 in 50 mm steps, 승인 2026-10-23). Absent = 2000. */
+  columnDiameterMm?: number;
   copingLengthMm: number;
   /** 끝단 높이 h1, 테이퍼 높이 h2, Level 폭 (mm). Absent = the P1 values (1000 · 1500 · 2100). */
   copingH1Mm?: number;
diff --git a/src/main.ts b/src/main.ts
index a38f24f..f39eb9b 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -17,6 +17,7 @@ import { DEFAULT_MATERIAL, FCK_MEMBERS, fyOf, GRADE_CHOICES, isAssumedMaterial,
 import { buildQuantity, quantityRows, type Quantity, type QuantityLine } from './core/rebar/quantity';
 import { parseDrawingRows } from './core/drawing-import';
 import { parseBearingElevationTable, parseSolplateTable, readDxfTexts } from './core/dxf-table';
+import { columnDiameterIssues } from './core/column-check';
 import { P1_COPING_GEOM } from './core/coping-geom';
 import { applyBearingRules, BEARING_ELEVATION_REF, BEARING_RULES_DEFAULT, type BearingBasis, type BearingRules, type DrawingBearing, DRAWING_TOL_MM, drawingCopingCheck, SOLPLATE_REF, SOLPLATE_T_RULE, solplateTs, withDrawingBasis } from './core/bearing-ref';
 
@@ -69,7 +70,7 @@ const CRS = '국토좌표계 (x = 동, y = 북)';
 const MEMBERS: { id: MemberId; label: string; keys: EditableKey[]; note: string }[] = [
   { id: 'bearing', label: '교량받침·받침블럭·솔플레이트', keys: [], note: '받침 상세(C3)는 Q16 결정 후 추가합니다. 지금은 표시만 합니다.' },
   { id: 'coping', label: '코핑', keys: ['copingWidthMm', 'copingLengthMm', 'copingH1Mm', 'copingH2Mm', 'levelWidthMm'], note: '폭, 길이, 끝단 높이 h1, 테이퍼 높이 h2, Level 폭을 입력합니다. 계단 위치·단차선 입력은 다음 단계에서 추가합니다.' },
-  { id: 'column', label: '기둥', keys: [], note: '기둥 지름 입력은 다음 단계에서 추가합니다. 높이와 위치는 계산값입니다.' },
+  { id: 'column', label: '기둥', keys: ['columnDiameterMm'], note: '기둥 직경 D를 입력합니다 (1200–3000 mm, 50 mm 단위). 높이와 위치는 계산값입니다.' },
   { id: 'footing', label: '확대기초 · 말뚝', keys: ['pileLengthMm'], note: '말뚝 길이를 입력합니다. 열별 본수는 다음 단계에서 추가합니다.' },
   { id: 'ground', label: '지반·지층', keys: [], note: '지반고는 입력값을 표시만 합니다. 지층 입력은 다음 단계에서 추가합니다.' },
 ];
@@ -196,6 +197,7 @@ animate();
 function withCopingDefaults(input: PierInput): PierInput {
   return {
     ...input,
+    columnDiameterMm: input.columnDiameterMm ?? 2000,
     copingH1Mm: input.copingH1Mm ?? P1_COPING_GEOM.h1,
     copingH2Mm: input.copingH2Mm ?? P1_COPING_GEOM.h2,
     levelWidthMm: input.levelWidthMm ?? P1_COPING_GEOM.levelWidth,
@@ -218,6 +220,9 @@ function buildEntry(rawInput: PierInput, checked: boolean): Pier {
     try {
       model = buildPier(modelInput);
       validatePier(input, model, issues);
+      for (const message of columnDiameterIssues(modelInput.columnDiameterMm ?? 2000, model)) {
+        issues.push({ code: 'D01', level: 'E', pier: input.name, message });
+      }
     } catch (err) {
       issues.push({ code: '미실시', level: 'E', pier: input.name, message: (err as Error).message });
     }
diff --git a/tests/column-diameter.test.ts b/tests/column-diameter.test.ts
new file mode 100644
index 0000000..44d1ee7
--- /dev/null
+++ b/tests/column-diameter.test.ts
@@ -0,0 +1,32 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { columnDiameterIssues } from '../src/core/column-check';
+import { columnInputFor, columnRebar } from '../src/core/rebar/column';
+import { loadP1 } from './helpers';
+
+describe('기둥 직경 D 입력 (승인 2026-10-23)', () => {
+  it('the default (absent) is 2000 mm and gives the P1 results', () => {
+    const m = buildPier(loadP1());
+    expect(m.columnDiameterMm).toBe(2000);
+    expect(columnInputFor(m).D).toBe(2000);
+    expect(columnRebar(columnInputFor(m)).mainP1).toBe(230);
+  });
+
+  it('a larger D (2500) changes the column radius and the rebar, and the model follows the input', () => {
+    const m = buildPier({ ...loadP1(), columnDiameterMm: 2500 });
+    expect(m.columnDiameterMm).toBe(2500);
+    expect(m.columns[0].diameter).toBeCloseTo(2.5, 6);
+    const col = columnRebar(columnInputFor(m));
+    expect(col.r1).toBeCloseTo(2500 / 2 - 125 - 0, 0); // r1 = D/2 − c, the outer bar radius
+  });
+
+  it('the checks: range and 50 mm steps, D below the Level width, D + 2c within the coping width, arc spacing', () => {
+    const m = buildPier(loadP1());
+    expect(columnDiameterIssues(2000, m)).toEqual([]);
+    expect(columnDiameterIssues(2050, m)).toEqual([]);
+    expect(columnDiameterIssues(3200, m).length).toBe(1);
+    expect(columnDiameterIssues(2025, m).length).toBe(1);
+    expect(columnDiameterIssues(2100, m).some((x) => x.includes('Level 폭'))).toBe(true);
+    expect(columnDiameterIssues(2050, m).some((x) => x.includes('코핑 폭'))).toBe(false);
+  });
+});