diff --git a/src/core/solplate.ts b/src/core/solplate.ts
new file mode 100644
index 0000000..36c394d
--- /dev/null
+++ b/src/core/solplate.ts
@@ -0,0 +1,55 @@
+/**
+ * 솔플레이트 기하 (1단계, 승인 2026-10-23). Pure: the solplate of one bearing, from its block (position, rowSkew) and the
+ * 공통 규칙 values. The solplate sits on the bearing (h3) and tapers along the 교축 방향 (s): the thick end T1 is on the
+ * 시점 side (s = −L/2), the thin end T2 on the 종점 side (s = +L/2). Lengths in m, as in the model.
+ */
+import { blockFramePoint, type BlockModel } from './geometry';
+
+export interface SolplateParams {
+  /** 받침 평면 A (mm, along 교축 s) and B (mm, along the 교각축 직각 n). */
+  A: number;
+  B: number;
+  /** 솔플 여유 (mm): L = A + margin, W = B + margin. */
+  margin: number;
+  /** 중심 두께 T (mm). */
+  T: number;
+  /** 기울기 S (%), negative as drawn: thickness changes by L·|S|/2 from the centre. */
+  slopePct: number;
+}
+
+export interface SolplateGeometry {
+  /** Length along 교축 (m) and width along 교각축 직각 (m). */
+  L: number;
+  W: number;
+  /** Thickness at the 시점 end (T1) and the 종점 end (T2) (m). */
+  T1: number;
+  T2: number;
+  /** Underside (EL m): the bearing top = block top (EL.X) + h3. */
+  bottom: number;
+  /** Eight corners: [s, y, n] in the model's local frame (EL m). */
+  corners: [number, number, number][];
+  /** Volume (m³): L·W·(T1 + T2)/2. */
+  volume: number;
+}
+
+/** Solplate of the bearing at `block`, with the bearing height h3 (mm) and the 공통 규칙 parameters. */
+export function solplateGeometry(block: BlockModel, h3: number, p: SolplateParams): SolplateGeometry {
+  const L = (p.A + p.margin) / 1000;
+  const W = (p.B + p.margin) / 1000;
+  // Thickness change from the centre to each end: L·|S|/2, with S in % (mm).
+  const drop = ((p.A + p.margin) * Math.abs(p.slopePct)) / 200 / 1000;
+  const T = p.T / 1000;
+  const T1 = T + drop;
+  const T2 = T - drop;
+  const bottom = block.padTop + h3 / 1000;
+  const thickAt = (a: number) => T1 + ((T2 - T1) * (a + L / 2)) / L;
+  const corners: [number, number, number][] = [];
+  for (const a of [-L / 2, L / 2]) {
+    for (const c of [-W / 2, W / 2]) {
+      const [s, n] = blockFramePoint(block, a, c);
+      corners.push([s, bottom, n]);
+      corners.push([s, bottom + thickAt(a), n]);
+    }
+  }
+  return { L, W, T1, T2, bottom, corners, volume: (L * W * (T1 + T2)) / 2 };
+}
diff --git a/tests/solplate.test.ts b/tests/solplate.test.ts
new file mode 100644
index 0000000..9d85cbb
--- /dev/null
+++ b/tests/solplate.test.ts
@@ -0,0 +1,43 @@
+import { describe, expect, it } from 'vitest';
+import { buildPier } from '../src/core/geometry';
+import { solplateGeometry } from '../src/core/solplate';
+import { loadP1 } from './helpers';
+
+const model = buildPier(loadP1());
+const front1 = model.blocks.filter((b) => b.row === '전열')[0];
+const params = { A: 700, B: 640, margin: 100, T: 30, slopePct: -1.91 };
+
+describe('솔플레이트 기하 (1단계, 승인 2026-10-23)', () => {
+  it('L = A + 100 and W = B + 100 along the 교축 and the 교각축 직각', () => {
+    const g = solplateGeometry(front1, 181, params);
+    expect(g.L).toBeCloseTo(0.8, 6);
+    expect(g.W).toBeCloseTo(0.74, 6);
+  });
+
+  it('the thick end T1 is on the 시점 side (−s): T1 ≈ 38, T2 ≈ 22 mm, and T1 + T2 = 2T', () => {
+    const g = solplateGeometry(front1, 181, params);
+    expect(g.T1 * 1000).toBeCloseTo(37.64, 1);
+    expect(g.T2 * 1000).toBeCloseTo(22.36, 1);
+    expect((g.T1 + g.T2) * 1000).toBeCloseTo(60, 6);
+    // The top at the 시점 corners is the thick side.
+    const topAtStart = g.corners.filter((c) => Math.abs(c[1] - (g.bottom + g.T1)) < 1e-9);
+    expect(topAtStart.length).toBe(2);
+  });
+
+  it('the underside is the bearing top: block top (EL.X) + h3', () => {
+    const g = solplateGeometry(front1, 181, params);
+    expect(g.bottom).toBeCloseTo(front1.padTop + 0.181, 6);
+  });
+
+  it('volume = L·W·T (the taper averages to the centre thickness)', () => {
+    const g = solplateGeometry(front1, 181, params);
+    expect(g.volume).toBeCloseTo(g.L * g.W * 0.03, 9);
+  });
+
+  it('the solplate is centred on the block and follows the row direction', () => {
+    const g = solplateGeometry(front1, 181, params);
+    const c = g.corners.reduce((acc, p) => [acc[0] + p[0] / 8, acc[1] + p[2] / 8] as [number, number], [0, 0]);
+    expect(c[0]).toBeCloseTo(front1.s, 6);
+    expect(c[1]).toBeCloseTo(front1.n, 6);
+  });
+});