diff --git a/src/lib/plant-hours-schema.test.ts b/src/lib/plant-hours-schema.test.ts
new file mode 100644
index 0000000..31fc27e
--- /dev/null
+++ b/src/lib/plant-hours-schema.test.ts
@@ -0,0 +1,139 @@
+import { describe, it, expect } from "vitest";
+import {
+  computePlantHoursFields,
+  validatePlantHoursRow,
+  type PlantHoursRowInput,
+} from "./plant-hours-schema";
+
+const baseVehicle = {
+  id: "veh-1",
+  qr_code_id: "QR-A",
+  description: "Generator X",
+  cost_per_hour: 185,
+  unit_type: "plant" as const,
+  site: "site-1",
+  site_project_code: "P-001",
+};
+
+const validRow: PlantHoursRowInput = {
+  date_logged: "2026-08-28",
+  vehicle_qr: "QR-A",
+  vehicle_desc: "Generator X",
+  start_date: "2026-08-24",
+  end_date: "2026-08-28",
+  start_time: "06:00",
+  end_time: "17:00",
+  start_hours: 1200,
+  end_hours: 1231,
+  hours: 31,
+  notes: "",
+  hour_meter_budget: 40,
+  cost_per_hour: 185,
+};
+
+describe("computePlantHoursFields", () => {
+  it("computes hours from end_hours − start_hours", () => {
+    expect(
+      computePlantHoursFields({ ...validRow, hours: undefined }).hours,
+    ).toBe(31);
+  });
+  it("computes value = hours * cost_per_hour rounded to 2dp", () => {
+    expect(computePlantHoursFields(validRow).value).toBe(5735);
+    // 31 * 185 = 5735 exactly
+  });
+  it("flags over_budget when hours > hour_meter_budget", () => {
+    expect(computePlantHoursFields(validRow).over_budget).toBe(false);
+    expect(
+      computePlantHoursFields({ ...validRow, hour_meter_budget: 20 })
+        .over_budget,
+    ).toBe(true);
+  });
+  it("flags hours_mismatch when operator hours differ by > 0.05", () => {
+    expect(computePlantHoursFields(validRow).hours_mismatch).toBe(false);
+    expect(
+      computePlantHoursFields({ ...validRow, hours: 30 }).hours_mismatch,
+    ).toBe(true);
+  });
+});
+
+describe("validatePlantHoursRow", () => {
+  it("returns no errors for a valid row + matching vehicle", () => {
+    const r = validatePlantHoursRow(validRow, baseVehicle, new Set());
+    expect(r.errors).toEqual([]);
+    expect(r.warnings).toEqual([]);
+  });
+
+  it("rejects unknown QR", () => {
+    const r = validatePlantHoursRow(
+      { ...validRow, vehicle_qr: "QR-XYZ" },
+      baseVehicle,
+      new Set(),
+    );
+    expect(r.errors[0]).toMatch(/unknown_vehicle_qr/);
+  });
+
+  it("rejects end_date < start_date", () => {
+    const r = validatePlantHoursRow(
+      { ...validRow, end_date: "2026-08-23" },
+      baseVehicle,
+      new Set(),
+    );
+    expect(r.errors[0]).toMatch(/bad_date_range/);
+  });
+
+  it("rejects end_hours < start_hours", () => {
+    const r = validatePlantHoursRow(
+      { ...validRow, end_hours: 1199 },
+      baseVehicle,
+      new Set(),
+    );
+    expect(r.errors[0]).toMatch(/end_hours_lt_start_hours/);
+  });
+
+  it("rejects bad time format", () => {
+    const r = validatePlantHoursRow(
+      { ...validRow, start_time: "not a time" },
+      baseVehicle,
+      new Set(),
+    );
+    expect(r.errors[0]).toMatch(/bad_time_format/);
+  });
+
+  it("rejects when cell cost_per_hour blank AND vehicle has none", () => {
+    const r = validatePlantHoursRow(
+      { ...validRow, cost_per_hour: null },
+      { ...baseVehicle, cost_per_hour: 0 },
+      new Set(),
+    );
+    expect(r.errors[0]).toMatch(/missing_cost_per_hour/);
+  });
+
+  it("falls back to vehicle.cost_per_hour when cell blank", () => {
+    const r = validatePlantHoursRow(
+      { ...validRow, cost_per_hour: null },
+      baseVehicle,
+      new Set(),
+    );
+    expect(r.errors).toEqual([]);
+    expect(r.computed.cost_per_hour).toBe(185);
+  });
+
+  it("rejects duplicate week", () => {
+    const r = validatePlantHoursRow(
+      validRow,
+      baseVehicle,
+      new Set(["QR-A|2026-08-24"]),
+    );
+    expect(r.errors[0]).toMatch(/duplicate_week/);
+  });
+
+  it("warns (does not error) on plant row for fuel-typed vehicle", () => {
+    const r = validatePlantHoursRow(
+      validRow,
+      { ...baseVehicle, unit_type: "fuel" },
+      new Set(),
+    );
+    expect(r.errors).toEqual([]);
+    expect(r.warnings[0]).toMatch(/plant_row_for_fuel_vehicle/);
+  });
+});
\ No newline at end of file
diff --git a/src/lib/plant-hours-schema.ts b/src/lib/plant-hours-schema.ts
new file mode 100644
index 0000000..46236ef
--- /dev/null
+++ b/src/lib/plant-hours-schema.ts
@@ -0,0 +1,150 @@
+/**
+ * Plant-hours row validator + derived-field helpers.
+ *
+ * The import route calls validatePlantHoursRow() once per row.
+ * It returns errors (which block insertion) and warnings (which
+ * surface in the Attention Required panel but don't block).
+ *
+ * Pure — no I/O, no PocketBase calls. The route owns the vehicle
+ * cache and the duplicate-set; this file just consumes them.
+ */
+import type { UnitType } from "./pb-types";
+
+export interface PlantHoursRowInput {
+  date_logged: string;
+  vehicle_qr: string;
+  vehicle_desc: string;
+  start_date: string;
+  end_date: string;
+  start_time: string | null;
+  end_time: string | null;
+  start_hours: number;
+  end_hours: number;
+  hours: number | null | undefined;
+  notes: string;
+  hour_meter_budget: number;
+  cost_per_hour: number | null;
+}
+
+export interface PlantHoursVehicleLookup {
+  id: string;
+  qr_code_id: string;
+  description: string;
+  cost_per_hour: number;
+  unit_type: UnitType;
+  site: string;
+  site_project_code: string;
+}
+
+export interface PlantHoursComputedFields {
+  hours: number;
+  cost_per_hour: number;
+  value: number;
+  over_budget: boolean;
+  hours_mismatch: boolean;
+  vehicle_unit_type_at_import: UnitType;
+}
+
+export interface PlantHoursValidationResult {
+  errors: string[];
+  warnings: string[];
+  computed: PlantHoursComputedFields;
+}
+
+const HOURS_MISMATCH_TOLERANCE = 0.05;
+
+/** Compute derived fields. Assumes the row passed basic type parsing. */
+export function computePlantHoursFields(
+  row: PlantHoursRowInput,
+  vehicleUnitType: UnitType = "plant",
+): PlantHoursComputedFields {
+  const hours = row.end_hours - row.start_hours;
+  const operatorHours = row.hours ?? hours;
+  const costPerHour = row.cost_per_hour ?? 0;
+  const value = Math.round(hours * costPerHour * 100) / 100;
+  return {
+    hours,
+    cost_per_hour: costPerHour,
+    value,
+    over_budget: hours > row.hour_meter_budget,
+    hours_mismatch:
+      Math.abs(operatorHours - hours) > HOURS_MISMATCH_TOLERANCE,
+    vehicle_unit_type_at_import: vehicleUnitType,
+  };
+}
+
+/**
+ * Validate one parsed row against the vehicle lookup and the
+ * set of (vehicle_qr, start_date) keys already imported in this batch.
+ */
+export function validatePlantHoursRow(
+  row: PlantHoursRowInput,
+  vehicle: PlantHoursVehicleLookup | undefined,
+  duplicatesThisBatch: Set<string>,
+): PlantHoursValidationResult {
+  const errors: string[] = [];
+  const warnings: string[] = [];
+
+  if (!vehicle || row.vehicle_qr !== vehicle.qr_code_id) {
+    errors.push(`unknown_vehicle_qr: ${row.vehicle_qr || "(empty)"}`);
+    return {
+      errors,
+      warnings,
+      computed: computePlantHoursFields(row),
+    };
+  }
+
+  if (row.end_date < row.start_date) {
+    errors.push("bad_date_range: end_date is before start_date");
+  }
+  if (row.end_hours < row.start_hours) {
+    errors.push(
+      `end_hours_lt_start_hours: ${row.end_hours} < ${row.start_hours}`,
+    );
+  }
+  if (
+    row.start_time !== null &&
+    !/^([01]\d|2[0-3]):[0-5]\d$/.test(row.start_time)
+  ) {
+    errors.push(`bad_time_format: start_time "${row.start_time}"`);
+  }
+  if (
+    row.end_time !== null &&
+    !/^([01]\d|2[0-3]):[0-5]\d$/.test(row.end_time)
+  ) {
+    errors.push(`bad_time_format: end_time "${row.end_time}"`);
+  }
+
+  // Cost-per-hour resolution.
+  let resolvedCostPerHour: number;
+  if (row.cost_per_hour !== null && row.cost_per_hour > 0) {
+    resolvedCostPerHour = row.cost_per_hour;
+  } else if (vehicle.cost_per_hour > 0) {
+    resolvedCostPerHour = vehicle.cost_per_hour;
+  } else {
+    errors.push("missing_cost_per_hour: cell empty and vehicle has no rate");
+    resolvedCostPerHour = 0;
+  }
+
+  const dupKey = `${vehicle.qr_code_id}|${row.start_date}`;
+  if (duplicatesThisBatch.has(dupKey)) {
+    errors.push(
+      `duplicate_week: ${vehicle.qr_code_id} already logged for ${row.start_date}`,
+    );
+  }
+
+  // Type-mismatch warning (lenient — doesn't block insertion).
+  if (vehicle.unit_type === "fuel") {
+    warnings.push(
+      `plant_row_for_fuel_vehicle: ${vehicle.qr_code_id} is typed as fuel; row was imported as audit-only data`,
+    );
+  }
+
+  const rowForCompute: PlantHoursRowInput = {
+    ...row,
+    cost_per_hour: resolvedCostPerHour,
+  };
+  const computed = computePlantHoursFields(rowForCompute, vehicle.unit_type);
+
+  return { errors, warnings, computed };
+}
\ No newline at end of file