diff --git a/src/sync/syncService.ts b/src/sync/syncService.ts
new file mode 100644
index 0000000..9fd05c1
--- /dev/null
+++ b/src/sync/syncService.ts
@@ -0,0 +1,90 @@
+import { Measurement } from '../types'
+import { initDatabase, getAllMeasurements, upsertMeasurement } from '../database/db'
+import { getWebDAVConfig } from '../screens/SettingsScreen'
+
+const WEBDAV_PATH = 'weegschaal/measurements.json'
+
+type SyncResult = { local: number; remote: number; merged: number }
+
+export function mergeMeasurements(local: Measurement[], remote: Measurement[]): Measurement[] {
+  const map = new Map<string, Measurement>()
+
+  for (const m of local) {
+    map.set(m.id, m)
+  }
+
+  for (const m of remote) {
+    const existing = map.get(m.id)
+    if (!existing || new Date(m.updatedAt) > new Date(existing.updatedAt)) {
+      map.set(m.id, { ...m, syncStatus: 'synced' })
+    }
+  }
+
+  return Array.from(map.values())
+}
+
+async function fetchRemote(config: { url: string; username: string; password: string }): Promise<Measurement[]> {
+  const url = `${config.url.replace(/\/+$/, '')}/${WEBDAV_PATH}`
+  const response = await fetch(url, {
+    headers: {
+      Authorization: 'Basic ' + btoa(`${config.username}:${config.password}`),
+    },
+  })
+
+  if (response.status === 404) {
+    return []
+  }
+
+  if (!response.ok) {
+    throw new Error(`WebDAV fetch failed: ${response.status}`)
+  }
+
+  return response.json()
+}
+
+async function uploadRemote(
+  config: { url: string; username: string; password: string },
+  measurements: Measurement[]
+): Promise<void> {
+  const url = `${config.url.replace(/\/+$/, '')}/${WEBDAV_PATH}`
+  const body = JSON.stringify(measurements)
+
+  const response = await fetch(url, {
+    method: 'PUT',
+    headers: {
+      Authorization: 'Basic ' + btoa(`${config.username}:${config.password}`),
+      'Content-Type': 'application/json',
+    },
+    body,
+  })
+
+  if (!response.ok) {
+    throw new Error(`WebDAV upload failed: ${response.status}`)
+  }
+}
+
+export async function syncAll(): Promise<SyncResult> {
+  const config = await getWebDAVConfig()
+  if (!config) {
+    throw new Error('Geen WebDAV configuratie. Stel deze eerst in in Instellingen.')
+  }
+
+  await initDatabase()
+
+  const local = await getAllMeasurements()
+  const remote = await fetchRemote(config)
+
+  const merged = mergeMeasurements(local, remote)
+
+  for (const m of merged) {
+    await upsertMeasurement(m)
+  }
+
+  await uploadRemote(config, merged)
+
+  return {
+    local: local.length,
+    remote: remote.length,
+    merged: merged.length,
+  }
+}
diff --git a/tests/syncService.test.ts b/tests/syncService.test.ts
new file mode 100644
index 0000000..fae08ca
--- /dev/null
+++ b/tests/syncService.test.ts
@@ -0,0 +1,57 @@
+import { mergeMeasurements } from '../src/sync/syncService'
+import { Measurement } from '../src/types'
+
+jest.mock('expo-sqlite', () => ({
+  openDatabaseAsync: jest.fn(),
+}))
+
+const base: Measurement = {
+  id: 'id-1',
+  weight: 75,
+  bmi: 22,
+  bodyFatPercentage: 15,
+  fatMass: 10,
+  musclePercentage: 40,
+  muscleMass: 30,
+  visceralFat: 8,
+  bmr: 1600,
+  measuredAt: '2024-06-01T08:00:00.000Z',
+  createdAt: '2024-06-01T08:00:00.000Z',
+  updatedAt: '2024-06-01T08:00:00.000Z',
+  syncStatus: 'local',
+}
+
+describe('mergeMeasurements', () => {
+  it('returns empty array when both are empty', () => {
+    expect(mergeMeasurements([], [])).toEqual([])
+  })
+
+  it('keeps local items not on remote', () => {
+    const result = mergeMeasurements([base], [])
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe('id-1')
+  })
+
+  it('keeps remote items not on local', () => {
+    const remote = { ...base, syncStatus: 'synced' as const }
+    const result = mergeMeasurements([], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].id).toBe('id-1')
+  })
+
+  it('picks the newer version when both have same id', () => {
+    const local = { ...base, weight: 75, updatedAt: '2024-06-02T08:00:00.000Z' }
+    const remote = { ...base, weight: 76, updatedAt: '2024-06-03T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].weight).toBe(76)
+  })
+
+  it('picks local when local is newer', () => {
+    const local = { ...base, weight: 77, updatedAt: '2024-06-04T08:00:00.000Z' }
+    const remote = { ...base, weight: 76, updatedAt: '2024-06-03T08:00:00.000Z' }
+    const result = mergeMeasurements([local], [remote])
+    expect(result).toHaveLength(1)
+    expect(result[0].weight).toBe(77)
+  })
+})