diff --git a/functions/src/scripts/correct-bet-probability.ts b/functions/src/scripts/correct-bet-probability.ts
new file mode 100644
index 00000000..d61157ff
--- /dev/null
+++ b/functions/src/scripts/correct-bet-probability.ts
@@ -0,0 +1,53 @@
+import * as admin from 'firebase-admin'
+import * as _ from 'lodash'
+
+import { Bet } from '../../../common/bet'
+import { getProbability } from '../../../common/calculate'
+import { Contract } from '../../../common/contract'
+
+type DocRef = admin.firestore.DocumentReference
+
+// Generate your own private key, and set the path below:
+// https://console.firebase.google.com/u/0/project/mantic-markets/settings/serviceaccounts/adminsdk
+// const serviceAccount = require('../../../../../../Downloads/dev-mantic-markets-firebase-adminsdk-sir5m-b2d27f8970.json')
+const serviceAccount = require('../../../../../../Downloads/mantic-markets-firebase-adminsdk-1ep46-351a65eca3.json')
+
+admin.initializeApp({
+  credential: admin.credential.cert(serviceAccount),
+})
+const firestore = admin.firestore()
+
+async function migrateContract(contractRef: DocRef, contract: Contract) {
+  const bets = await contractRef
+    .collection('bets')
+    .get()
+    .then((snap) => snap.docs.map((bet) => bet.data() as Bet))
+
+  const lastBet = _.sortBy(bets, (bet) => -bet.createdTime)[0]
+  if (lastBet) {
+    const probAfter = getProbability(contract.totalShares)
+
+    await firestore
+      .doc(`contracts/${contract.id}/bets/${lastBet.id}`)
+      .update({ probAfter })
+
+    console.log('updating last bet from', lastBet.probAfter, 'to', probAfter)
+  }
+}
+
+async function migrateContracts() {
+  const snapshot = await firestore.collection('contracts').get()
+  const contracts = snapshot.docs.map((doc) => doc.data() as Contract)
+
+  console.log('Loaded contracts', contracts.length)
+
+  for (const contract of contracts) {
+    const contractRef = firestore.doc(`contracts/${contract.id}`)
+
+    console.log('contract', contract.question)
+
+    await migrateContract(contractRef, contract)
+  }
+}
+
+if (require.main === module) migrateContracts().then(() => process.exit())