diff --git a/CHANGELOG.md b/CHANGELOG.md
index 7af38545f..28dec6d5b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -25,6 +25,7 @@ function encodeMuxedAccountToAddress(account: xdr.MuxedAccount, supportMuxing: b
 function encodeMuxedAccount(gAddress: string, id: string): xdr.MuxedAccount;
 ```
 
+- Adds a helper function `Transaction.getClaimableBalanceId(int)` which lets you pre-determine the hex claimable balance ID of a `createClaimableBalance` operation prior to submission to the network ([#482](https://github.com/stellar/js-stellar-base/pull/482)).
 
 ### Breaking Changes
 
diff --git a/src/operations/set_trustline_flags.js b/src/operations/set_trustline_flags.js
index 51c7dfb21..0a872e563 100644
--- a/src/operations/set_trustline_flags.js
+++ b/src/operations/set_trustline_flags.js
@@ -42,7 +42,7 @@ export function setTrustLineFlags(opts = {}) {
   const attributes = {};
 
   if (typeof opts.flags !== 'object' || Object.keys(opts.flags).length === 0) {
-    throw new Error('opts.flags must be an map of boolean flags to modify');
+    throw new Error('opts.flags must be a map of boolean flags to modify');
   }
 
   const mapping = {
diff --git a/src/transaction.js b/src/transaction.js
index f268260bc..c2960b05c 100644
--- a/src/transaction.js
+++ b/src/transaction.js
@@ -1,4 +1,5 @@
 import map from 'lodash/map';
+
 import xdr from './generated/stellar-xdr_generated';
 import { hash } from './hashing';
 
@@ -6,7 +7,10 @@ import { StrKey } from './strkey';
 import { Operation } from './operation';
 import { Memo } from './memo';
 import { TransactionBase } from './transaction_base';
-import { encodeMuxedAccountToAddress } from './util/decode_encode_muxed_account';
+import {
+  extractBaseAddress,
+  encodeMuxedAccountToAddress
+} from './util/decode_encode_muxed_account';
 
 /**
  * Use {@link TransactionBuilder} to build a transaction object. If you have an
@@ -213,4 +217,53 @@ export class Transaction extends TransactionBase {
 
     return envelope;
   }
+
+  /**
+   * Calculate the claimable balance ID for an operation within the transaction.
+   *
+   * @param   {integer}  opIndex   the index of the CreateClaimableBalance op
+   * @returns {string}   a hex string representing the claimable balance ID
+   *
+   * @throws {RangeError}   for invalid `opIndex` value
+   * @throws {TypeError}    if op at `opIndex` is not `CreateClaimableBalance`
+   * @throws for general XDR un/marshalling failures
+   *
+   * @see https://github.com/stellar/go/blob/d712346e61e288d450b0c08038c158f8848cc3e4/txnbuild/transaction.go#L392-L435
+   *
+   */
+  getClaimableBalanceId(opIndex) {
+    // Validate and then extract the operation from the transaction.
+    if (
+      !Number.isInteger(opIndex) ||
+      opIndex < 0 ||
+      opIndex >= this.operations.length
+    ) {
+      throw new RangeError('invalid operation index');
+    }
+
+    let op = this.operations[opIndex];
+    try {
+      op = Operation.createClaimableBalance(op);
+    } catch (err) {
+      throw new TypeError(
+        `expected createClaimableBalance, got ${op.type}: ${err}`
+      );
+    }
+
+    // Always use the transaction's *unmuxed* source.
+    const account = StrKey.decodeEd25519PublicKey(
+      extractBaseAddress(this.source)
+    );
+    const operationId = xdr.OperationId.envelopeTypeOpId(
+      new xdr.OperationIdId({
+        sourceAccount: xdr.AccountId.publicKeyTypeEd25519(account),
+        seqNum: new xdr.SequenceNumber(this.sequence),
+        opNum: opIndex
+      })
+    );
+
+    const opIdHash = hash(operationId.toXDR('raw'));
+    const balanceId = xdr.ClaimableBalanceId.claimableBalanceIdTypeV0(opIdHash);
+    return balanceId.toXDR('hex');
+  }
 }
diff --git a/test/unit/transaction_test.js b/test/unit/transaction_test.js
index af988dafc..4804da257 100644
--- a/test/unit/transaction_test.js
+++ b/test/unit/transaction_test.js
@@ -563,6 +563,77 @@ describe('Transaction', function() {
       );
     });
   });
+
+  describe('knows how to calculate claimable balance IDs', function() {
+    const address = 'GA7QYNF7SOWQ3GLR2BGMZEHXAVIRZA4KVWLTJJFC7MGXUA74P7UJVSGZ';
+
+    const makeBuilder = function(source) {
+      return new StellarBase.TransactionBuilder(source, {
+        fee: StellarBase.BASE_FEE,
+        networkPassphrase: StellarBase.Networks.TESTNET,
+        withMuxing: true
+      }).setTimeout(StellarBase.TimeoutInfinite);
+    };
+
+    const makeClaimableBalance = function() {
+      return StellarBase.Operation.createClaimableBalance({
+        asset: StellarBase.Asset.native(),
+        amount: '100',
+        claimants: [
+          new StellarBase.Claimant(
+            address,
+            StellarBase.Claimant.predicateUnconditional()
+          )
+        ]
+      });
+    };
+
+    const paymentOp = StellarBase.Operation.payment({
+      destination: address,
+      asset: StellarBase.Asset.native(),
+      amount: '100'
+    });
+
+    it('calculates from transaction src', function() {
+      let gSource = new StellarBase.Account(address, '1234');
+
+      let tx = makeBuilder(gSource)
+        .addOperation(makeClaimableBalance())
+        .build();
+      const balanceId = tx.getClaimableBalanceId(0);
+      expect(balanceId).to.be.equal(
+        '00000000536af35c666a28d26775008321655e9eda2039154270484e3f81d72c66d5c26f'
+      );
+    });
+
+    it('calculates from muxed transaction src as if unmuxed', function() {
+      let gSource = new StellarBase.Account(address, '1234');
+      let mSource = new StellarBase.MuxedAccount(gSource, '5678');
+      let tx = makeBuilder(mSource)
+        .addOperation(makeClaimableBalance())
+        .build();
+
+      const balanceId = tx.getClaimableBalanceId(0);
+      expect(balanceId).to.be.equal(
+        '00000000536af35c666a28d26775008321655e9eda2039154270484e3f81d72c66d5c26f'
+      );
+    });
+
+    it('throws on invalid operations', function() {
+      let gSource = new StellarBase.Account(address, '1234');
+      let tx = makeBuilder(gSource)
+        .addOperation(paymentOp)
+        .addOperation(makeClaimableBalance())
+        .build();
+
+      expect(() => tx.getClaimableBalanceId(0)).to.throw(
+        /createClaimableBalance/
+      );
+      expect(() => tx.getClaimableBalanceId(1)).to.not.throw();
+      expect(() => tx.getClaimableBalanceId(2)).to.throw(/index/);
+      expect(() => tx.getClaimableBalanceId(-1)).to.throw(/index/);
+    });
+  });
 });
 
 function expectBuffersToBeEqual(left, right) {
diff --git a/types/index.d.ts b/types/index.d.ts
index 9de2bd1c2..a5c155383 100644
--- a/types/index.d.ts
+++ b/types/index.d.ts
@@ -903,6 +903,8 @@ export class Transaction<
     minTime: string;
     maxTime: string;
   };
+
+  getClaimableBalanceId(opIndex: number): string;
 }
 
 export const BASE_FEE = '100';