diff --git a/conservancy_beancount/data.py b/conservancy_beancount/data.py
index 8e191dfbf6677cc57602ab4f1d21d09fdc621fdf..b16e60e8856a9fdf0cc18ea77a72d4178080eb1d 100644
--- a/conservancy_beancount/data.py
+++ b/conservancy_beancount/data.py
@@ -260,6 +260,31 @@ class Posting(BasePosting):
         return self.account.is_cash_equivalent() and self.is_debit(threshold, default)
 
 
+def balance_of(txn: Transaction,
+               *accounts: str,
+               default: Optional[DecimalCompat]=None,
+) -> Optional[decimal.Decimal]:
+    """Return the balance of specified postings in a transaction.
+
+    Given a transaction and a series of account names, balance_of returns the
+    balance of the amounts of all postings under those account names.
+
+    Account names are matched using Account.is_under. Refer to that docstring
+    for details about what matches.
+
+    If any of the postings have no amount, returns default.
+    """
+    if default is not None:
+        default = decimal.Decimal(default)
+    retval = decimal.Decimal(0)
+    for post in txn.postings:
+        if Account(post.account).is_under(*accounts):
+            if post.units.number is None:
+                return default
+            else:
+                retval += post.units.number
+    return retval
+
 def iter_postings(txn: Transaction) -> Iterator[Posting]:
     """Yield an enhanced Posting object for every posting in the transaction"""
     for index, source in enumerate(txn.postings):