How to set allocation rules for shared expenses

1. List what is shared

Start with the last two or three months of statements, and sort costs into shared, not shared, and sometimes shared. The last group causes the arguments: takeout ordered for the house, a hardware store run, a gift from everyone. Decide those now, while no specific bill is at stake.

2. Pick a split method for each cost

3. Write a match for each rule

A rule needs a test that a transaction either passes or fails. From most to least reliable:

• Card or account. "Everything on the card ending 7734 is shared." A dedicated house card removes most of the work.
• Merchant name. "Merchant contains CITY POWER." Use the name as it appears on your statement, since it often differs from the brand.
• Amount and day. "$2,700 around the 1st." This catches rent paid as a plain bank transfer, which won't carry a merchant name.
• Category. "Groceries." It's the broadest test, and banks sometimes label a charge differently than you would.

4. Decide which rule wins

Rules will overlap. A pharmacy run on the house card matches both the card rule and your exception for personal items. Apply the first match in this order:

• Named exceptions (one person's gym, one person's parking)
• Merchant rules
• Card or account rules
• Category rules
• Everything else goes to review

Net refunds against the same rule in the same month, so a returned grocery order lowers shared groceries.

5. Decide how payments are allocated

When someone pays you back, the payment needs a rule too:

• Who sent it. List the sender names as they appear on your statement, such as "SAM K". Payment apps often show the app and the sender in the description.
• What it pays first. Apply each payment to the oldest open balance unless the payer names a bill.
• What happens to extra. Carry an overpayment forward as a credit, and don't refigure past months.
• What counts besides transfers. Note cash or a bill someone paid directly, since neither shows up as money coming in.

6. Send everything else to review

Don't guess at charges no rule covers. Put them on a short list for a person to decide. When the same merchant appears twice, turn it into a rule. After a few months, the list usually gets short.

Here is the full set as a note you can save:

Household: me, Sam, Priya. I pay every bill.

Exceptions (first)
- Merchant contains "IRONWORKS GYM": mine only

Merchant rules
- "CITY POWER": equal
- "FIBERNET": equal
- $2,700 transfer around the 1st: rent, me 40%, Sam 30%, Priya 30%

Card rules
- Card ending 7734 (house card): equal

Payments
- Incoming from "SAM K" or "PRIYA N" count toward what they owe
- Apply to the oldest balance first; extra carries forward

Everything else
- Not shared. List anything that looks shared for me to review.

A worked month

With those rules and these made-up amounts:

Sam sent $900, so Sam still owes $181. Priya sent $1,081 and is settled. Next month, Sam's first payment goes to that $181 before anything newer.

Running the rules automatically

Rules written this way are easy for an AI agent to follow. Connect the accounts that pay shared bills with BankBridge, keep the note where the agent can read it, such as a Claude Project, and ask:

"Apply our shared expense rules to last month. Show each charge with the rule you used and each person's share. Subtract incoming payments, oldest balance first, and list anything no rule covered."

The agent uses list_transactions for the accounts and search_transactions for merchants and senders, then does the math in the chat. Showing the rule on each line lets everyone check the result.

The allocation is automatic, but the money is not. BankBridge is read-only, so the agent can't send or request a payment. You get a total and a breakdown to paste into a payment request. The monthly routine, including who needs an account, is in splitting shared expenses with an AI agent.

Connect the accounts that pay your shared bills for $5 a month per bank and run your first allocation.