--- title: "How to set allocation rules for shared expenses" description: "Give each shared cost a match rule, a split method and an order, then decide how payments apply. A template and worked example an AI agent can run every month." slug: "how-to-set-rules-for-shared-expenses" url: "https://bankbridge.money/guides/how-to-set-rules-for-shared-expenses" category: "how-to" published: "2026-11-06" updated: "2026-11-06" --- # How to set allocation rules for shared expenses > Write one rule per shared cost: what it matches (a merchant, a card, a category), how it splits (equal, percentage, fixed amount), and who paid. Put specific rules before general ones, apply each payment to the oldest balance first, and send anything unmatched to a review list. An AI agent can then apply the rules to your real transactions every month. ## 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 Method Works for Example **Equal** Costs everyone uses about the same Internet split three ways **Percentage** Rent by room size, couples by income Rent 40/30/30 **Fixed amount** Predictable costs, or a person's capped share Parking $75 for one person **By days** Move-ins, move-outs and long trips 19 of 30 days of a third **Payer only** Things that look shared but aren't One person's gym on the house card ## 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: Charge Total Me Sam Priya Rent (40/30/30) $2,700 $1,080 $810 $810 Electric (equal) $141 $47 $47 $47 Internet (equal) $60 $20 $20 $20 House card (equal) $612 $204 $204 $204 **Share of the month** **$3,513** **$1,351** **$1,081** **$1,081** 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](https://bankbridge.money/guides/claude-projects-for-personal-finance), 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](https://bankbridge.money/guides/split-shared-expenses-with-an-ai-agent). [Connect the accounts that pay your shared bills](https://bankbridge.money/login) for $5 a month per bank and run your first allocation. ## FAQ ### What is the fairest way to split shared expenses? There is no single fair method. Equal splits suit costs everyone uses the same way, such as internet. Percentages suit rent by room size or a couple splitting by income. Pick a method per cost, write it down, and agree on it before the first bill. ### How should a partial payment be allocated? Apply it to the oldest open balance first, unless the payer names a specific bill. That keeps old amounts from lingering and gives everyone one running number. Write the rule down so nobody has to decide it again each month. ### How do I automate shared expense rules? Save the rules as a plain-text note, connect the accounts that pay shared bills to an AI agent with BankBridge, and ask for the month's allocation. The agent reads your transactions and applies the rules. It can't send or request money, because BankBridge is read-only. ### What if a charge matches two rules? Put the more specific rule first, so a merchant rule beats a category rule and an exception beats both. An agent applies the first match and can show which rule it used on each line, which makes mistakes easy to find.