split-ynab

Automatically split transactions in YNAB.

Overview

My partner and I use a combination of a personal credit cards, a shared credit card (via authorized users), and Venmo for shared expenses. I use YNAB to track my spending (and they do not). We roughly follow the "Option Two" as outlined in YNAB's blog post on the subject. This integration makes the bookkeeping easier. Our setup is:

1. We have a shared credit card account that is used for expenses which we split evenly (groceries, restaurants, etc.). This is an account in my name which my partner is an authorized user of.
2. In my budget I have a "Splitting" category which I've funded with $1000. This is the amount that I'm willing to spend on shared expenses before my partner needs to pay me back.
3. When either of us makes a purchase on that account, it will be synced to my YNAB via the built-in import. I then classify the full amount of that purchase to the relevant category. So a $30 purchase at Trader Joe's will show up (initially) as coming entirely from my "Groceries" budget category.
4. This program is run on a periodic schedule and sees that a new expense has been added to the shared credit card account. It updates the transaction to be a split transaction. In this example, it will update the $30 purchase so that $15 come from my "Groceries" budget category, and $15 come from my "Splitting" category.
5. At the end of the month when we get the bill for the credit card, I pay the full amount and charge my partner for their portion via Venmo manually. The amount they owe me is $1000 minus the current amount in my "Splitting" category. When I get repaid, that money goes to refilling the "Splitting" category.

We like this setup because it reduces the number of Venmo transactions we make, and I like that this keeps my YNAB accurate with my spending, even when I pay for shared things. I also like that it keeps YNAB as the single source of truth for my spending, rather than using another app (like Splitwise) to track shared expenses.

Configuration

Configuration is read from a config.yml file at the root of the application. An example config might look like:

---
ynabToken: "your-token-here"
budgetId: "00000000-1111-2222-3333-444455556666"
splitCategoryId: "deadbeef-1111-2222-3333-444455556666"
accounts:
  - id: "01010101-1111-2222-3333-444455556666" # Defaults to 50/50
    exceptFlags: ["green"]
  - id: "02020202-1111-2222-3333-444455556666"
    exceptFlags: ["green"]
    defaultPercentTheirShare: 20
flags:
  - color: "orange" # Defaults to 50/50
  - color: "purple"
    percentTheirShare: 30

ynabToken is your YNAB API token. You can get generate one on the YNAB developer settings page.

budgetId is the ID of the budget you want to use. You can get this from the URL of the budget in the YNAB web app. For example, if your budget URL is https://app.ynab.com/00000000-1111-2222-3333-444455556666/budget, then your budget ID is 00000000-1111-2222-3333-444455556666.

splitCategoryId is the ID of the category you want to assign the other person's portion of the split transactions to, the "Splitting" category in the example above. Getting this is a little trickier than the above, you'll have to issue an API call to list your categories and find the one you want. For example, you can use curl and jq to find the ID of the category named "Splitting" like so (replacing <your_budget_id> and <your_ynab_token> with the values you obtained above):

curl --request GET \
  --url https://api.ynab.com/v1/budgets/<your_budget_id>/categories \
  --header 'Authorization: Bearer <your_ynab_token>' \
  | jq '.data.category_groups[].categories[] | select(.name=="Splitting")'

The accounts and flags sections are used to determine which transactions should be split, and how to split them.

To have an account's transactions be split by default, first obtain the account's ID from the YNAB API, then add an entry to the accounts section. You can optionally specify exceptFlags: any transactions on the account with the given flags will not be split. I use this in cases where I don't want to split a transaction, like if I've bought something for myself on the shared credit card. You can also optionally specify defaultPercentTheirShare: this is the percentage of the transaction that should be considered the other person's responsibility, and defaults to 50. But if you have a different arrangement, like if you pay 70% of the shared expenses and your partner pays 30%, you would specify defaultPercentTheirShare: 30.

flags is used for two things:

1. To allow transactions outside the specified accounts to be split
2. To override the default split percentage for a given transaction

For all transactions in your YNAB, if it matches one of these flags it will be split at the given rate, defaulting to 50/50. This is useful if pay for something on your personal credit card, but you want to split it with your partner. It's also useful if you want to split a transaction at a different rate than the default for a given account, like if you pay 70% of the internet bill but it comes out of the shared credit card account.

Running Locally

Assuming you have Go installed (if not, see the Go docs), clone the repo, add a config.yml following the section above, and run:

cd split-ynab
go mod download
go run cmd/split-ynab/main.go

Deploying to AWS

This project uses AWS CDK to define all its necessary AWS resources. If you have an AWS account, you can easily deploy this project to the cloud where it will run every hour.

See the CDK documentation for how to install the CDK on your machine. Once CDK is installed:

cd split-ynab
cdk bootstrap
cdk deploy

And the application will be deployed to AWS!

The cost of doing should be zero, assuming you haven't gone past your AWS free tier limits:

• The "always free" tier of AWS Lambda provides 1 million executions per month, which is more than enough to run this every hour.
• The "always free" tier of AWS DynamoDB provides 25 GB of storage, this application needs only a few bytes to store a single key/value pair.
• The configuration file and various CDK assets are stored in S3. S3 provides 5GB of free storage for a year, then $0.023 per GB per month after that. Currently the total size of my bucket is 14KB, cost should round to zero.