This document describes how to do YNAB-style budgeting with the help of the avail_ext fava extension.
Enable the rebudget beancount plugin and the avail_ext fava extension
by adding the following lines to your beancount file:
plugin "refried.plugins.rebudget"
2020-01-01 custom "fava-extension" "refried.extensions.avail_ext"
This will add a new report "Budget" to fava. The rebudget plugin is
required for it to function properly.
The "Budget" extension will display all of your Income and Expense accounts, by month. In the style of YNAB, each account has three columns: "Budgeted", "Spent", and "Available".
Normal transactions will show up under the "Spent" columns of their respective Expense accounts. No need to do anything special!
To (re)budget between Expense accounts, simply create a new transaction transferring money between Income/Expense accounts. Due to the nature of double-entry bookkeeping, the signs will be the opposite of what you might expect!
For example, suppose you have $3000 in unbudgeted income and you want to fill out your budget for the upcoming month:
2020-02-01 "Budgeting for February"
Income:To-be-budgeted 2,000 USD
Expenses:Obligations:Rent -1,000 USD
Expenses:Obligations:Utilities -100 USD
Expenses:Necessities:Groceries -200 USD
Expenses:Fun-money:Video-games -700 USD
Remember, the signs appear swapped! This takes $2000 out of the "To be budgeted" account and moves money into the expense account categories.
Later, if you realize that you need more money for groceries (and don't need nearly as much for video games), you can rebudget money like so:
2020-02-13 "Need more money for fancy home dinner plans"
Expenses:Necessities:Groceries -300 USD
Expenses:Fun-money:Video-games 300 USD
Understandably, this will wreak havoc with your (non-budgeting) expense reports. This is where the rebudget plugin comes in!
For any transaction that don't involve an Assets or Liabilities account, such as the ones above, the rebudget plugin will
automatically apply the tag #rebudget. So by filtering out the #rebudget tag you will have the correct data to use for your usual reports!
(In fava you can use the search filter -#rebudget.)
If you also want YNAB-style account views with "Working" vs "Cleared" totals and cleared vs uncleared postings, see the YNAB-style account views document.