@ashdragon@mastodon.social A digital spreadsheet. But to be fair I'm not really budgeting, just checking my expenses at the end of the month to have a broad idea of my expenses.
I can do this now, because I started paying most of the stuff using my debit card; this is apparently possible here in Spain where they accept even very small payments below 1€ to be done with the card.

What I do is the following: download my bank statement, copy expenses to a spreadsheet and broadly categorize each expense. This doesn't take more than 5 minutes per month. Then I have a formula set in place that will calculate how much money I spent, what percentage of my income I got as savings and what percentage of my savings I have invested.

Indeed I can do this because I pretty much keep in mind more or less how much I spent through the month. It works quite well for me as I rarely spend more than 50% of my salary.