Four lines
Write four numbers for one month: what arrives (income after tax), what is fixed (rent, bills, loan payments), everything else you spend, and what is left.
In the example: income 2,800 euros. Fixed costs 1,400. Everything else 800. Spending in total 2,200. What is left: 600.
Why "everything else" is one line
Splitting it into twenty categories feels thorough and usually gets abandoned by March. One honest total, checked against your bank statement once a month, is more useful than twenty guesses.
The share that is left
600 euros out of 2,800 is 21 percent. Some people use a rule such as 50 percent needs, 30 percent wants and 20 percent savings. It is a common rule of thumb, not a standard, and plenty of households cannot or need not follow it.
From budget to scenario
The 600 euros left is the monthly amount the calculator sets aside. Change "everything else" to 700 and the monthly amount becomes 700. That is the whole connection between a budget and a savings roll.
Try this example in the calculator
Not financial advice. All amounts are in euros (EUR) and are examples.