Step 1: write down your real net income
Not the gross amount, but what actually goes into your account each month. If your income varies (freelance work, commissions), use an average of the last 3-6 months to avoid overestimating.
Step 2: list fixed expenses
Housing (rent or mortgage), utilities (power, water, internet), insurance, subscriptions you always pay. These are hard to lower from one month to the next.
Step 3: list variable expenses
Food, transport, leisure, unplanned purchases. This is usually where the real room for adjustment is if the budget doesn't add up.
Step 4: do the subtraction
Income − fixed expenses − variable expenses = what's left available. If the result is negative, you're spending more than you earn, and it's worth reviewing variable expenses first before fixed ones.
Step 5: decide how much to save, not "whatever's left"
A more effective approach than "saving whatever's left at the end of the month" is deciding a fixed percentage in advance (for example, 10-20% of income) and treating it as another fixed expense, setting it aside at the start of the month instead of at the end.
Do it with the tool
The monthly budget calculator does this subtraction for you, separating fixed and variable expenses, and tells you what percentage of your income the leftover amount represents.