In short: It's a simple sum: net income minus fixed costs minus your savings rate equals your free-to-spend budget. First list all recurring costs, spread irregular expenses (insurance, repairs) across the month, and subtract your savings rate up front. Whatever's left is yours to spend this month.
How to Calculate Your Monthly Budget – What's Left Over?
How much money is actually left at the end of the month? One simple sum gives you the answer – and it's easier than you think.
Add up your net income: pay after tax, plus any child benefit, side income or rental income – everything that truly lands in your account.
Capture your fixed costs cleanly: rent, utilities, insurance, phone, subscriptions, loan payments. Easiest way: scan your last three bank statements.
Spread irregular costs across the month: add up yearly expenses (car insurance, holidays, Christmas) and divide by 12 – so December doesn't catch you out.
Subtract your savings rate before you count what's left – save first, don't save from the leftovers. The 20% in the 50/30/20 rule is a handy guide.
Free to spend = net income − fixed costs − saving. That's your room to breathe for wants and daily life.
Needs · 50%
Wants · 30%
Saving & debt · 20%
50/30/20 – a simple split of your take-home pay.
Try it yourself
50 % needs—
30 % wants—
20 % saving & debt—
Illustrative, rounded.
Kontoo calculates your "free this month" live and forecasts how your balance runs to month-end: set it up now. To play with your savings rate, try the compound-interest calculator.
In depth
Capture fixed costs cleanly – and miss none
The most common mistake when working out a budget is estimating fixed costs from memory – you almost always forget the small recurring ones: streaming, cloud storage, club fees, a magazine subscription, that second insurance policy. Far more reliable is scanning your last three bank statements: every recurring debit is a fixed-cost candidate. As you go, deliberately separate true fixed costs (the same amount every month) from variable costs like groceries or fuel – those fluctuate and belong in your free-to-spend budget, not the fixed sum. You only get a realistic starting figure once this list is complete; better to spend 20 minutes once than to plan every month with a flattering number.
Smooth out irregular costs – the divide-by-12 trick
Budgets rarely break on everyday spending; they break on the big one-off items: car insurance in January, a summer holiday, gifts in December, the annual utilities bill. These amounts feel "unpredictable", but they aren't – they just don't arrive every month. So scale them down: example – about €1,800 in yearly costs for such items works out to €150 a month you set aside. Skip this and you'll cover the spikes on your overdraft and undo the whole year's savings. This monthly reserve belongs in the calculation as its own fixed-cost line – kept separate from your emergency fund, which stays reserved for genuine emergencies.
A worked example
Illustrative figures, not official values: a single person takes home €2,400 a month. Their fixed costs – rent €900, utilities/internet/phone €120, insurance €90, subscriptions €40, a divide-by-12 reserve for yearly bills €150 – add up to €1,300. They subtract a savings rate of €360 (that's 15% of net). What's left is €2,400 − €1,300 − €360 = €740 free to spend on groceries, leisure and wants. Measured against the 50/30/20 rule, that's roughly 54% needs, 31% wants and 15% saving – close, but the savings rate still has room to grow. Adjustment points like that only become visible once the sum is down in black and white.
Education, not advice. How we work and check figures: Editorial. Figures as of 2026, last reviewed 07/04/2026.
Frequently asked questions
How do I calculate my monthly budget?
Simple: net income minus fixed costs minus your savings rate. List every recurring cost, divide yearly expenses by 12, subtract your savings rate – whatever remains is your free-to-spend budget for the month.
How much money is left each month?
As much as remains from your net income after fixed costs and saving. Example: €2,400 net − €1,300 fixed costs − €360 saving = €740 free to spend. Without a clean fixed-cost list, most people overestimate this figure.
What is free-to-spend (disposable) income?
The money left after all fixed commitments and your savings rate – free to use for groceries, leisure and wants. Formula: net − fixed costs − saving. Some include the savings rate; it's cleaner to subtract it first.
Does the savings rate count as a fixed cost?
For the calculation, treat it like one: save first, then plan the rest ("pay yourself first"). That way saving isn't a leftover that happens to remain at month-end – or doesn't.
What do I do with irregular expenses?
Spread them across the month: add up all yearly costs (insurance, holidays, repairs) and divide by 12. Set that amount aside monthly so the big bills don't knock your budget off balance.
What can I afford?
A rough guide is the 50/30/20 rule: about 50% of net income for needs, 30% for wants, 20% for saving. What you can truly afford over time is shown by your free-to-spend budget – not your current account balance.
All lessons · Glossary · Editorial · Kontoo does the math and explains – this is general education, not tax, legal or financial advice.
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