Short on the 14th, fine by the 28th
The rest of this site mostly answers long questions. How much will I have at 60? When can I stop working? Those matter enormously, and they are the reason most of these tools exist. But almost nobody bounces a payment in 2050. They bounce it on the 14th, because the mortgage went out on the 1st, council tax on the 5th, energy on the 8th, and payday is not until the 28th. The month balances perfectly. The sequence does not.
Timing failures carry real price tags: an overdraft fee, a returned direct debit charge, a missed credit card payment and the interest that follows, charges the FCA has spent years pressing banks over. None of them is caused by spending too much. This calendar exists to catch them a week before they happen.
Ten lines, one chart
Start with the balance in your current account on the 1st, then add every movement you can predict, income first, then each recurring payment, with the day of the month it lands. You do not need every coffee; that is what the expense diary is for. You need the big, dated, non-negotiable items: rent or mortgage, council tax, utilities, insurance, phone, loan and card payments, childcare, season tickets, and any regular transfer out to savings. The whole thing takes a few minutes because these are the payments that repeat identically each month, and after the first build it becomes a two-minute check. Once a month is plenty, on payday, alongside your allocation.
Everything you type stays in this browser. There is no bank connection to grant (nor could you grant one even if you wanted to), which also means there is no account access to be breached.
The chart underneath plots your running balance across all 31 days: green where you are positive, amber marking the lowest day, red for anything below zero. Hover a bar to see what lands that day. One health warning belongs right here, though, because the whole chart rests on it: this is a flat 31-day month with no weekday logic, and real payments do not work that way. A direct debit due on a Saturday usually leaves on the following working day, salaries often arrive on the last working day rather than a fixed date, and February is short. Treat the day numbers as approximate and give yourself a day or two of slack either side of any tight spot.
If your income is irregular, enter the amount you are confident of receiving and its likely date, not your best case. A trough calculated on optimistic income is worse than no trough at all.
The trough, not the ending
Your end-of-month balance is close to meaningless as a safety measure, because it is measured after your salary has arrived. What decides whether a payment clears is the trough: the lowest your balance goes at any point in the month. If the trough is £40, you are one unexpected charge from an overdraft, even though the month ended comfortably. If it is negative, the chart tells you exactly which day it happens and what caused it.
The gap between your trough and zero is the buffer you are genuinely running. Most people are shocked by how thin it is. Reading the calendar day by day works through what to do once you can see the dip coming.
Remember what the chart cannot know: it only shows what you enter. Variable spending (food, fuel, going out) is not there unless you add it as a line, so your real balance will track lower than the green line for most of the month. Some people add a single "daily spending" line on the 1st for a realistic worst case; others prefer the chart to show only fixed commitments and read the trough as a ceiling rather than a forecast. Either is defensible, as long as you know which you are looking at. It is a single-account view too. If your bills leave one account and your income arrives in another, or you juggle several cards, the chart will not reflect where the money actually is, so model the account the direct debits come out of.
Fixes that cost nothing
When several bills cluster before payday, you rarely need to earn or spend a penny differently. Most providers will change a direct debit date on request, and shifting two bills from the 5th to the 20th can lift the trough by hundreds. If you move money to savings on the 1st, move half on the 1st and half after payday instead. Discretionary payments, the extra against a card or the investing transfer, are the easiest things to push later in the month rather than earlier. And if the trough is chronically thin whatever you move, the answer is a bigger buffer, which is what the emergency fund calculator is for. If it is chronically negative rather than thin, that is a debt problem more than a timing one, and StepChange's free debt advice is the right starting point.
There is no rollover between months. Each run starts from the opening balance you type, so the picture is one month at a time by design: a short-term tool that does not attempt to be anything else. That also means it cannot see anything irregular (an annual renewal, a tax bill, a one-off repair), and those are exactly the payments that break a month, which is why they deserve a funded pot of their own rather than a hopeful gap in the calendar. For the long view, that is what the FIRE calculator does.
Once the shape of the month works, deciding the size of each payment is a different job; that is the payday allocator. And if the culprit is a cluster of recurring charges you no longer value, the subscription audit is the faster fix.