How to track your spending in Mexico

There's one financial feature that structures a lot of people's spending in Mexico and that almost no finance app handles well: meses sin intereses — interest-free monthly instalments.

If you use them, your monthly budget isn't set by this month's decisions. It was set by decisions you made up to eighteen months ago.

MSI, and why they break expense tracking

Buying at 12 or 18 MSI is free in interest terms. The cost is a different kind: you commit months that don't exist yet.

The logging mistake is always the same. If you record the full purchase on the day you make it, that month is inflated and the next twelve look light. If you don't record it, it turns up on the statement by itself and there's nothing left to decide.

What works: log the monthly payment as a recurring expense for as many months as it lasts, not the purchase. With that, the coming months already contain the MSI before they arrive, and the question before buying changes — it stops being "can I afford the payment?" and becomes "can I afford it alongside every MSI I already have running?".

One number worth checking before any big purchase: how much of your next statement is already committed. It's covered in interest-free instalments.

The cut-off date matters more than it looks

Buying the day before the cut-off or the day after can mean up to thirty extra days to pay. It isn't a trick for spending more: it's information worth having once you've already decided to buy. How the two dates work is in closing date vs due date.

SPEI, cash and everyday apps

Between SPEI transfers, cash, cards and whichever wallets each person uses, no single source has your complete month. The bank can't see what you paid in cash, and across much of the country cash is still a large share of spending.

So a method that's indifferent to payment method pays off: you log it when you pay, whatever you paid with. The alternative — waiting for the bank to import everything — leaves out precisely what's hardest to remember. Covered in tracking without connecting a bank.

The aguinaldo isn't a good month

December brings the year-end bonus and it also brings December's spending. Treating it as extra income for the month is what makes January the hardest month of the year.

What works: when it lands, allocate it before touching it. Part to the emergency fund, part to the foreseeable first-quarter expenses — property tax, vehicle fees, school fees, insurance — and whatever's left is spending. In that order, not the reverse.

If you invoice

Two things that fix more than anything else:

1. Separate the work account from the personal one. Without it you don't know what you earn or what you spend, only what's left. The mechanics are in separating personal and work expenses.
2. Set tax aside on the day you're paid. Tax money looks like your money while it sits in the account, and that's the whole problem. If your invoicing moves month to month, irregular income lays out the base-month method.

In one line

Log the monthly payment rather than the purchase, check how much of your next statement is already committed before taking on new MSI, use a method that works the same for cash as for SPEI, and allocate the aguinaldo before you touch it.

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