Budget Tracker Methodology — Zero-Based & 50/30/20 Budgeting Explained

This methodology handbook documents the math, design principles, and category classification rules behind the CalculatorAI Budget Tracker.

1. Core Budgeting Frameworks

The Budget Tracker combines two complementary personal finance systems:

Zero-Based Budgeting (ZBB)

In Zero-Based Budgeting, every unit of income is assigned a target before the month begins:

$$\text{Unallocated Balance} = \text{Total Planned Income} - \text{Total Planned Expenses}$$

When $\text{Unallocated Balance} = 0$, your budget is fully balanced ("Every dollar has a job").

The 50 / 30 / 20 Allocation Rule

Expenses are grouped into three primary classifications:

• Needs (50%): Non-negotiable survival and fixed expenses (Housing, Utilities, Groceries, Minimum Debt Payments, Transportation).
• Wants (30%): Lifestyle and discretionary choices (Dining Out, Entertainment, Hobbies, Shopping, Subscriptions).
• Savings & Extra Debt (20%): Wealth building and future security (Emergency Fund, Retirement Contributions, Investment Portfolios, Extra Debt Payoff).

$$\text{Group Ratio %} = \frac{\text{Group Planned Spend}}{\text{Total Planned Expenses}} \times 100$$

2. Multi-Currency Conversion Architecture

Line items and categories store native currencies (USD, EUR, GBP, etc.). Aggregated surfaces apply live FX rates via useFxConvert:

$$\text{Display Amount} = \text{Native Amount} \times \text{FX Rate}(\text{Native} \to \text{Display})$$

All form updates and direct row edits retain the native currency to avoid compounding rounding drift.

3. How Planned Income Is Counted

Income can be described in two places: as a category target ("Salary — 6,500 a month") and as one or more line items filed under that category ("Salary deposit — 6,500").

These are two views of the same money, not two sources of it. So the rule is items first:

• If a line item exists for an income category, the items decide that category's planned income and the category target is ignored.
• If a category has a target but no line items yet, the target counts.

Adding both would double every salary that had been entered twice, which in turn doubles the unallocated balance — the single number zero-based budgeting is built around.

$$\text{Planned Income} = \sum{\text{income items}} + \sum{\text{income category targets with no items}}$$

4. Importing an Existing Budget

Most people arrive with a budget that already exists somewhere — a spreadsheet, another app, a bank statement, or a sheet of paper. The import reads it and returns a list of proposed lines, each with a name, a category, a Needs / Wants / Savings & Debt / Income classification, planned and actual amounts, and whether it repeats monthly.

Two rules govern it:

• Totals are never imported. Subtotals, section headers and "balance carried forward" rows are skipped, because the tracker computes its own totals and importing them would count everything twice.
• Nothing is saved until you confirm. The extracted lines are shown for review with each row switchable off, and the month they belong to is editable before anything is written.

Amounts are always read as magnitudes; whether money moves in or out is decided by the group, not by a minus sign.

5. Sending a Budget Line to the Expense Ledger

A budget line is a plan. Once the payment has actually happened, it also belongs in the expense ledger as a fact.

Handing one over opens the expense form prefilled — in the line's own stored currency, never the converted figure you happen to be looking at — with the budget group mapped onto the closest matching expense category. It is always a prefilled form, never a silent write: the tracker does not create records in another tracker on your behalf.

6. Free and Pro

The free tier covers 8 monthly line items and 10 custom categories. Both are counted from the rows you actually have, so deleting one frees a slot again, and the example budget a new account starts with never counts against either.

Removing the caps, importing an existing budget and the AI budget copilot are Pro. Everything about reading and leaving is free on every plan: your existing data stays visible if Pro lapses, and CSV export — with every field the form collects — is never gated.