Instrument 01 · Plan
Zero-Based Budget Calculator Every Unit Gets A Job.
A zero-based budget gives every unit of income a job — bills, spending, savings or debt — until income minus everything assigned equals zero. It does not mean spending it all: savings is a job too. Enter what actually lands in your account, then assign until To Assign reads zero.
Runs in your browserNothing uploaded or storedLast reviewed
Use net pay, not gross — a plan built on money that never arrives fails in week three. Changing this rescales every unlocked envelope and keeps your shape.
The lock keeps an envelope's amount fixed — Auto Assign and income changes will route around it. Nature (Need / Want / Extra) is what turns a list of numbers into a judgement you can act on.
Left to assign
Reading Your Result
What The Number Is Telling You.
- To Assign above zero
- Money without a job. Give it one — savings counts.
- To Assign at zero
- The plan is complete. Every unit is spoken for.
- To Assign below zero
- You have assigned more than you earn. Take it back from a flexible category.
The Model
The Formula, In Full.
The arithmetic this instrument runs, with a worked example. Its test cases are published on the methodology page
To Assign = Income − Σ Assigned
Category balance = Assigned − Spent (a negative balance must be covered from another category or from To Assign)Worked example. Income 4,200 → Rent 1,450 · Groceries 520 · Transport 180 · Utilities 160 · Insurance 140 · Extra debt payment 400 · Emergency fund 500 · Sinking funds 350 · Personal 300 · Subscriptions 90 · Giving 110 = 4,200. To Assign: 0.
Assumptions and edge cases
- Income is take-home pay that has actually arrived, not salary before tax.
- One month at a time; next month starts from next month's income.
- Savings and extra debt payments are categories like any other.
| Edge case | Behaviour |
|---|---|
| To Assign negative | Shown in the alarm colour with the exact over-assignment; never clamped to zero. |
| Income blank or zero | Figures show an em dash; no division is attempted. |
| A category left at zero | Valid. A zero assignment is a decision. |
| Irregular income | Budget last month's income (see FAQ). |
Questions
Zero-Based Budget, Answered Plainly.
What is a zero-based budget?
A zero-based budget assigns every unit of income to a specific job until income minus assignments equals zero. The jobs include bills, everyday spending, savings, sinking funds and extra debt payments. It is called zero-based because you start each month from zero and justify every allocation, rather than adjusting last month's numbers. Nothing is left unassigned, which is what makes overspending visible the moment it happens.
How do I make a zero-based budget?
Start with the take-home pay that has actually arrived this month. Fund fixed bills first, then minimum debt payments, then savings and sinking funds, then flexible spending such as groceries and fuel. Anything left goes to a goal or to extra debt payments. Adjust until To Assign reads exactly zero. The calculator above does the arithmetic as you type, so the only decisions left are yours.
Does zero-based budgeting mean spending all my money?
No. Zero refers to unassigned money, not to your bank balance. Money assigned to an emergency fund, a holiday or extra debt payments is still yours — it simply has a job. A zero-based budget with a large savings line is a budget that saves on purpose rather than by accident.
What happens if I overspend a category?
Cover it immediately by moving money from another category, or from To Assign if anything is left. The overspend is real money already gone, so pretending it did not happen only moves the problem to next month. Covering it forces the trade-off into the open: the extra spent on dining out came from somewhere, and you choose where.
How do I do a zero-based budget with irregular income?
Budget the money you already have, not the money you expect. The most reliable method is to live one month behind: this month's budget uses last month's total income, which has already arrived. Until you get there, budget each payment as it lands, funding essentials first. A buffer category that holds one month of essentials smooths the gaps between irregular payments.
What is the difference between zero-based budgeting and the 50/30/20 rule?
The 50/30/20 rule sets the shape of a budget: half to needs, 30 per cent to wants, 20 per cent to savings. A zero-based budget is the plan inside that shape, with every unit assigned to a named category. Many people start with 50/30/20 to see whether their fixed costs fit, then move to zero-based budgeting to run the month. The 50/30/20 calculator checks the shape in a minute.
What are sinking funds in a zero-based budget?
Sinking funds are categories that save a little each month for a known future cost: car insurance due in March, a holiday in August, a laptop that will need replacing. Dividing the cost by the months remaining turns an annual shock into a monthly line. In a zero-based budget they are ordinary categories that happen to roll over, and the savings goal calculator works out each monthly figure.