A household budget

A budget answers two questions: how much is left once the bills are paid, and whether the spending stayed inside the limit you set. This note keeps the month in three parts, adds each one up without a single +, and ends with two checks, lines that pass or fail rather than answer with a number.

# Income
Salary: £2,450                             // £2,450.00
Side work: £320                            // £320.00

# Bills
Rent: £950 #fixed                          // £950.00
Council tax: £160 #fixed                   // £160.00
Energy: £118 #fixed                        // £118.00
Phone: £22 #fixed                          // £22.00

# Spending
spent += £64                               // £64.00
spent += £38.50                            // £102.50
spent += £71.20                            // £173.70
spent                                      // £173.70

# Month
:income = total of section "Income"        // £2,770.00
:bills = total of #fixed                   // £1,250.00
:left = income - bills - spent             // £1,346.30
left / 4                                   // £336.57
check left >= £500                         // ✓
check spent <= £150                        // ERROR: check failed: £173.70 is more than £150.00

Change a bill or add a line of spending and the month follows. The last check fails on purpose: the shopping went past £150, and the note shows both amounts rather than leaving you to notice.

What each part is doing

# Income is a heading, and a heading starts a section: the lines under it, down to the next heading at the same level or above. total of section "Income" adds up every figure in that section, so a new source of income is one more line under the heading. See sections.

Salary: £2,450 is a label and an amount. The text before the colon is for you; the engine reads the amount after it.

#fixed is a category tag. It marks a bill as belonging to a group without changing what it is worth, and total of #fixed adds up every line carrying it, wherever it sits. A tag is the tool when the lines you want to add are not all in one place. See category tags.

spent += £64 keeps a running total: each line adds to spent and shows the total so far, the way a till receipt does. The first one starts from zero. See variables.

:left = income - bills - spent names the answer so that the lines below can use it. left / 4 is roughly a week’s share of it.

check left >= £500 is a check: a comparison that answers with a tick when it holds and with an error, naming both sides, when it does not. It is the line that tells you, at a glance, whether the month worked. See checks.

Changing the shape of the month

A second tag keeps a second group apart: mark the subscriptions #subs and total of #subs sits beside total of #fixed. A section can hold subsections (## Food, ## Travel), and the section’s total includes them. What the note does not do is know your bank balance: every figure is one you typed, so the answers are as current as the lines are.