Budgeting

1 Book Topics

  • Mindset shift: budgets are about conscious allocation, not restriction; “room for error” is the most underrated concept (Housel)
  • Expense categories: fixed (predictable), variable (fluctuate), periodic (annual/irregular); each requires a different strategy
  • Sinking funds: set aside 1/12 of annual periodic costs each month to avoid “budget emergencies”
  • Zero-based budgeting: every dollar gets a job; income − all allocations = 0; best for rebuilding from debt
  • Percentage-based budgeting: Sethi’s Conscious Spending Plan (50–60% fixed, 10%+ investing, 20–35% guilt-free); low maintenance at steady income
  • Best tool: the one you’ll actually use; automatic imports, easy categorization, and a summary view matter most

2 Percentages

The foundational budgeting calculation: what share of income does each expense represent?

Formula: (Expense ÷ Income) × 100

Rent ($1,500) as a share of a $5,000 paycheck

Rent ($1,500) as a share of a $5,000 paycheck

We can create a simple function to calculate the percentage of income that an expense represents. The fmt_pct() function formats the result as a percentage.

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percent_of_income <- function(expense, income) {
    fmt_pct(expense / income)
}
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percent_of_income(expense = 1500, income = 5000)
#> [1] "30%"

In Python, we store intermediate values explicitly before formatting; this makes the calculation steps transparent and easier to debug.

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def percent_of_income(expense, income):
    ratio = expense / income
    return fmt_pct(ratio)

percent_of_income(expense=1500, income=5000)
#> '30.0%'

Assuming Expense is in cell B2 and Income is in cell C2:

=(B2 / C2) * 100

Or, more simply, enter =B2 / C2 and format the cell as a Percentage using the button in the toolbar.

3 Annualizing and Monthly-izing

Scale any spending habit across time periods to see the true cost.

Annual → Monthly: ÷ 12

Monthly → Annual: × 12

Weekly → Monthly: × 4.33

Daily → Annual: × 365

The same $5/day habit across four time periods

The same $5/day habit across four time periods

Two of the most common conversions are annualizing daily, weekly, and monthly amounts and converting annual, weekly, or daily amounts to a monthly equivalent.

to_annual <- function(amount, period = c("daily", "weekly", "monthly")) {
  period  <- match.arg(period)
  factors <- c(daily = 365, weekly = 52, monthly = 12)
  fmt_dollar(amount * factors[[period]])
}
to_monthly <- function(amount, period = c("daily", "weekly", "annual")) {
  period  <- match.arg(period)
  factors <- c(daily = 30.44, weekly = 4.33, annual = 1 / 12)
  fmt_dollar(amount * factors[[period]])
}
to_annual(amount = 5, period = "daily")
#> [1] "$1,825.00"
to_monthly(amount = 150, period = "weekly")
#> [1] "$649.50"
def to_annual(amount, period="daily"):
    factors = {"daily": 365, "weekly": 52, "monthly": 12}
    return fmt_dollar(amount * factors[period])
def to_monthly(amount, period="daily"):
    factors = {"daily": 30.44, "weekly": 4.33, "annual": 1 / 12}
    return fmt_dollar(amount * factors[period])
print(to_annual(amount=5, period="daily"))
#> $1,825.00
print(to_monthly(amount=150, period="weekly"))
#> $649.50

Let’s start with the daily value in cell A1, then convert it to the cost per week in cell A2:

=A1 * 7

Next we can convert the week to the monthly cost in cell A3:

=A2 * 4.33

Finally, we get the annual cost in cell A4:

=A1 * 365

Format cells A1 - A4 as Currency or Accounting.

To convert a weekly value in cell A1 to a monthly value, place the following formula in cell A2:

=A1 * 4.33

To convert the monthly cost to an annual cost, enter the following in cell A3:

=A2 * 12

Format cells A1 - A3 as Currency or Accounting.

4 Opportunity Cost

Every dollar spent is also what that dollar could have become. The hidden cost of any purchase is its future value if invested instead.

Formula: amount × (1 + r)^n

$25,000 invested at 7% — value at each checkpoint

$25,000 invested at 7% — value at each checkpoint
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opportunity_cost <- function(amount, rate, years) {
  (amount * (1 + rate)^years) |>
    fmt_dollar()
}
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opportunity_cost(amount = 25000, rate = 0.07, years = 30)
#> [1] "$190,306.38"

In Python, we store intermediate values explicitly before formatting; this makes the calculation steps transparent and easier to debug.

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def opportunity_cost(amount, rate, years):
    fv = amount * (1 + rate) ** years
    return fmt_dollar(fv)
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opportunity_cost(amount=25000, rate=0.07, years=30)
#> '$190,306.38'

Assuming the initial amount is in cell A2, the annual rate (as a decimal, e.g., 0.07) is in B2, and the number of years is in C2:

=A2*(1+B2)^C2

5 Break-Even

Useful for evaluating memberships, refinances, or any purchase with an upfront cost and recurring savings.

Formula: Upfront Cost ÷ Monthly Savings = months to break even

$60/month savings accumulating toward a $500 upfront cost

$60/month savings accumulating toward a $500 upfront cost
months_to_break_even <- function(upfront_cost, monthly_savings) {
  upfront_cost / monthly_savings
}
months_to_break_even(upfront_cost = 500, monthly_savings = 60)
#> [1] 8.333333
def months_to_break_even(upfront_cost, monthly_savings):
    return upfront_cost / monthly_savings
months_to_break_even(upfront_cost=500, monthly_savings=60)
#> 8.333333333333334

Assuming the upfront cost is in cell A2 and the monthly savings amount is in cell B2:

=A2 / B2