Params

A param is a named numeric value that can change over time. Params let you express things like salary, contribution limits, or interest rates in one place and reference them throughout your entries and assertions.

Constant params

param <name> = <expression>

param interest_rate = 5%
param retirement_rate = 0.16
param max_401k = 24_500 per year

A % after a value divides it by 100, so 5% is 0.05. per year makes the value a rate (see Rates).

The expression is re-evaluated at the start of each simulated day. For expressions built from numbers and other constant params, the value never changes. A param that reads an account balance follows that balance as it changes.

Time-varying params

param <name> {
  from <date> [to <date>] = <expression>
  from <date> [to <date>] = <expression>
  ...
}

param salary {
  from 2025-12-31 to 2026-04-01 = 115_000 per year
  from 2026-04-01               = 130_000 per year
}

Each interval specifies a from date (inclusive) and an optional to date (exclusive). The simulator uses whichever interval covers the current day. Intervals must not overlap. An interval without a to clause extends indefinitely.

On a day that no interval covers (before the first interval, in a gap between intervals, or after the last one ends), the param has no value. Using it on such a day is an error, so add an interval with the value you want, for example = 0, to cover those days.

A more complete example:

param beth_salary {
  from 2026-01-01 to 2027-01-01 = 160_000 per year
  from 2027-01-01 to 2028-01-01 = 190_000 per year
  from 2028-01-01 to 2029-01-01 = 225_000 per year
  from 2029-01-01 to 2030-01-01 = 255_000 per year
}

Rates

per day, per week, per month, per quarter or per year after a value makes it a rate: an amount per period, like a salary or a yearly contribution limit. Write a rate the way you’d say it, and let entries work out how much of it each firing posts (see Entries):

param salary   = 120_000 per year
param rent     = 3_000 per month
param interest = 5% per year

per binds tighter than any other operator, so salary - 500 per month subtracts 500 a month, and (a + b) per year needs its parentheses.

saldo checks how rates combine before it simulates anything:

A rate and a total over the same period, like a yearly limit and a .ytd total, can be added, subtracted and compared: the rate counts in full. Other amounts, like account balances, only mix with rates in an entry’s postings, where a rate means the firing’s share of it.

A param takes on the kind of its value, so param gross = salary + bonus is a rate too. per converts rates between months, quarters and years, or between days and weeks, but not from weeks or days to months or years, since those aren’t a fixed number of weeks or days.

Using params in expressions

Reference a param by name in any expression:

jim_salary * 0.16
Liabilities:Loan * interest_rate
min(salary * rate, max_401k - retirement_contribution.ytd)

A time-varying param automatically returns the right value for the current date, so you never need to branch on time in your expressions.

Aggregations

Named legs on entries (see Entries) accumulate into period-to-date buckets that you can read in any expression:

These reset automatically at the start of each year, quarter, or month.

min(salary * 0.16, max_401k - retirement_contribution.ytd)