Calculator methodology
The exact rules behind every number.
A complete public statement of how Cairn's child support, spousal support, and tax engine work. Citations to source authority, and the cross-validation discipline against DivorceMate Tools One.
These are the rules the free calculator runs — see your numbers in five minutes.
On this page
What's covered here.
Last reviewed: August 2026 · Ontario family law and the federal support tables. Reviewed against source authority on this date.
Engine · Child support
Child support — Federal Guidelines + Schedule 1
Cairn's child support engine applies the Federal Child Support Guidelines (SOR/97-175 — the federal regulation that sets the tables). The base monthly amount comes from Schedule 1, Ontario column, indexed to the payor's gross annual income (T1 line 15000) and the number of children. For sole-care arrangements (kids living mostly with one parent), this table amount is determinative. The recipient's income does not change the base amount.
Section 7 special expenses — childcare, post-secondary, extraordinary educational and extracurricular expenses, health-related expenses over $100/year not covered by insurance, dental — are added on top and shared between parents in proportion to their gross incomes.
Shared-parenting arrangements under section 9 (each parent has 40%+ of the time) compute each parent's table amount at their income and set the two amounts off against each other. Split parenting time under section 8 computes each parent's table amount for the children in the other parent's care and offsets the two.
Engine · Spousal support
Spousal support — Spousal Support Advisory Guidelines (SSAG)
Cairn's spousal support engine applies the Spousal Support Advisory Guidelines (2008; Revised User's Guide 2016). Two formulas:
Without-children formula. Amount range = (1.5% × years × gross-income difference) up to (2% × years × gross-income difference); years stop counting at 25, so the range tops out at 37.5–50% of the gross-income difference. The recipient is never left with more than 50% of the couple's combined net income. Duration = 0.5–1.0 × years, indefinite after 20 years. The Rule of 65 (years + recipient's age at separation ≥ 65, with at least 5 years of relationship) also makes duration indefinite.
With-children formula. Operates on individual net disposable income (INDI — what each parent has left after tax, CPP/EI, the Ontario Health Premium and their child-support contribution, plus the child benefits they receive). Spousal support is the range of amounts that leaves the recipient with 40–46% of combined INDI. Benefits (Canada Child Benefit, Ontario Child Benefit, Canada Groceries and Essentials Benefit) are income-tested on net income, as the CRA does. Duration is the longer of the relationship-length test and the children's-age test.
Parenting arrangement variants, as the SSAG Revised User's Guide sets out: shared parenting (§8.6 — both parents' full table amounts deducted, and the range widened to include the point where both households have equal income), split parenting (§8.7), and custodial payor (§8.9 — children mainly with the higher earner: grossed-up child support removed from the payor's income, then the without-children formula).
Cairn outputs the low / mid / high amount and the SSAG duration range. The mid is shown as the headline number; low and high are visible underneath. We don't pretend a single point estimate is the answer — the Guidelines explicitly produce a range.
Engine · Tax
Ontario tax engine — federal + provincial + CPP + EI
The Cairn tax engine implements current-year federal and Ontario provincial tax brackets, the Basic Personal Amount (federal and Ontario), the eligible-dependant credit, the Ontario surtax, CPP base contributions up to the Year's Maximum Pensionable Earnings (YMPE — the CRA-set cap on first-tier CPP contributions), CPP2 (the second-tier contribution introduced 2024, applied between the YMPE and the Year's Additional Maximum Pensionable Earnings or YAMPE), and EI on insurable earnings up to the maximum. Following the CRA T4032ON method, the base CPP contribution and EI are tax credits while the enhanced CPP and CPP2 contributions are deductions (self-employed parents pay both halves); the Canada employment amount, the Ontario tax reduction, the LIFT credit and the Ontario Health Premium are applied. Results are pinned to a published 2026 Ontario tax table in automated tests.
Spousal support is treated as deductible to the payor (line 22000) and taxable to the recipient (line 12800). Child support is neither deductible nor taxable.
Tax tables are updated annually. The current set is sourced from CRA T4032ON, and we maintain an internal runbook for the annual roll-forward.
Engine · Source deductions
T1213 source-deduction view (spousal support payors)
For payors of spousal support, Cairn shows two paycheque scenarios. Default withholding: tax is withheld on gross pay ignoring the spousal support deduction; the deduction returns as a year-end refund. T1213-approved: tax is withheld on gross pay minus the annualized spousal support deduction; cash stays with the payor each pay period.
The Cairn calculator does not file T1213 for you — it shows the gap and explains how. Filing the form, attaching proof, and waiting for CRA's Letter of Authority to your employer is your responsibility. The /calculator/t1213-source-deduction page has a 7-step filing guide.
Sources
Engine · Validation
How the numbers are checked
Every rate, bracket, table and benefit amount is checked against the official source — CRA, the Ontario Ministry of Finance, and Justice Canada's child support tables and SSAG publications. Tax figures are re-checked each December and benefit amounts each July, when they change.
Automated tests lock in the results before any release: tax results against a published 2026 Ontario tax table, child support against the official tables, the SSAG worked example, and a real case calculated professionally in DivorceMate Tools One (the software most Ontario family lawyers license) — that professional figure falls inside Cairn's range. We do not license DivorceMate, and Cairn's output is an estimate, not a DivorceMate calculation. The last full methodology review against source authority was September 24, 2026.
Sources
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